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Fluorescent dissolved organic matter as a multivariate biogeochemical tracer of submarine groundwater discharge in coral reef ecosystems
... Offshore coral islands which experience tidally modulated SGD input, release nutrients via vertical pore water upwelling, tidal pumping, and temperature-driven convection, which can in turn, lead to sustained productivity within coral reefs (Santos et al., 2010). Tidally dominated SGD inputs can be chronic sources of nutrients to tropical reefs (Nelson et al., 2015;Jiang et al., 2021); this contrasts with fluvial nutrient inputs, which vary greatly with precipitation (Amato et al., 2016;Richardson et al., 2017a;Richardson et al., 2017b). Hence, in coastal areas with tidally dominated SGD inputs, SGD may comprise a significant component of reef nutrient inputs but have highly variable biogeochemistry on small temporal scales (i.e., hours) and spatial scales (i.e., meters) (Swarzenski et al., 2017;Luijendijk et al., 2020;Silbiger et al., 2020). ...
... 2 Materials and methods 2.1 Study site description and experimental design 157.7624°W) close to the shoreline (Nelson et al., 2015). The reef flat is dominated by macroalgae. ...
... On reefs with mostly freshwater SGD that experience semidiurnal tidal patterns, intermediate salinities can occur up to four times a day, creating a periodicity that could produce consistently elevated productivity on reefs with SGD. Consistent with previous surveys of spatial distribution of SGD at this site (Nelson et al., 2015;Richardson et al., 2017a;Richardson et al., 2017b;Lubarsky et al., 2018), we showed that groundwater can have an effect on productivity up to 200 meters offshore (Figure 1; Supplemental Figure 4); it is important to understand SGD's full spatial extent in SGD is the main and largest source of freshwater and inorganic nutrients to this site. These relationships were found using linear regressions obtained from salinity vs. nutrients plots. ...
Submarine groundwater discharge (SGD) in high volcanic islands can be an important source of freshwater and nutrients to coral reefs. High inorganic nutrient content is generally thought to augment primary production in coastal systems but when this is delivered via a freshwater vector as is the case with SGD in this study, the effects on productivity are unclear. In the current literature, there is limited evidence for a direct association between SGD and primary productivity of reefs. To elucidate the response of primary productivity to SGD, we conducted spatially and temporally explicit in situ benthic chamber experiments on a reef flat along a gradient of SGD. We found significant quadratic relationships between C-uptake and SGD for both phytoplankton and the most abundant macroalga, Gracilaria salicornia , with uptake maxima at SGD-derived salinities of ~21−22 (24.5−26.6 μmol NO 3 -L ⁻¹ ). These results suggest a physiological tradeoff between salinity tolerance and nutrient availability for reef primary producers. Spatially explicit modeling of reefs with SGD and without SGD indicate reef-scale G. salicornia and phytoplankton C-uptake decreased by 82% and 36% in the absence of SGD, respectively. Thus, nutrient-rich and low salinity SGD has significant effects on algal C-uptake in reef systems.
... Springs have been reported in many regions: Hawaii (Nelson et al., 2015), Indonesia (Oehler et al., 2018) and Florida (Luzius et al., 2018), to name a few. They have been identified as potential sources of nutrients (Carruthers et al., 2005;Null et al., 2014;Swarzenski et al., 2017;Oehler et al., 2019a) but their impact varies with seasonal changes in precipitation (Luzius et al., 2018). ...
... Depending on the surrounding terrestrial environment and hydrological regime, for example in systems with a strong DOM surface signal, the SGD-derived DOM signal may be masked. The residence time in the coastal aquifer also plays an important role: In comparison, diffuse SGD appears to have greater potential of accumulating DOC along its flow paths than spring-type SGD, mainly because longer residence times favor reducing conditions with lower abundances of electron acceptors, as well as additional release of DOM due to particulate organic matter degradation and desorption from mineral phases (e.g., Seidel et al., 2014;Nelson et al., 2015;Linkhorst et al., 2017). However, submarine springs can be seasonally or locally enriched with high concentrations of DOC (Luzius et al., 2018;Pain et al., 2019;Adyasari et al., 2021) which may discharge rapidly into the adjacent water column. ...
... However, DOC concentrations in the groundwater springs were rather low and may not provide enough substrate for the local microbial communities. FDOM was used as an SGD tracer in coastal environments in previous studies (Kim et al., 2012;Nelson et al., 2015;Kim and Kim, 2017) and in addition applied to differentiate for example terrestrial, anthropogenic or microbial sources of DOM. The handheld device used in this study has emission and excitation spectra targeting mainly terrestrial, "humic-like" FDOM fractions (Coble, 1996). ...
Submarine groundwater discharge (SGD) connects fresh groundwater and marine ecosystems and conveys terrestrially derived dissolved organic matter (DOM) and nutrients from land to sea. The connectivity of terrestrial and marine ecosystems via SGD depends strongly on local environmental settings. For example, SGD composition is modified on its transit through the coastal aquifer, with spring-type SGD from highly permeable aquifers presumably being less affected than diffuse discharge systems from sedimentary environments. In our study, we investigated spring-type SGD near Sahlenburg/Cuxhaven, Northern Germany, which passes through fine, unconsolidated tidal sediments before entering the coastal ocean. We characterized groundwater, surface water and seawater endmembers from different seasons and assessed the potential of tidal sediments impacting the biogeochemistry of “fast-track”, point-source groundwater discharge systems. In addition to physicochemical parameters and nutrients, we analyzed the DOM molecular composition via ultrahigh-resolution mass spectrometry (FT-ICR-MS). Our data revealed a widespread physicochemical and geochemical influence of the groundwater springs on the tidal flat, producing low salinity and low dissolved organic carbon (DOC), and high nitrate and high oxygen concentrations not only in the springs, but also in adjacent porewater. From near- to offshore, salinity and DOC concentrations in springs decreased whereas nitrate and oxygen concentrations increased, resembling an inverse estuarine pattern. Furthermore, high nitrate values suggest anthropogenic sources (e.g., agricultural influence) in the surrounding watershed and may stimulate primary productivity in the tidal flat. Humic-like fluorescent DOM (FDOM) abundances and DOM molecular fingerprints indicated inputs of terrestrial DOM from nearshore saltmarsh plants, as well as from the nearby Elbe and Weser estuaries. Our study demonstrated that SGD had a strong geochemical impact even in the vicinity of large rivers, with productive springs actively hindering sulfate and nitrate reduction by flushing otherwise anoxic systems with oxygen. We posit that the geochemical influence of groundwater springs in tidal flats is underestimated because it can extend far beyond their visual discharge points.
... SGD is the flow of water-meteoric, marine or a composite of both-from the land through the marginal seabed and into the coastal ocean [1,2]. SGD is a widespread phenomenon on near shore coral reefs as evidenced by reports from Hawai'i [3][4][5][6], Mo'orea [7,8], Florida [9], Israel [10], the Great Barrier Reef [11], Jamaica [12], Mexico [13] and Japan [14]. While there is an extensive body of literature describing high fluxes of SGD to coral reefs and other coastal ecosystems [15,16], there is a critical need to better understand the biological and ecological consequences of SGD [17]. ...
... SGD can be a source of high nutrient, low pH and cool water to coastlines [6,11,18,19], which can benefit oligotrophic systems [20] or put coral reefs at risk of eutrophication [15], depending on the watershed conditions and local environmental context. While SGD is natural, it can also act as a conduit for anthropogenic pollutants and lead to reef degradation [21]. ...
... While SGD is natural, it can also act as a conduit for anthropogenic pollutants and lead to reef degradation [21]. Additionally, SGD can be fresh or saline and often has different concentrations of total alkalinity (TA), silicate and dissolved organic compounds than seawater [6,19,22]. Because each of these biogeochemical parameters can affect biological and ecological processes on coral reefs, it is important to understand how the unique environments created by SGD affect coral reef ecosystems. ...
Submarine groundwater discharge (SGD) influences near-shore coral reef ecosystems worldwide. SGD biogeochemistry is distinct, typically with higher nutrients, lower pH, cooler temperature and lower salinity than receiving waters. SGD can also be a conduit for anthropogenic nutrients and other pollutants. Using Bayesian structural equation modelling, we investigate pathways and feedbacks by which SGD influences coral reef ecosystem metabolism at two Hawai'i sites with distinct aquifer chemistry. The thermal and biogeochemical environment created by SGD changed net ecosystem production (NEP) and net ecosystem calcification (NEC). NEP showed a nonlinear relationship with SGD-enhanced nutrients: high fluxes of moderately enriched SGD (Wailupe low tide) and low fluxes of highly enriched SGD (Kūpikipiki'ō high tide) increased NEP, but high fluxes of highly enriched SGD (Kūpikipiki'ō low tide) decreased NEP, indicating a shift toward microbial respiration. pH fluctuated with NEP, driving changes in the net growth of calcifiers (NEC). SGD enhances biological feedbacks: changes in SGD from land use and climate change will have consequences for calcification of coral reef communities, and thereby shoreline protection.
... Besides nutrients and other inorganic species, SGD and rivers may also carry dissolved organic matter (DOM) to coral reefs. Recent works have shown significant inputs of groundwaterderived chromophoric and fluorescent DOM (CDOM and FDOM, respectively) in various coastal reef environments (Tedetti et al., 2011;Nelson et al., 2015;Kim and Kim, 2017;Webb et al., 2019). Humic-like fluorophores have been identified as major components of groundwater FDOM, and it has been proposed to use FDOM to differentiate groundwater types and to track their dispersal into reef waters (Tedetti et al., 2011;Nelson et al., 2015;Kim and Kim, 2017). ...
... Recent works have shown significant inputs of groundwaterderived chromophoric and fluorescent DOM (CDOM and FDOM, respectively) in various coastal reef environments (Tedetti et al., 2011;Nelson et al., 2015;Kim and Kim, 2017;Webb et al., 2019). Humic-like fluorophores have been identified as major components of groundwater FDOM, and it has been proposed to use FDOM to differentiate groundwater types and to track their dispersal into reef waters (Tedetti et al., 2011;Nelson et al., 2015;Kim and Kim, 2017). In addition to direct allochthonous inputs, an autochthonous production of FDOM may occur in the reef waters due to the SGDinduced eutrophication or other processes (Blanco et al., 2008;Tedetti et al., 2011;Nelson et al., 2015). ...
... Humic-like fluorophores have been identified as major components of groundwater FDOM, and it has been proposed to use FDOM to differentiate groundwater types and to track their dispersal into reef waters (Tedetti et al., 2011;Nelson et al., 2015;Kim and Kim, 2017). In addition to direct allochthonous inputs, an autochthonous production of FDOM may occur in the reef waters due to the SGDinduced eutrophication or other processes (Blanco et al., 2008;Tedetti et al., 2011;Nelson et al., 2015). CDOM/FDOM in coral reef waters could influence the reef biota by supplying carbon, but also by modifying the seawater optical properties and subsequently the attenuation (and availability) of solar UV and visible radiations in the water column (Coble, 2007;Nelson and Siegel, 2013;Sempéré et al., 2015;Martias et al., 2018). ...
We investigated the influence of freshwater discharges on biogeochemistry and benthic communities of La Saline fringing reef, the largest coral reef system of La Réunion Island (France, Indian Ocean). Sampling was conducted in February–March 2012 along a land-ocean continuum, including rivers, groundwater, reef waters, and oceanic waters. Our results indicated the discharges of basalt groundwater (enriched in nitrates) in the southern part of the reef, and the discharges of sand groundwater and river waters (enriched in organic matter) in sites located northward. These discharges mainly impacted fore reef surface waters, while groundwater resurgences were more diffuse and not clearly detected in depth. We highlighted the relationship Si(OH)4/NO3– to identify basalt groundwater discharges. We also put forward the relationships humic-like fluorescence/NH4⁺ and tryptophan-like fluorescence/PO43– to identify sand groundwater discharges and wastewater pollutions, respectively. We observed the eutrophication of the southern part of the reef through higher phytoplankton biomasses, a massive presence of fleshy algal formations and a low coral coverage. The spatial homogeneity of the benthic community structures onto the entire slope contrasted with the spatial heterogeneity of chemical variables in water. This reflected the different spatial and temporal scales involved in biological and chemical processes in coral reef ecosystems.
... This is especially true in volcanic high island systems surrounded by oligotrophic waters (Moosdorf et al. 2015). Most of the literature concerning SGD has been focused on characterizing its solutes and chemical composition, with the implication that these distinct water masses can be important vectors for inorganic nutrients, organic matter, metals, microbes, and pharmaceuticals (Nelson et al. 2015;Richardson et al. 2017b). However, fewer studies have looked at the impacts of SGD on coastal communities. ...
... Maunalua Bay, located on the southeastern shore of O'ahu, has algal-dominated fringing reefs with localized, tidally modulated SGD. The fringing reefs have well-characterized and biogeochemically distinct SGD from each other close to the seeps (e.g., different inorganic nutrient concentrations, different carbonate chemistry) (Nelson et al. 2015;Richardson et al. 2017a;Lubarsky et al. 2018). The bay experiences a mixed semi-diurnal tidal regime such that the SGD fluxes vary on several cyclical temporal scales. ...
... There is a close to conservative relationship between SGDderived inorganic nutrients (nitrate, phosphate, and silicate) and salinity at Black Point and Wailupe (linear regressions statistics described in table 1 of La Valle et al. 2019, included as Table S2). SGD fluxes and nutrient delivery was described in detail in Holleman (2011), Nelson et al. (2015, Richardson et al. (2017a, b), Lubarsky et al. (2018), and summarized in La The SGD at both sites is high in inorganic N and P but approximately twice as concentrated at Black Point, possibly due to the high density of nearshore cesspools in this area (Richardson et al. 2017b). Salinity and 222 Rn surveys of the bay's coastline indicated negligible surface water inputs (Richardson et al. 2017a) suggesting that SGD is the dominant source of nutrients and terrestrial freshwater inputs throughout the year, excluding rain. ...
Submarine groundwater discharge (SGD) is an important transporter of solutes and fresh water in coastal systems worldwide. In high island systems with a mixed semidiurnal tidal cycle driving SGD, coastal biogeochemistry is temporally and spatially variable. Past studies have shown that SGD covaries with the local species composition, diversity, and richness of biological communities on a scale of meters. Empirical orthogonal function analyses (EOF)—a method analogous to principal components analysis which finds spatial patterns of variability and their time variation period—were used to define both the spatial and temporal variation in SGD using spatially resolved time series of salinity. The first two EOFs represented variability at the tidal 12‐h period and the daily 24‐h period, respectively, and were responsible for more than 50% of the SGD‐derived salinity variability. We used the first two EOFs to explore spatiotemporally explicit patterns in SGD variability and their relationships with benthic community structure in reef systems. Distance‐based linear models found significant relationships between multivariate community structure and variability in SGD at different periods. Taxa‐specific logistic regressions showed that zoanthids and turf are more likely to be present in areas with high tidally driven SGD variability, while the inverse relationship is true for the invasive rhodophyte Acanthophora spicifera, calcifying macroalgae, the native rhodophyte Pterocladiella sp., the cyanobacteria Lyngbya sp., and the invasive chlorophyte Avrainvillea amadelpha. These results show that benthic communities vary with respect to SGD derived salinity at the scale of hundreds of meters resulting in spatially heterogeneous biotic patches.
... Samples were analyzed on an Attune Acoustic Focusing Cytometer with Autosampler Attachment (Life Technologies, Eugene, OR, United States). Samples were run at a flow rate of 100 µL min −1 on standard sensitivity; 150 µL of sample was aspirated, 75 µL was counted and data was collected only from the last 50 µL (event rates were empirically determined to be steady only after 25 µL of continuous sample injection per Nelson et al., 2015). ...
... Samples for fluorescence spectroscopy were measured using an Horiba Aqualog scanning fluorometer following the methods of Nelson et al. (2015), including scan time and resolution, spectral data processing, inner filter correction, Raman unit standardization, blank subtraction and PARAFAC modeling (Stedmon and Bro, 2008;Lawaetz and Stedmon, 2009;Kothawala et al., 2013). Scans were processed using a Matlab (v2007b) script written and specified by Nelson et al. (2015) and Quinlan et al. (2018); most recent version available at doi: 10.5281/zenodo.3479841), ...
... Samples for fluorescence spectroscopy were measured using an Horiba Aqualog scanning fluorometer following the methods of Nelson et al. (2015), including scan time and resolution, spectral data processing, inner filter correction, Raman unit standardization, blank subtraction and PARAFAC modeling (Stedmon and Bro, 2008;Lawaetz and Stedmon, 2009;Kothawala et al., 2013). Scans were processed using a Matlab (v2007b) script written and specified by Nelson et al. (2015) and Quinlan et al. (2018); most recent version available at doi: 10.5281/zenodo.3479841), modified to additionally capture the peak present at Excitation 240 nm and Emission 300 nm (phenylalanine-like: Lakowicz, 2010). ...
Crustose coralline algae (CCA) are critical members of the coral reef ecosystem, yet they remain poorly studied. Recent research on CCA has shown that only a few species play a significant role in the settlement of coral larvae through either the production of chemical settlement cues or the facilitation of specific microbial communities that are hypothesized to influence coral settlement. Thus, defining how DOM exudates differ between CCA species and the bacterioplankton communities these exudates facilitate is important for understanding the role of CCA in invertebrate settlement. We conducted single day exudation experiments on two species of CCA to compare tissue microbiome community structure, DOM production and the effect of DOM on the bacterioplankton community. We collected exudates from Hydrolithon reinboldii and Porolithon onkodes in both filter-sterilized seawater and unfiltered seawater from Kāne‘ohe Bay, Hawai‘i. Our results demonstrate that while both species exude equivalent quantities of dissolved organic carbon they differ in the composition of fluorescent DOM and fostered distinct microbial communities. P. onkodes exudates facilitate more microbial OTUs associated with coral disease, whereas H. reinboldii facilitated OTUs known to produce antimicrobial compounds. Our results highlight species-specific differences in the composition of fDOM exudates of CCA and the effect of those on microbial community structure.
... So far, only two publications report FT-ICR-MS data of DOM composition in the STE Beck et al. 2017), whereas CDOM analyses are more widely applied (Kim et al. 2012(Kim et al. , 2013Suryaputra et al. 2015;Nelson et al. 2015;Couturier et al. 2016). Overall, these qualitative DOM studies confirmed supply of marine, labile DOM to the STE by tides and waves, indicated by enrichment patterns of labile organic matter, such as sugars, amino acids, and protein-like CDOM, at deposition sites of algal debris or in areas where seawater infiltration occurred (Fig. 12.3; Kim et al. 2012;Seidel et al. 2015). ...
... On the other hand, strong negative correlations of humic-like CDOM fluorescence and absorbance with salinity have been attributed to a terrestrial source of aromatic DOM compounds. As a result, humic-like CDOM fluorescence has been used as SGD tracer in areas with productive submarine springs (Fig. 12.3; Nelson et al. 2015;Kim and Kim 2017). Compared to aliphatic, proteinlike CDOM, the aromatic, humic-like DOM compounds are less biodegradable and more photodegradable and absorb sunlight. ...
... Low salinities are indicated by cool colors, and high relative abundances of terrestrially-derived humic substances (or HIX) are indicated by warm colors. Modified from (a) Seidel et al. (2015) and (b) Nelson et al. (2015) with permissions from Elsevier Furthermore, not many studies have looked at the microbial diversity of STE, using modern sequencing technologies. A previous molecular study from the island of Spiekeroog by Beck et al. (2017) was based on denaturing gradient gel electrophoresis (DGGE). ...
Terrestrial and marine environments merge at the land-sea transition zone. This zone is important as ~38% of the world’s population live by and depend on the coastal regions, and oceans are considerably affected by it. Furthermore, terrestrial and marine groundwater and seawater mix in the subterranean estuary (STE), where submarine groundwater discharge (SGD), i.e., discharging fresh groundwater and recirculated seawater, results in significant solute fluxes to the sea. With this article, we focus on advances of geochemical, microbiological, and technological aspects related to fresh groundwater, SGD, and STE in sandy coastal areas, using the barrier island Spiekeroog as a case study area. Previous studies showed that the fresh groundwater composition in sandy coastal aquifers is governed by calcareous shell dissolution, cation exchange, and organic matter degradation. Biogeochemical reactions in the STE further modify the water composition of SGD, with a dependence on residence time. Microbial communities, which are present in coastal sediments and usually follow salinity and redox gradients, are the driver for the degradation of organic matter. Regarding organic matter sources in the STE, it is evident that dissolved organic matter is primarily of marine origin and that SGD delivers degraded dissolved organic matter back into the ocean. Furthermore, recent studies used radiotracers, such as radium and radon, and seepage meters as reliable tools to quantify rates and fluxes associated with SGD. We conclude that, despite the advances being made, the complexity and interactions of the different processes at land-sea transition zones require multidisciplinary scientific approaches.
... Optical signatures associated with river CDOM may be lost close to the river mouth through mixing with oligotrophic marine waters (Del Castillo et al., 2000;Jaffé et al., 2004). Additionally, groundwater has been found to exhibit higher levels of CDOM than adjacent surface seawater (Nelson et al., 2015;Webb et al., 2019). ...
... SGD can be a source of CDOM to coral reefs (Kim and Kim, 2017;Nelson et al., 2015;Webb et al., 2019). However, limited data are available on CDOM in SGD with related investigations focusing on nearshore reefs (Nelson et al., 2015). ...
... SGD can be a source of CDOM to coral reefs (Kim and Kim, 2017;Nelson et al., 2015;Webb et al., 2019). However, limited data are available on CDOM in SGD with related investigations focusing on nearshore reefs (Nelson et al., 2015). Here, we hypothesise that SGD in coral reefs provides a source of CDOM to surface waters that attenuates heat and light. ...
Excessive ultra-violet (UV)exposure acts synergistically with ocean warming to trigger coral bleaching. Photoinhibition in corals may be reduced by chromophoric dissolved organic matter (CDOM)absorbing UV light. This study assesses whether submarine groundwater discharge (SGD)delivers CDOM to the coral reef lagoon of One Tree Island (Great Barrier Reef). SGD was traced using radon ( ²²² Rn), a naturally occurring tracer. Time series observations revealed tidal radon and diel CDOM trends at One Tree Island. CDOM exhibited a strong negative correlation with PAR (R ² = 0.50), and a weak positive correlation with ²²² Rn (R ² = 0.13). Removal of time and light as drivers of CDOM resulted in residuals linked to radon at low tide and at night, but not at high tide or during the day. We conclude that CDOM was driven primarily by PAR fluctuations and light-driven oxidation with only minor contributions via ²²² Rn-traced SGD in this lagoon.
... Another possible application for such an observatory is to search for submarine groundwater discharges (SGD) in coastal waters. SGD consist of an inflow of fresh groundwater and recirculated seawater from the sea floor into the ocean [1] (Fig. 1). The localization of SGDs is a technical effort; however marine scientists are interested in their localization, because the SGDs discharge nutrients into the coastal environment [2,3]. ...
... Where s is in meter, r is a random number in the range [0,1] and a is the parameter alpha that can have any value in the range [1,2]. When using Levy-flight as a search algorithm, the value of alpha has to be chosen off-line by the user before the search. ...
... The performance of the search algorithms could be effected by the chosen value for the maximum step size. To take the impact of the maximum step size into consideration, the experiments for the different algorithms were carried out using different values of s from the range [1,400]. The performance of Levy-flight with different values of alpha [1.0, 1.5, 2.0] were compared. ...
... Nutrient concentrations are also used to assess water quality relative to sewage pollution. Elevated nutrients are typically detected in sewage polluted areas (Wei and Huang, 2010;Nelson et al., 2015;Amato et al., 2016). However, numerous non-sewage watershed sources affect nutrient concentrations. ...
... These results suggest that Puakō's shoreline and surface waters, as well as some benthic waters may be contaminated with sewage. Similar to FIB, nutrient concentrations are often spatially variable within water bodies with sewage pollution, with highest values generally observed along shorelines where homes have OSDS, including those in Hawai'i (Wei and Huang, 2010;Nelson et al., 2015;Amato et al., 2016;Wiegner et al., 2016). Our study and a concurrent one found that nutrient concentrations at Puakō followed this pattern, with highest concentrations documented along the shoreline, and then decreasing with increasing distance offshore (Couch et al., 2014b). ...
... and C. perfringens concentrations suggesting they may be from sewage (Table 2), whereas benthic nutrient concentrations were not correlated with FIB (Table 3). Compared to most other studies conducted in Hawai'i, shoreline NO 3 − + NO 2 − concentrations were five to ten times greater than those reported on other islands or locations on Hawai'i Island with known sewage inputs (Wiegner et al., 2013;Nelson et al., 2015;Wiegner et al., 2016;Wiegner et al., 2017). Additionally, shoreline nutrient concentrations (NO 3 − + NO 2 − , NH 4 + , and PO 4 3+ ) measured at Puakō as part of this study and Abaya et al., 2018 are the highest reported to date, and in some cases, they are 40 times greater than previously reported values (Knee et al., 2010). ...
... Maunalua Bay is an 8 km long embayment on the south shore of O'ahu, Hawai'i with several well-documented SGDsprings (Dimova et al. 2012;Swarzenski et al. 2013;Nelson et al. 2015;Richardson et al. 2017a). Land-use in Maunalua has shifted dramatically in the past century, from sparsely populated, rural developments in the early 1900s, toward rapid urbanization beginning in the 1960s (Wolanski et al. 2009). ...
... Two sites with previously identified SGD (Swarzenski et al. 2013;Nelson et al. 2015;Richardson et al. 2017a) were selected in Maunalua Bay. The sites, Black Point and Wailupe (Fig. 1), are shallow reef flats located 3 km apart, each with a discrete, nearshore SGD seep. ...
... Interestingly, although TA correlated highly with SGD, the TA gradient was opposite between the sites, with TA in SGD at Black Point higher than the marine endmember, and lower than the marine endmember at Wailupe (Supporting Information Fig. 6). This is consistent with previous studies (Nelson et al. 2015;Richardson et al. 2017b) and is likely due to the fact that SGD at each site is derived from separate aquifers (Richardson et al. 2017a,b). TA values converge at the lowest values of %SGD, indicating that we captured the marine endmember at both sites. ...
Submarine groundwater discharge (SGD) is an important source of nutrients to many coastal reefs, yet there is little information on how SGD impacts key coral reef processes. Here, we investigated the effect of SGD on coral growth and bioerosion rates from Porites lobata nubbins and blocks of calcium carbonate (CaCO 3) on two reef flats in Maunalua Bay, O'ahu. Over a 6-month (coral nubbins) and yearlong (CaCO 3 blocks) deployment period, we combined multiple metrics of coral growth (buoyant weight, surface area, and linear extension) and bioerosion with a suite of co-measured physicochemical parameters that are indicators for SGD (carbonate chemistry, dissolved inorganic nutrients, temperature, salinity, and water motion). All coral growth metrics showed a modal response to SGD, and the percent change in buoyant weights and nub-bin surface area were negatively related to pH variation. SGD negatively affected coral survival, indicating that at high levels of SGD, salinity stress could be killing corals, but at mid-levels SGD-associated nutrients could be increasing growth rates. SGD had a positive effect on bioerosion, most likely due to the positive effect of increased nutrients on bioeroding organisms. Further, coral accretion rates were two orders of magnitude higher than bioerosion rates; however, given the low coral cover on these reef flats, the total carbon-ate accreted by corals is much lower than suggested by rates alone. These results indicate that corals can thrive on SGD-impacted reefs if isolated from secondary stressors, so active management to reduce macroal-gae and sedimentation could allow coral recovery in Maunalua Bay.
... Hence, the great variety of CDOM sources and of transformation processes makes the coastal CDOM a complex and heterogeneous pool of organic molecules displaying a broad range of structure and reactivity (Benner et al., 2005;Cao et al., 2016). Whereas most research dealing with the dynamics of CDOM in coastal waters has focused on temperate or high latitude environments, the sources and dynamics of CDOM in tropical coral reef areas remain much less known (Tedetti et al., 2011;Rochelle-Newall et al., 2014;Nelson et al., 2015), although tropical environments have received special attention in recent years due to coral bleaching in the context of global change (Weishaar et al., 2003;Biscéré et al., 2017;Wolanski et al., 2017). ...
... Regarding the Chl a concentrations recorded here after a rainfall event (up to 1.44 μg L −1 ; already reported by Tenorio et al., 2005), the eastern lagoon of New Caledonia shows quite a high phytoplankton biomass compared with other tropical reef areas, such as the Réunion Island (0.14-0.32 μg L −1 ; Tedetti et al., 2011) and Maunalua Bay (Hawaï, 0.06-0.29 μg L −1 ; Nelson et al., 2015). In this study, tyrosinelike fluorophore was not well correlated to Chl a, suggesting that tyrosine-like fluorophore in the lagoon is not a part of the DOM produced by marine phytoplankton (Romera-Castillo et al., 2010). ...
... The presence of tryptophan-like fluorophore in the waters surrounding the coral reef could be linked to the biological activity of the coral reef ecosystem, as observed by in the Moorea lagoon, French Polynesia. Indeed, the dominant hermatypic coral holobiont Porites spp. in the South Pacific and the microalgae associated exude labile DOC with fluorescent properties similar to those of tryptophan-like material (Nelson et al., 2015), influencing the microbial activity and altering bacterioplankton growth and community structure around coral reefs. Tryptophan-like released in waters surrounding the coral reef would be then subjected to the effect of the tide inducing local seawater inflows and plumes of tryptophan from the open ocean to the coast even though no evidence of relevant maritime influence is obvious. ...
The eastern lagoon of New Caledonia (NC, Southwest Pacific), listed as a UNESCO World Heritage site, hosts the world's second longest double-barrier coral reef. This lagoon receives river inputs, oceanic water arrivals, and erosion pressure from ultramafic rocks, enriched in nickel (Ni) and cobalt (Co). The aim of this study was to characterize colored dissolved organic matter (CDOM), as well as to determine its main sources and its possible relationships (through the use of Pearson correlation coefficients, r) with biogeochemical parameters, plankton communities and trace metals in the NC eastern lagoon. Water samples were collected in March 2016 along a series of river/lagoon/open-ocean transects. The absorption coefficient at 350 nm (a(350)) revealed the influence of river inputs on the CDOM distribution. The high values of spectral slope (S275-295, >0.03 m(-1)) and the low values of specific ultraviolet absorbance (SUVA(254), <4 L mg-C-1 m(-1)) highlighted the photodegradation of CDOM in surface waters. The application of parallel factor analysis (PARAFAC) on excitation-emission matrices (EEMs) allowed the identification of four CDOM components: (1) one humic-and one tyrosine-like fluorophores. They had terrestrial origin, exported through rivers and undergoing photo-and bio-degradation in the lagoon. These two fluorophores were linked to manganese (Mn) in southern rivers (r = 0.46-0.50, n = 21, p < 0.05). (2) A tryptophan-like fluorophore, which exhibited higher levels offshore. It would be potentially released from the coral reef. (3) A second tyrosine-like ("tyrosine 2-like") fluorophore. Linked to Prochlorococcus cyanobacteria (r = 0.39, n = 47, p < 0.05), this fluorophore would have an oceanic origin and enter in the lagoon through its southern and northern extremities. It also displayed relationships with Ni and Co content (r = 0.53-0.54, n = 21, p < 0.05). This work underlines the diversity of CDOM sources in the NC eastern lagoon.
... To answer these questions, we conducted endmember sampling of seawater, groundwater, and porewater over four sampling campaigns across different seasons and tied their molecular DOM characteristics to electron acceptor distributions and dynamics . In addition, we investigated the co-evolution of DOC and DOM with the accumulation of the groundwater-tracer dissolved silicate (DSi; Oehler et al., 2019) and a DOM fluorescence marker which is known to accumulate in aged groundwater (Nelson et al., 2015;Kim and Kim, 2017). Finally, we investigated DOC and DOM release from the shallow STE over a tidal cycle in two different seasons. ...
... Environmental parameters indicative of relative water residence time in the STE in a qualitative sense, i.e., O 2 , Fe, and Si(OH) 4 (Reckhardt et al., 2015;Oehler et al., 2019;Ahrens et al., 2020) revealed increasing relative contributions of highly unsaturated and aromatic compounds to the DOM pool with increasing porewater age. Furthermore, FDOM increased with increasing water residence times, making this fluorescence component a potential groundwater tracer as previously proposed in other coastal environments (Kim et al., 2012;Nelson et al., 2015;Kim and Kim, 2017). It should be noted that Si(OH) 4 and FDOM concentrations are highly influenced by the relative contributions of the fresh groundwater endmember (Supplementary Table 1; Figure 6; Ehlert et al., 2016), and that is difficult to separate patterns of accumulation along the flowpath from additions of our fresh groundwater endmember due to its large concentration ranges (Figure 3). ...
Advective flows of seawater and fresh groundwater through coastal aquifers form a unique ecohydrological interface, the subterranean estuary (STE). Here, freshly produced marine organic matter and oxygen mix with groundwater, which is low in oxygen and contains aged organic carbon (OC) from terrestrial sources. Along the groundwater flow paths, dissolved organic matter (DOM) is degraded and inorganic electron acceptors are successively used up. Because of the different DOM sources and ages, exact degradation pathways are often difficult to disentangle, especially in high-energy environments with dynamic changes in beach morphology, source composition, and hydraulic gradients. From a case study site on a barrier island in the German North Sea, we present detailed biogeochemical data from freshwater lens groundwater, seawater, and beach porewater samples collected over different seasons. The samples were analyzed for physico-chemistry (e.g., salinity, temperature, dissolved silicate), (reduced) electron acceptors (e.g., oxygen, nitrate, and iron), and dissolved organic carbon (DOC). DOM was isolated and molecularly characterized via soft-ionization ultra-high-resolution mass spectrometry, and molecular formulae were identified in each sample. We found that the islands’ freshwater lens harbors a surprisingly high DOM molecular diversity and heterogeneity, possibly due to patchy distributions of buried peat lenses. Furthermore, a comparison of DOM composition of the endmembers indicated that the Spiekeroog high-energy beach STE conveys chemically modified, terrestrial DOM from the inland freshwater lens to the coastal ocean. In the beach intertidal zone, porewater DOC concentrations, lability of DOM and oxygen concentrations, decreased while dissolved (reduced) iron and dissolved silicate concentrations increased. This observation is consistent with the assumption of a continuous degradation of labile DOM along a cross-shore gradient, even in this dynamic environment. Accordingly, molecular properties of DOM indicated enhanced degradation, and “humic-like” fluorescent DOM fraction increased along the flow paths, likely through accumulation of compounds less susceptible to microbial consumption. Our data indicate that the high-energy beach STE is likely a net sink of OC from the terrestrial and marine realm, and that barrier islands such as Spiekeroog may act as efficient “digestors” of organic matter.
... The search for submarine groundwater discharges (SGD) in coastal waters is one of the possible applications for such an observatory. Marine scientists are interested in locating and analysing these discharges because the nutrients discharged by SGD have a significant influence on the marine ecosystem (Dugan, et al., 2010;Moore W. , 2010;Nelson, et al., 2015). AUVs can be used for the exploration of medium sized areas and measure some parameters, for example conductivity, temperature or nutrients to locate a SGD . ...
... The aim of the AUV is to localise a point of interest, for example SGDs. There is a constant input of different substances, i.e. nutrients or fluorescent dissolved organic matter (FDOM) (Nelson, et al., 2015) due to SGDs to the marine environment. While substances are discharged into the ocean, the concentration of these substances will be a function of the position and the time because of mixing processes in the water (Stedmon, et al., 2010). ...
... This change was most distinct at the two shallow sites. The nTA and nDIC values from the second sampling period were also enriched relative to a range of values reported from nearshore Oahu sites (Drupp et al., 2013) but consistent with coastal sites from Maunalua Bay, Oahu, with known inputs of SGD (Nelson et al., 2015;Richardson et al., 2017). ...
... Using a SGD flux rate of 87 cm d −1 at the primary seep site (Swarzenski et al., 2016), and SGD nitrate endmember concentration of 117 µmol L −1 (Prouty et al., 2017b), the nitrate flux from the primary vent site is 712 mol d −1 , clearly demonstrating excess nutrient loading. Elevated SGD endmember nutrient concentrations are consistent with those observed from Black Point, Maunalua Bay, where effluent from proximal on-site sewage disposal is linked to excess nitrogen loads (Nelson et al., 2015;Richardson et al., 2017). As described above, an offshore gradient in nutrient concentrations was observed with enriched nutrients at the shallow sites compared to the deeper sites, consistent with a decrease in coral δ 15 N values away from the vent (Prouty et al., 2017a). ...
Constraining coral reef metabolism and carbon chemistry dynamics are fundamental for understanding and predicting reef vulnerability to rising coastal CO2 concentrations and decreasing seawater pH. However, few studies exist along reefs occupying densely inhabited shorelines with known input from land-based sources of pollution. The shallow coral reefs off Kahekili, West Maui, are exposed to nutrient-enriched, low-pH submarine groundwater discharge (SGD) and are particularly vulnerable to the compounding stressors from land-based sources of pollution and lower seawater pH. To constrain the carbonate chemistry system, nutrients and carbonate chemistry were measured along the Kahekili reef flat every 4 h over a 6-day sampling period in March 2016. Abiotic process – primarily SGD fluxes – controlled the carbonate chemistry adjacent to the primary SGD vent site, with nutrient-laden freshwater decreasing pH levels and favoring undersaturated aragonite saturation (Ωarag) conditions. In contrast, diurnal variability in the carbonate chemistry at other sites along the reef flat was driven by reef community metabolism. Superimposed on the diurnal signal was a transition during the second sampling period to a surplus of total alkalinity (TA) and dissolved inorganic carbon (DIC) compared to ocean endmember TA and DIC measurements. A shift from positive net community production and positive net community calcification to negative net community production and negative net community calcification was identified. This transition occurred during a period of increased SGD-driven nutrient loading, lower wave height, and reduced current speeds. This detailed study of carbon chemistry dynamics highlights the need to incorporate local effects of nearshore oceanographic processes into predictions of coral reef vulnerability and resilience.
... Nutrient concentrations are also commonly used to assess water quality. It has been shown that nutrient concentrations are significantly higher nearshore with known sewage pollution (Lapointe et al., 1990;Nelson et al., 2015). However, measuring nutrients at the shoreline alone as a sewage indicator is not informative enough for management actions because of their numerous non-sewage watershed sources. ...
... concentrations received a medium weight (weight = 2) as HDOH uses this FIB to assess marine recreational water safety specifically for sewage pollution, but not the highest weight because concentrations fluctuate over short time scales (min to h) and have other sources, like soils, in tropical areas (Hardina and Fujioka, 1991;Byappanahalli and Fujioka, 1998;Byappanahalli and Fujioka, 2004). Nutrient concentrations received the lowest weight (weight = 1) since sewage pollution is known to increase them, but nutrients can also come from other sources within the watershed and concentrations can vary over short time scales (Lapointe et al., 1990;David et al., 2013;Nelson et al., 2015). The equation for calculating the overall sewage pollution score for each station was: (C. ...
Sewage pollution is contributing to the global decline of coral reefs. Identifying locations where it is entering waters near reefs is therefore a management priority. Our study documented shoreline sewage pollution hotspots in a coastal community with a fringing coral reef (Puakō, Hawai'i) using dye tracer studies, sewage indicator measurements, and a pollution scoring tool. Sewage reached shoreline waters within 9 h to 3 d. Fecal indicator bacteria concentrations were high and variable, and δ 15 N macroalgal values were indicative of sewage at many stations. Shoreline nutrient concentrations were two times higher than those in upland groundwater. Pollution hotspots were identified with a scoring tool using three sewage indicators. It confirmed known locations of sewage pollution from dye tracer studies. Our study highlights the need for a multi-indicator approach and scoring tool to identify sewage pollution hotspots. This approach will be useful for other coastal communities grappling with sewage pollution.
... At every time point, 1 mL of each sample was fixed with 16 μL of 32% paraformaldehyde PFA. Samples were run on an Attune Acoustic Focusing Cytometer (Applied Biosystems, Part No. 4445280ASR) at the University of Hawaiʻi at Mānoa to enumerate bacterial cell counts86 . For sample collection details and flow cytometer settings, see the supplementary methods(Fig. ...
Coral bleaching is a well-documented and increasingly widespread phenomenon in reefs across the globe, yet there has been relatively little research on the implications for reef water column microbiology and biogeochemistry. A mesocosm heating experiment and bottle incubation compared how unbleached and bleached corals alter dissolved organic matter (DOM) exudation in response to thermal stress and subsequent effects on microbial growth and community structure in the water column. Thermal stress of healthy corals tripled DOM flux relative to ambient corals. DOM exudates from stressed corals (heated and/or previously bleached) were compositionally distinct from healthy corals and significantly increased growth of bacterioplankton, enriching copiotrophs and putative pathogens. Together these results demonstrate how the impacts of both short-term thermal stress and long-term bleaching may extend into the water column, with altered coral DOM exudation driving microbial feedbacks that influence how coral reefs respond to and recover from mass bleaching events.
... Despite retention and remineralization processes in the subterranean estuary, DOC transport via SGD into the coastal ocean column can be substantial and even rival riverine inputs (Goñi and Gardner 2003;Kim et al. 2012;Santos et al. 2009). In areas with elevated fresh groundwater contributions, SGD is furthermore a substantial source of coloured dissolved organic matter (Kim et al. 2013;Nelson et al. 2015). ...
Submarine groundwater discharge (SGD) at the interface of land and sea is likely an important part of the global hydrological cycle and has started to attract the attention of a growing interdisciplinary scientific community. While before the year 2000 only a few papers about that topic are listed in the ISI Web of Science, by now about 100 publications per year address the topic. Submarine groundwater discharge has been defined as ‘direct groundwater outflow across the ocean-land interface into the ocean’ (Church, 1996), later refined to ‘any and all flow of water on continental margins from the seabed to the coastal ocean’ (Burnett et al., 2003), consisting of fresh terrestrial groundwater of modern origin (‘meteoric water’), connate water, and recirculated seawater (Fig.1). Although the majority of the SGD flux is derived from recirculated seawater, the term “groundwater discharge” often tends to be misleadingly reduced on its fresh terrestrial groundwater proportion. Additional confusion may also be caused by the use of different synonyms for this proportion which comprise “freshwater discharge”, “submarine spring” (if discharge occurs in spatially focused form, such as in karst environments), “freshwater spring” or “Vrulja” (Bögli, 1980; Milne, 1897; Kohout, 1966; Fleury et al., 2007; d´Elia et al., 1981). The processes controlling the fluxes of SGD belong to topic 13 of the currently unsolved problems in hydrology (Blöschl et al., 2019).
... SGD in these systems has proven to be more difficult to characterize and nearly impossible to quantify. Tracers such as radon and radium [9,10], dissolved silica [11,12], dissolved organic matter [13], temperature [6,14] have been used to help quantify discharge, but the tremendous dilution due to large volumes of moving coastal waters has limited the accuracy of these methods. In addition, because freshwater floats on seawater it can be difficult to obtain representative water samples to measure these tracers. ...
Groundwater discharge from high tropical islands can have a significant influence on the biochemistry of reef ecosystems. Recent studies have suggested that a portion of groundwater may underflow the reefs to be discharged, either through the reef flat or toward the periphery of the reef system. Understanding of this potential discharge process is limited by the characterization of subsurface reef structures in these environments. A geophysical method was used in this study to profile the reef surrounding the high volcanic island of Mo’orea, French Polynesia. Boat-towed continuous resistivity profiling (CRP) revealed electrically resistive features at about 10–15 m depth, ranging in width from 30 to 200 m. These features were repeatable in duplicate survey lines, but resolution was limited by current-channeling through the seawater column. Anomalous resistivity could represent the occurrence of freshened porewater confined within the reef, but a change in porosity due to secondary cementation cannot be ruled out. Groundwater-freshened reef porewater has been observed near-shore on Mo’orea and suggested elsewhere using similar geophysical surveys, but synthetic models conducted as part of this study demonstrate that CRP alone is insufficient to draw these conclusions. These CRP surveys suggest reefs surrounding high islands may harbor pathways for terrestrial groundwater flow, but invasive sampling is required to demonstrate the role of groundwater in terrestrial runoff.
... Mode of delivery of human-derived pollutants into near-shore environments can range from a specific point-source such as a sewage outfall, to diffuse release across a system (Smith et al., 1981;Carpenter et al., 1998;Dailer et al., 2010;Rey-Villiers et al., 2021) as is the case for agricultural run-off (Howarth, 2008;Hale et al., 2015). While agricultural run-off is often considered the largest source of diffuse nitrogen pollution (Andrello et al., 2022), recent monitoring has revealed that ground water discharge presents a prominent and often diffuse source of pollution into marine ecosystems, accounting for 20-50 % of total exogenous nitrogen inputs at some sites (Paytan et al., 2006;Nelson et al., 2015;Xing et al., 2017); whilst sewage discharge has been shown to occur in waters surrounding at least 104 of 112 distinct reef geographies (including 80 countries, 6 states and 26 territories) (Wear and Thurber, 2015b). ...
Near-shore coral reefs are at high-risk of exposure to pollution from terrestrial activities. Pollution impacts can vary with site-specific factors that span sources, rainfall and oceanographic characteristics. To effectively manage pollution, we need to understand how these factors interact. In this study, we detect terrestrially derived nutrient inputs on near-shore reefs at Norfolk Island, South Pacific by analysis of dissolved inorganic nitrogen (DIN) and stable isotopes. When compared to a reef site with predominantly oceanic inputs, we found that both the lagoon and a small reef adjacent to a catchment have signatures of human-derived DIN shown through depleted δ15N signatures in macroalgae. We find pollution exposure of reef sites is associated with known and unknown sources, rainfall and mixing of water with the open ocean. In characterising exposure of reef sites we highlight the role of site-specific context in influencing pollution exposure for benthic communities even in remote island systems.
... Data from the three field sites in this study suggest that salinity contrasts between sediment porewater and surface waters may often be adequate to enable measurement of groundwater inflow to surface waters even in many, if not most, freshwater systems. FDOM may also be a potential tracer for groundwater discharge in some systems, as FDOM signatures may be usable for distinguishing groundwater of different sources from ambient surface waters [60]. ...
Aquatic eddy covariance (AEC) is an in situ technique for measuring fluxes in marine and freshwater systems that is based on the covariance of velocity and concentration measurements. To date, AEC has mainly been applied to the measurement of benthic oxygen fluxes. Here, development of a fast multiple-channel sensor enables the use of AEC for measurement of benthic fluxes of fluorescent material, salt, and heat at three distinct sites in Massachusetts, USA, including the Connecticut River, the Concord River, and Upper Mystic Lake. Benthic fluxes of salt, useful as a tracer for groundwater input (submarine groundwater discharge), were consistent with independent measurements made with seepage meters. Eddy fluxes of heat were consistent with the balance of incoming solar radiation and thermal conduction at the sediment surface. Benthic eddy fluxes of fluorescent dissolved organic material (FDOM) revealed a substantial net downward flux in the humic-rich Concord River, suggesting that microbial consumption of dissolved organic carbon in the sediment was significant. Simultaneous measurement of several fluxes expands the utility of AEC as a biogeochemical tool while enabling checks for mutual consistency among data channels.
... Therefore, there must be other nitrate sources to the coral reef to maintain the relatively high nitrate concentrations we observed. Potential sources include groundwater inputs (Nelson et al., 2015) or sediment resuspension (Erler et al., 2014). Similar trends of lower oceanic concentrations were seen for nitrite and phosphate, but differences were much smaller (Table 1). ...
Shallow-water coral reefs hold large quantities of acrylate and its precursor dimethylsulfoniopropionate (DMSP), but production and removal processes for these compounds are poorly characterized. Here we determined the concentrations and cycling of acrylate and DMSP in a transect from a coral reef ecosystem to the open ocean, 2 km beyond the reef in Mo'orea, French Polynesia, during April 2018. Concentrations of dissolved acrylate and DMSP were low throughout the reef-ocean transect, ranging from 0.8-3.9 nM and 0.2-3.0 nM, respectively, with no difference observed between the coral reef and open ocean when comparing mean concentrations (± std dev) of dissolved acrylate (1.7 ± 0.7 vs 2.3 ± 0.8 nM) or DMSP (0.9 ± 0.7 vs 1.3 ± 0.6 nM). In the coral reef, dissolved acrylate was rapidly taken up by the heterotrophic community with a fast turnover time averaging~6 h, six times faster than in the open ocean, and nearly as fast as the average turnover time of dissolved DMSP (~3 h). A clear diel trend was observed for the heterotrophic consumption of dissolved acrylate and DMSP in the coral reef, with higher uptake rate constants during daylight hours, synchronized with the larger daytime release of acrylate and DMSP from the coral compared to the nighttime release of these compounds. We also measured photochemical production rates of acrylate in Mo'orean waters, but rates were one to two orders of magnitude slower compared to its rates of biological consumption. Coral and macroalgae were the main sources of dissolved acrylate and DMSP to the reef ecosystem. Our results indicate there is rapid turnover of acrylate and DMSP in the coral reef with a tight coupling between production and removal pathways that maintain dissolved concentrations of these two Frontiers in Marine Science
... Absorption and fluorescence spectroscopies are effective techniques for tracing DOM transport from the watershed to the open ocean (Baker, 2001;Guo et al., 2014;Li et al., 2019;Wang et al., 2021;Qu et al., 2022). They are thus widely used to characterize the sources, levels, composition, and flux of colored and fluorescent DOM (CDOM and FDOM) in groundwater (Chen et al., 2010;Tedetti et al., 2011;Nelson et al., 2015;Kim and Kim, 2017). Webb et al. (2019) determined high variability in SGD-derived DOC and CDOM fluxes in 12 coastal systems in eastern Australia and Cook Island, including tidal freshwater wetlands, estuaries, mangroves, coral reefs, coastal lakes, a saltmarsh, and a residential canal estate. ...
Groundwater discharge and river runoff are two important ways for allochthonous dissolved organic matter (DOM) to enter the ocean, but they vary spatially. Currently, the source, composition, and efflux of groundwater and riverine dissolved organic matter to the tropical northern South China Sea remain poorly understood, preventing an accurate estimate of coastal carbon budgets. In July 2021, nine groundwater and six river water samples were collected along the coast of western Guangdong and were characterized using dissolved organic carbon (DOC) analysis, UV-visible, and fluorescence spectroscopy techniques. Atypical absorption spectra were strongly observed in groundwaters, which were mainly attributed to the high-level nitrates. An exponential-Gaussian equation showed that the average contribution of nitrate to total groundwater absorbances was up to 36% at ∼ 300 nm. Groundwater showed lower levels of DOC, colored dissolved organic matter (CDOM), and fluorescent dissolved organic matter (FDOM) as compared to river water, whereas the humic-like components dominated both groundwater and river water FDOM pools. Higher values of spectral slope S 350–400, fluorescence index, biological index, and the ratio of two humic-like fluorescence jointly reveal that groundwater DOM mainly originated from microbial activities, whereas the river water DOM had considerable contribution from terrestrial soil. High precipitation, natural or agricultural land types, and a lack of seawater intrusion are expected to cause low levels of groundwater DOM in western Guangdong. Among 52 rivers from across the globe, high watershed productivity and precipitation may lead to high–moderate DOM levels in river water in western Guangdong. The DOM fluxes via fresh groundwater discharge in western Guangdong are comparable in magnitude to those by river runoff, accounting for 16%–18% of the Pearl River DOM fluxes. Based on the slopes of linear correlations between CDOM and DOC obtained in groundwater samples and 52 rivers, as well as the data on water fluxes and DOC levels, the global CDOM fluxes via groundwater discharge and river runoff are estimated to be 3.5–12.2 × 10¹² m² yr⁻¹ and 4.3 ± 0.3 × 10¹⁴ m² yr⁻¹, respectively, highlighting the importance of considering groundwater discharge and river runoff in coastal carbon budgets.
... Other tracers of groundwater discharge include stable isotopes (δ 18 O and δ 2 H; Godoy et al., 2013;Rocha et al., 2016), silica (Oehler et al., 2019), methane (Cable et al., 1996a;Dulaiova et al., 2010;Kim and Hwang, 2002), and dissolved organic matter (DOM) (Nelson et al., 2015). Systematic review of articles published containing the keywords 'submarine groundwater discharge' or 'submarine springs' and where methods to assess SGD (y-axis) are described. ...
Natural tracers – any substance present in the environment in small but measurable amounts – are a central tool in many scientific disciplines, providing information about processes and systems. In hydrology and oceanography, these scientific objects have become fundamental for understanding processes that are invisible or have disparate temporal scales, some of them differing greatly from the human time scale. One of the research topics that has been most closely linked to the use of natural tracers is the assessment of submarine groundwater discharge (SGD). This process, which involves the discharge of terrestrial and marine groundwater from coastal aquifers to the coastal ocean, has been recognized as an important process modulating the chemical budgets of the coastal ocean, controlling coastal ecosystems, and providing significant ecosystem services to society. The radioactive tracers of radium isotopes and radon represent the most extensive and widespread tool for investigating the magnitude and implications of this process in a wide variety of environments, from small coves to the entire ocean. However, reporting SGD estimates by means of these tracers is complex and requires profound knowledge regarding fundamental steps in the process of quantifying SGD using these radionuclides, from the tracer measurement techniques to the estimation of groundwater and solute fluxes. This Thesis explores the use of radium isotopes and radon as tracers of SGD by addressing a set of research gaps dealing with (1) the analytical techniques for measuring and quantifying these radionuclides, (2) their geochemical behavior in groundwater systems, and (3) their applications as tracers for both groundwater systems and the coastal ocean. Regarding the analytical techniques, this Thesis includes an assessment of the quantification systematics of the RaDeCC system, the most widely used counter for quantifying short-lived Ra isotopes, providing quantification limits and guidelines. The work represents a significant advance in pursuing better and more precise SGD estimates, as well as for any hydrological and oceanographic application of these tracers. Additionally, the Thesis presents a comprehensive analysis of the behavior of Ra isotopes and Rn in groundwater through a novel transport model of radionuclides. This model enables the use of these tracers for identifying SGD pathways, constraining the tracer concentration in the discharging groundwater, and evaluating the groundwater flow characteristics. Finally, this dissertation presents one of the first works evaluating the variations of SGD and associated nutrient fluxes induced by extreme precipitation events using Ra isotopes as tracers. The work emphasizes the relevance that these episodic events may have for coastal ecosystems, their relative significance for annual SGD estimates, and their implications in future climate change scenarios. Overall, the works presented in this Thesis contribute to improving the current knowledge both about the use of radium isotopes and radon as tracers of environmental processes and about the magnitude and implications of submarine groundwater discharge.
... During the study period in August 2015, salinity was within the maximum growth range of some algae species of 18-28, as adapted and modified from Amato (2015;Amato et al. 2018, see below) for 2.6 AE 2.2 daylight hours per day, with some days having 0 and some as much as 8 h in that salinity range during daylight hours (Fig. 3). The spatial extent of the area experiencing these freshened conditions can be estimated from TIR images (Fig. 2), salinity and nutrient spatial surveys (Nelson et al. 2015;Richardson et al. 2017a), and spatially distributed salinity sensor deployments (Lubarsky et al. 2018). The area affected by the springs at low tide extends to about 10,000 m 2 . ...
Coastal groundwater‐dependent ecosystems benefit from lowered salinity, nutrient‐rich submarine groundwater discharge (SGD). Across Pacific islands marine macroalgae appear to have been challenged by and adapted to the stress of lowered salinity with a trade‐off of nutrient subsidies delivered by SGD. Human alterations of groundwater resources and climate change‐driven shifts brought modifications to the magnitude and composition of SGD. This paper discusses how native macroalgae have adapted to SGD nutrient and salinity gradients, but that invasive algae are outcompeting the natives near SGD with nutrient pollution. It is important to re‐evaluate land and water use practices by modifying groundwater sustainable yields and improving wastewater infrastructure to keep SGD reductions minimal and nitrogen inputs in optimal ranges. This task may be particularly challenging amidst global sea level rise and reductions in groundwater recharge, which threaten coastal groundwater systems and ecosystems dependent on them.
... 17,18 In recent days, DOM molecular characterization is employed to distinguish its origin and distribution in this ecosystem, which may be an important step toward understanding the role of DOM subsidies in reef ecosystem function. 19,20 However, the analysis of DOM molecular composition is limited due to its high complexity. 21 The recent advance of ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) allows the characterization of thousands of molecular formulas in the complex DOM mixtures from water samples. ...
... Hence, low DO values are expected near SGDs during the in situ surveys. Recent studies found enriched FDOM values in the region of SGDs [60]. Therefore, higher FDOM values are expected to be a proxy for SGDs in this study. ...
Submarine groundwater discharge (SGD) is an important pathway of nutrients into coastal areas. During the last decades, interest of researchers in SGDs has grown continuously. However, methods applied for SGD research usually focus on the aquifer or on the mixing processes on larger scales. The distribution of discharged water within the water column is not well investigated. Small remotely operated vehicles (ROV) equipped with environmental sensors can be used to investigate the spatial distribution of environmental parameters in the water column. Herein, a low-cost multi-sensor platform designed to investigate the spatial distribution of water quality properties is presented. The platform is based on an off-the-shelf underwater vehicle carrying various environmental sensors and a short-baseline localisation system. This contribution presents the results of SGD investigations in the area of Woodman Point (Western Australia). Various potential SGD plumes were detected using a skiff equipped with a recreational echo sounder. It was demonstrated that this inexpensive equipment could be used to detect and investigate SGDs in coastal areas. In addition, the low-cost multi-sensor platform was deployed to investigate the spatial distribution of environmental parameters including temperature (T), electric conductivity (EC), dissolved oxygen (DO), oxidation-reduction potential (ORP), pH, and dissolved organic matter fluorescence (FDOM). Three ROV surveys were conducted from different skiff locations. Analyses of the spatial distribution of the environmental parameters allowed the identification of nine potential SGD plumes. At the same locations, plumes were identified during the sonar surveys. In addition, fuzzy logic was used for the fusion of salinity, DO, and FDOM readings in order to enhance SGD detection capability of the designed multi-sensor system. The fuzzy logic approach identified 293 data points as potential within a SGD plume. Average minimum-distance between these points and the identified SGD plumes was 0.5 m and 0.42 m smaller than the minimum-distance average of the remaining data points of survey one and three respectively. It was shown that low-cost ROVs, equipped with environmental sensors, could be an important tool for the investigation of the spatio-temporal behaviour of SGD sites. This method allows continuous mapping of environmental parameters with a high spatial and temporal resolution. However, to obtain deeper insights into the influence of SGDs on the nearshore areas, this method should be combined with other well-established methods for SGD investigation, such as pore water sampling, remote sensing, or groundwater monitoring.
... Metabolomic analyses of DOM can provide insight into the prevalence and distribution of POPs and other xenobiotics that are amenable to the extraction and analysis methods employed (Lara-Martín et al., 2020). Even natural organic inputs from streams, estuaries and groundwater sources have been shown to exhibit unique organic signatures (e.g., Nelson et al., 2015), and untargeted metabolomics of this DOM as it is modified in the marine environment can better define the connectivity of coastal systems and the potential for allochthonous organic matter to play a role in reef nutrient and food web dynamics. ...
The rapidly advancing field of metabolomics encompasses a diverse suite of powerful analytical and bioinformatic tools that can help to reveal the diversity and activity of chemical compounds in individual organisms, species interactions, and entire ecosystems. In this perspective we use examples from studies of coral reefs to illustrate ways in which metabolomics has been and can be applied to understand coastal ecosystems. Examples of new insights that can be provided by metabolomics include resolving metabolite exchange between plants, animals and their microbiota, identifying the relevant metabolite exchanges associated with the onset and maintenance of diverse, microbial mutualisms characterizing unknown molecules that act as cues in coral, reproduction, or defining the suites of compounds involved in coral-algal competition and microbialization of algal-dominated ecosystems. Here we outline sampling, analytical and informatic methods that marine biologists and ecologists can apply to understand the role of chemical processes in ecosystems, with a focus on open access data analysis workflows and democratized databases. Finally, we demonstrate how these metabolomics tools and bioinformatics approaches can provide scientists the opportunity to map detailed metabolic inventories and dynamics for a holistic view of the relationships among reef organisms, their symbionts and their surrounding marine environment.
... Previous studies have used FDOM humic-like components identified through PARAFAC analysis to quantify the transport of terrestrial DOM to the coast through SGD (Nelson et al., 2015; Leote et al. (2008) and in this study. Kim and Kim, 2017;Cho et al., 2021). ...
Due to the widespread pollution of coastal groundwaters with fertilizers, submarine groundwater discharge (SGD) is often thought to be a large dissolved inorganic nitrogen (DIN) source to the ocean. Whether this N is autochthonous or allochthonous to the subterranean estuary (STE), the availability of large quantities of DIN can nevertheless interact with the cycling of other elements, such as carbon (C). In previous studies, we documented the discharge of large quantities of freshwater and NO3– from the mouth of an STE into the Ria Formosa lagoon (SW Iberian Peninsula). For the period covered in this study (2009–2011), the same STE site was dominated by recirculating seawater due to a prolonged fall in piezometric head in the coupled coastal aquifers. Total SGD rates remained similarly high, peaking at 144 cm day–1 at the lower intertidal during fall. We observed a progressive increase of NO3– availability within the STE associated with the recovery of piezometric head inland. Interestingly, during this period, the highest SGD-derived dissolved organic C and DIN fluxes (112 ± 53 and 10 ± 3 mmol m–2 day–1, respectively) originated in the lower intertidal. NO3– enrichment in the STE influences the benthic reactivity of fluorescent dissolved organic matter (FDOM): when seawater recirculation drives STE dynamics, only small changes in the benthic distribution of recalcitrant humic-like FDOM are observed (from −2.57 ± 1.14 to 1.24 ± 0.19 10–3 R.U. “bulk” sediment h–1) in the absence of DIN. However, when DIN is available, these recalcitrant fractions of FDOM are actively generated (from 1.32 ± 0.15 to 11.56 ± 3.39 10–3 R.U. “bulk” sediment h–1), accompanied by the production of labile protein-like FDOM. The results agree with previous studies conducted with flow-through reactor experiments at the same site and suggest that DIN enrichment in the STE enhances the metabolic turnover of sedimentary organic matter up to the point of discharge to surface waters. DIN pollution of coastal aquifers may therefore promote a contraction of the residence time of particulate organic C within the STE, driving carbon from continental storage into the sea.
... In this study area, FDOM H showed strong negative correlation with salinity and overwhelmed by the overall dilution of the terrestrial FDOM H relative to the internal production rate of FDOM. Thus, it has been used as a good SGD tracer in areas with high SGD rate in volcanic islands 32,34 . All previous studies conducted in Jeju Island consistently showed good negative correlation between FDOM H and salinity 22,32 , which indicates apparent recalcitrant FDOM H sources from terrestrial inputs. ...
We examined the residence time, seepage rate, and submarine groundwater discharge (SGD)-driven dissolved nutrients and organic matter in Hwasun Bay, Jeju Island, Korea during the occurrence of a typhoon, Kong-rey, using a humic fluorescent dissolved organic matter (FDOM H )-Si mass balance model. The study period spanned October 4–10, 2018. One day after the typhoon, the residence time and seepage rate were calculated to be 1 day and 0.51 m day ⁻¹ , respectively, and the highest SGD-driven fluxes of chemical constituents were estimated (1.7 × 10 ⁶ mol day ⁻¹ for dissolved inorganic nitrogen, 0.1 × 10 ⁶ mol day ⁻¹ for dissolved inorganic phosphorus (DIP), 1.1 × 10 ⁶ mol day ⁻¹ for dissolved silicon, 0.5 × 10 ⁶ mol day ⁻¹ for dissolved organic carbon, 1.6 × 10 ⁶ mol day ⁻¹ for dissolved organic nitrogen, 0.4 × 10 ⁶ mol day ⁻¹ for particulate organic carbon, and 38 × 10 ⁶ g QS day ⁻¹ for FDOM H ). SGD-driven fluxes of dissolved nutrient and organic matter were over 90% of the total input fluxes in Hwasun Bay. Our results highlight the potential of using the FDOM H -Si mass balance model to effectively measure SGD within a specific area (i.e., volcanic islands) under specific weather conditions (i.e., typhoon/storm). In oligotrophic oceanic regions, SGD-driven chemical fluxes from highly permeable islands considerably contribute to coastal nutrient budgets and coastal biological production.
... Dissolved organic nitrogen (DON) was calculated as the 119 difference between TDN and the sum of ammonium, nitrate and nitrite. 120Samples for fluorescence spectroscopy were measured using an Horiba Aqualog scanning 121 fluorometer following the methods ofNelson et al. (2015) and processed using a Matlab 122 (v2007b) script (available at https://github.com/zquinlan/fDOMmatlab/script.md). Six 123 PARAFAC components were validated using split half validation and outlier analysis (Figure 124 S1). ...
Dissolved organic matter (DOM) composition is a key determinant of microbial community metabolism and trophic nutrient transfer. On coral reefs, four primary groups of benthic organisms dominate photosynthetic production: corals, macroalgae, microphytobenthos, and encrusting algae on rubble, all of which exude significant quantities of DOM. We conducted a mesocosm experiment to characterize and contrast DOM exudates from these four organismal groups under three levels of continuous inorganic nutrient enrichment. We measured bulk dissolved organic carbon and nitrogen and the multivariate spectral characteristics of fluorescent DOM (fDOM). Moderate nutrient enrichment enhanced DOM exudation by all producers. Corals exuded rapidly accumulating DOM with a markedly high concentration of aromatic amino acid-like fDOM components that clearly distinguishes them from algal exudates, which were dominated by humic-like fDOM components and did not accumulate significantly. Our results emphasize the differences between coral and algae in their potential to influence microbial communities and metabolism in reefs.
... The fDOM solutes of SGD can be analyzed and visualized with geospatial software to create maps of potential areas of SGD. Critically, fDOM has the potential to differentiate groundwater sources according to land-use, hydrology, or other factors, in combination with other biogeochemical parameters (Nelson et al. 2015). ...
Cesspools as onsite sewage disposal systems (OSDS) are widespread in the Hawaiian Islands and of concern due to their lack of primary treatment and direct discharge of pathogens and nutrients into groundwater. Approximately 88,000 cesspools in Hawai‘i release nearly 55 million gallons per day (mgd) of sewage into the ground. Here, we review the status of wastewater pollution, with an emphasis on cesspools, and associated impacts to water resources, nearshore ecosystems, and human health.
We present evidence supporting the creation of a cesspool conversion plan, highlighting the need to upgrade cesspools. Knowledge gaps in areas such as hydraulic/hydrologic modeling and technological limitations in identifying specific wastewater sources present barriers to addressing cesspool challenges. We show many of these constraints can be diminished. For example, limitations in identifying specific sources from wastewater indicators using %N and δ15N can be reduced with available land-use information
and potential pollution sources to clarify concentration and isotopic data. Resource management presents many challenges, including recognition of diverse societal views and values. To overcome discrepancies in available data, and varying societal values, the use of transparent, adaptable framework methods such as “structured decision-making” offers approaches for problem-solving. Such frameworks are consistent with a holistic management approach to OSDS that couple the natural and social sciences in identifying
and addressing barriers to reduce negative impacts. Maintaining momentum through adoption of clearly articulated short-, medium-, and long-term achievement benchmarks associated with such a management approach is recommended.
... Dissolved organic nitrogen (DON) was calculated as the 119 difference between TDN and the sum of ammonium, nitrate and nitrite. 120Samples for fluorescence spectroscopy were measured using an Horiba Aqualog scanning 121 fluorometer following the methods ofNelson et al. (2015) and processed using a Matlab 122 (v2007b) script (available at https://github.com/zquinlan/fDOMmatlab/script.md). Six 123 PARAFAC components were validated using split half validation and outlier analysis (Figure 124 S1). ...
Dissolved organic matter (DOM) composition is a key determinant of microbial community metabolism and trophic nutrient transfer. On coral reefs, four primary groups of benthic organisms dominate photosynthetic production: corals, macroalgae, microphytobenthos, and encrusting algae on rubble, all of which exude significant quantities of DOM. We conducted a mesocosm experiment to characterize and contrast DOM exudates from these four organismal groups under three levels of continuous inorganic nutrient enrichment. We measured bulk dissolved organic carbon and nitrogen and the multivariate spectral characteristics of fluorescent DOM (fDOM). Moderate nutrient enrichment enhanced DOM exudation by all producers. Corals exuded rapidly accumulating DOM with a markedly high concentration of aromatic amino acid-like fDOM components that clearly distinguishes them from algal exudates, which were dominated by humic-like fDOM components and did not accumulate significantly. Our results emphasize the differences between coral and algae in their potential to influence microbial communities and metabolism in reefs.
... Typically unseen and rarely measured, SGD represents a key pathway for solutes to coral reef ecosystems (Cuet et al. 2011;Cyronak et al. 2014;Garrison et al. 2003;Knee et al. 2010;Nelson et al. 2015;Paerl 1997;Paytan et al. 2006;Peterson et al. 2009;Povinec et al. 2012;Rad et al. 2007;Street et al. 2008;Tait et al. 2013). High volcanic islands generally produce large rates of fresh SGD due to steep seaward hydraulic gradients and aquifers with large hydraulic conductivities (Cuet et al. 2011;Cyronak et al. 2014;Haßler et al. 2019;Knee et al. 2016Knee et al. , 2010Povinec et al. 2012;Rad et al. 2007;Street et al. 2008;Tait et al. 2014Tait et al. , 2013Wang et al. 2014). ...
Submarine groundwater discharge (SGD) is widely acknowledged as a key driver of environmental change in tropical island coral reefs. Previous work has addressed SGD and groundwater-reef interactions at isolated submarine springs; however, there are still many outstanding questions about the mechanisms and distribution of groundwater discharge to reefs. To understand how groundwater migrates to reefs, a series of offshore 222Rn (radon) and submarine electrical resistivity (ER) surveys were performed on the tropical volcanic island of Mo’orea, French Polynesia. These surveys suggest that fresher water underlies the fringing reef, apparently confined by a <1-m-thick low-permeability layer referred to as a reef flat plate. Reef flat plates have been documented elsewhere in tropical reefs as thin, laterally continuous limestone units that form through the super-saturation of calcium carbonate in the overlying marine waters. In other tropical reefs, the reef flat plate is underlain by a highly permeable karstic limestone formation, but the submarine reef geology on Mo’orea is still uncertain. Numerical modeling of two-dimensional reef transects and SGD quantifications, based on water budget and radon/salinity mass balance, support the confining nature of the reef flat plates and indicate important implications for SGD impacts to tropical reefs. Except where incised by streams or local springs, reef flat plates may route SGD to lagoons or to the reef crest 100s of meters offshore. Because groundwater can transport pollutants, nutrients, and low pH waters, the reef flat plate may play an important role in the spatial patterns of reef ecology and coastal acidification.
... In the coral reef ecosystem of La Reunion Island (Indian ocean), fluorescence properties of CDOM revealed a dominance of autochthonous/biological material in oceanic waters, an increase in humic-like material in reef waters in contact with volcanic grounds, and the presence of anthropogenic compounds in some of the water bodies (Tedetti et al., 2011). Submarine ground waters entering the tropical lagoons were identified by their fluorescence signatures in Hawaii (Nelson et al., 2015). In a mesocosm representing ideally the Southwest Caledonian lagoon, CDOM fluorescence was linked to picophytoplankton abundance (Tedetti et al., 2016). ...
New Caledonia (Southwest Pacific), like all tropical Pacific Island countries, is impacted by weather events, climate change, and local anthropogenic forcing. Strong erosion of particles and dissolved organic matter (DOM) from ultramafic rocks, associated with trace metals dissemination (i.e., nickel, manganese and cobalt), potentially affects lagoon waters and coral reefs surrounding the main island. The CALIOPE (CALedonian Inherent Optical PropErties) cruises were performed along the Eastern Lagoon of New Caledonia (ELNC) (400 km, 13 transects from Bay to open ocean, 51 stations) during contrasted meteorological conditions: a dry period (October 2011), a windy situation (March 2014), and a strong rainy event (March 2016). CDOM absorption and fluorescence (FDOM), particulate absorption, backscattering, suspended particulate matter (SPM), total chlorophyll a (TChla), nutrients (NOx), pigment and phytoplankton composition were measured. Among the four CDOM fluorophores, the humic-like component (λEx/λEm: 235/460 nm), assimilated to a photoproduct of terrestrial organic matter, had relatively low fluorescence compared to protein-like fluorophores. As CDOM absorption, particulate absorption, backscattering, SPM, total chlorophyll a (TChla) and nutrient (NOx) concentrations, this humic-like material generally showed the highest values during rainfall events, the latter inducing an increase in riverine terrigeneous inputs and change toward higher phytoplankton size classes. The tyrosine 1-like (λEx/λEm: 220, 275/304 nm) and tryptophan-like fluorophores (λEx/λEm: 230, 300/352 nm) were strongly influenced by wind displaying a 7-fold and 3-fold increase, respectively in windy situation compared to calm conditions. These increases could be related to enhancements of autochthonous biological activities (highest mean concentrations of Synechococcus spp., phycoerythrin, pico- and nano-eukaryotes, heterotrophic bacteria and nanoplankton observed in wind condition) through the inputs of organic and mineral materials issued from the wind-induced sediment resuspension, atmospheric deposition and water mass mixing. By contrast, the tyrosine 2-like fluorophore (λEx/λEm: 245, 275/304 nm) substantially increased during rain events and presented the lowest values in wind conditions. These strong increases may be linked to the stimulation of planktonic activities due to riverine inputs. Therefore, this study emphasizes the significant differential influence of weather conditions (calm/wind/rain) on biogeochemistry and CDOM/FDOM distributions in the ELNC.
... Another natural SGD tracer involves dissolved organic matter (DOM). For example, SGD is often enriched in humic-like fluorescent DOM and Nelson et al. (2015) demonstrated the utility of using indices of humidification derived from DOM in discharging groundwater as a tracer of SGD dispersal in nearshore waters. ...
The number of studies concerning Submarine Groundwater Discharge (SGD) grew quickly as we entered the twenty-first century. Many hydrological and oceanographic processes that drive and influence SGD were identified and characterized during this period. These processes included tidal effects on SGD, water and solute fluxes, biogeochemical transformations through the subterranean estuary, and material transport via SGD from land to sea. Here we compile and summarize the significant progress in SGD assessment methodologies, considering both the terrestrial and marine driving forces, and local as well as global evaluations of groundwater discharge with an emphasis on investigations published over the past decade. Our treatment presents the state-of-the-art progress of SGD studies from geophysical, geochemical, bio-ecological, economic, and cultural perspectives. We identify and summarize remaining research questions, make recommendations for future research directions, and discuss potential future challenges, including impacts of climate change on SGD and improved estimates of the global magnitude of SGD.
... Furthermore, other factors may influence the distribution of microbial taxa in an active environment such as the land-ocean interface, e.g. organic carbon content, sediment grain size, or coastal regime (upwelling vs downwelling) (Herfort et al., 2017;Nelson et al., 2015). ...
Submarine groundwater discharge (SGD) is known to transport terrestrial nutrients and other potential pollutants to coastal areas around the world. However, SGD studies in tropical developing regions, such as Southeast Asia, are scarce, even though this area is hypothesized to be an SGD hotspot due to favorable meteorological and hydrological conditions. Jepara, a coastal city in northern Java, is characterized by a number of environmental and anthropogenic steering factors (e.g., precipitation rates, volcanic geology, coastal population density, and urban land use) that may support the notion of considerable SGD rates and its associated pollutant fluxes. Therefore, this thesis investigates SGD volumetric estimation with nutrient and microbial community composition to analyze the scale of land-based pollutants delivered by SGD to the nearshore water. Additionally, environmental and health impacts of SGD and suggested coastal water management in this region are also discussed. 222Rn was employed as a groundwater tracer in the coastal water to estimate SGD rates. Spatial and temporal 222Rn surveys were conducted in the estuaries and along the coastline of Jepara. The results indicate that terrestrial groundwater was discharged more at the estuaries than at the coastline. Fresh groundwater comprised up to 42% of total river discharge and 40% of total SGD at the coastline. SGD in this area was driven primarily by tidal pumping, with additional hydraulic gradient-driven fresh groundwater discharge at low tide. A combination of estuarine and coastal SGD results in total volumetric SGD rates of 6.6 x 105 m3 d-1. SGD rates in this area were comparable with those of other volcanic SGD studies and were higher than those in other sub-tropical or temperate region studies. SGD was confirmed to deliver terrestrial dissolved inorganic nitrogen (DIN) and dissolved silica (DSi) to the coastal system, and it also potentially acted as one of the landa ocean delivery pathways for fecal indicators and bacterial pathogens. It was also found that salinity and temperature were the most determinant variables that shaped microbial community composition in an SGD cross section. Nutrient and land-use analyses suggest that high nutrient pools in the coastal hydrological system originated from human activities, i.e., agriculture, livestock, and the sewage system. This result was also supported by microbial community analysis, where identification of fecal indicators and potential pathogens in the SGD compartment confirmed the occurrence of biological contamination. Nutrient levels and potential pathogens lead to coastal eutrophication and waterborne illnesses, which were reported from this area. From these observations, it was concluded that suitable coastal water pollution prevention at this study site should include terrestrial nitrogen containment along the riverbank and estuaries (e.g., a constructed wetland or riparian zones) and the development of a sewage system and a centralized wastewater treatment plant. Overall, this thesis shows a significant amount of contaminant discharge in the coastal area via SGD due to a combination of both environmental and anthropogenic factors. Moreover, it can be inferred that a combination of interdisciplinary geoscience research (e.g., hydrosphere, biosphere, anthroposphere) can provide a deeper understanding and assessment of SGD in a specific environment. Even though it is a local study, the methodology and results of this thesis can be replicated and thus provide assistance in other coastal urban cities in tropical regions and hence facilitate better evaluation and monitoring of tropical coastal water ecosystems in the future.
... Furthermore, other factors may influence the distribution of microbial taxa in an active environment such as the land-ocean interface, e.g. organic carbon content, sediment grain size, or coastal regime (upwelling vs downwelling) (Herfort et al., 2017;Nelson et al., 2015). Changes in microbial community composition due to seasonal or tidal cycles have been observed elsewhere (Grossart et al., 2004;Lee et al., 2017;Yeo et al., 2013), and further temporal observations in this area are needed to explain microbial community composition as a function of tidal phase or seasonal variation. ...
Submarine groundwater discharge (SGD) can be an important pathway for chemical or biological pollutants from land to the ocean around the world. However, studies on the microbial communities associated with SGD in Southeast Asia, which has been hypothesized as SGD hotspot, remain scarce. In this study, we examined the microbial community composition with 16S rRNA gene sequencing along the hydrological continuum of an SGD site in a tropical urban area of Indonesia. Of the observed parameters in this study, salinity and temperature were the most determinant variables explaining patterns in microbial community composition. The bacterial taxon Burkholderiaceae was predominantly found in low salinity samples, including those from terrestrial groundwater and brackish pore water, while cyanobacteria of the genus Synechococcus sp. CC9902 were indicative of saline SGD and seawater samples. The composition of microbial taxa in each sample pointed to the influence of shallow terrestrial groundwater in the beach pore water, while seawater recirculation dominated the SGD sampling points situated further offshore. We identified taxa containing fecal indicators and potential pathogens at the SGD compartments; however, while a likely explanation, we could not conclude with certainty that SGD was a conduit for these bacteria. Overall, the results from this study show that microbial community analysis can highlight hydrological processes and water quality at the SGD site; thus, they could be useful for environmental policymakers to formulate water management strategies in coastal areas.
... The co-occurrences of FDOM maxima with low salinity patches were likely connected to the characteristics of the measured fluorescence: the hand-held fluorometer used in this study targets terrestrially-derived, humic-like fluorescent DOM fractions ("peak-C", Coble, 1996) and this fluorescence signal has been used to trace land-derived DOM in SGD (Nelson et al., 2015;. Most likely sources of FDOM at the study site include leachates of recent terrestrial inland vegetation, or hydrolyzed DOM from Pleistocene marsh peats (Streif, 2002). ...
Submarine groundwater discharge (SGD) is a ubiquitous source of meteoric fresh groundwater and recirculating seawater to the coastal ocean. Due to the hidden distribution of SGD, as well as the hydraulic- and stratigraphy-driven spatial and temporal heterogeneities, one of the biggest challenges to date is the correct assessment of SGD-driven constituent fluxes. Here, we present results from a 3-dimensional seasonal sampling campaign of a shallow subterranean estuary in a high-energy, meso-tidal beach, Spiekeroog Island, Northern Germany. We determined beach topography and analyzed physico-chemical and biogeochemical parameters such as salinity, temperature, dissolved oxygen, Fe(II) and dissolved organic matter fluorescence (FDOM). Overall, the highest gradients in pore water chemistry were found in the cross-shore direction. In particular, a strong physico-chemical differentiation between the tidal high water and low water line was found and reflected relatively stable in- and exfiltrating conditions in these areas. Contrastingly, in between, the pore water compositions in the existing foreshore ridge and runnel system were very heterogeneous on a spatial and temporal scale. The reasons for this observation may be the strong morphological changes that occur throughout the entire year, which affect the exact locations and heights of the ridge and runnel structures and associated flow paths. Further, seasonal changes in temperature and inland hydraulic head, and the associated effect on microbial mediated redox reactions likely overprint these patterns. In the long-shore direction the pore water chemistry varied less than the along the cross-shore direction. Variation in long-shore direction was probably occurring due to topography changes of the ridge-runnel structure and a physical heterogeneity of the sediment, which produced non-uniform groundwater flow conditions. We conclude that on meso-tidal high energy beaches, the rapidly changing beach morphology produces zones with different approximations to steady-state conditions. Therefore, we suggest that zone-specific endmember sampling is the optimal strategy to reduce uncertainties of SGD-driven constituent fluxes.
... Photosynthesis and respiration rates are the leading metabolic drivers of pCO 2 (and hence pH and V arag ) on coral reefs [34,36 -39,65]. In a global meta-analysis, Cyronak et al. [37] documented that pCO 2 is increasing over threefold faster on coral reefs than the open ocean from heightened metabolic activity [37] due in large part to local disturbances such as organic matter and nutrient loading from terrestrial run-off [66], SGD [13,67] and surface run-off [13]. Calcification and dissolution rates are highly sensitive to changes in pH [11]; thus, shifts in the photosynthesis or respiration will influence NCC rates ( figure 1). ...
There is a long history of examining the impacts of nutrient pollution and pH on coral reefs. However, little is known about how these two stressors interact and influence coral reef ecosystem functioning. Using a six-week nutrient addition experiment, we measured the impact of elevated nitrate (NO-3) and phosphate (PO3-4) on net community calcification (NCC) and net community production (NCP) rates of individual taxa and combined reef communities. Our study had four major outcomes: (i) NCC rates declined in response to nutrient addition in all substrate types, (ii) the mixed community switched from net calcification to net dissolution under medium and high nutrient conditions, (iii) nutrients augmented pH variability through modified photosynthesis and respiration rates, and (iv) nutrients disrupted the relationship between NCC and aragonite saturation state documented in ambient conditions. These results indicate that the negative effect of NO-3 and PO3-4 addition on reef calcification is likely both a direct physiological response to nutrients and also an indirect response to a shifting pH environment from altered NCP rates. Here, we show that nutrient pollution could make reefs more vulnerable to global changes associated with ocean acidification and accelerate the predicted shift from net accretion to net erosion.
... Dissolved organic nitrogen (DON) was calculated as the difference between TDN and the sum of NH 1 4 1 NO 2 3 1 NO 2 2 . Samples for fluorescence spectroscopy were measured using an Horiba Aqualog scanning fluorometer following the methods of Nelson et al. (2015) and processed using a MATLAB (v2007b) script (https://github.com/zquinlan/fDOMmatlab/ script.md). ...
Dissolved organic matter (DOM) composition influences microbial community metabolism and benthic primary producers are a source of DOM in coral reefs. As reef benthic communities change, in part due to nutrient pollution, understanding impacts on reef microbial processes requires knowledge of DOM sources and composition. We conducted a multi-week mesocosm experiment dosing four coral reef benthic constituents with three levels of nitrate and phosphate to contrast exudate composition and quantify the effects of nutrient enrichment on exudate release. Moderate nutrient enrichment enhanced bulk dissolved organic carbon exudation by all producers. Corals exuded rapidly accumulating DOM with a markedly high concentration of aromatic amino acid-like fluorescent DOM components that clearly distinguishes them from algal exudates, which were dominated by humic-like fluorescent components and did not accumulate significantly. Our results indicate that corals and algae release DOM of different quality and the quantity of DOM release increases with inorganic nutrient availability.
Marine organisms are increasingly recognized as both responding to and driving biogeochemical changes in their environment. The addition of exogenous resources to the ocean, such as nutrients, that alter organismal physiology can lead to biogeochemical cascades wherein these solutes both alter water chemistry directly and indirectly by changing biological processes that influence water chemistry. To quantify how allochthonous nutrients drive biogeochemical cascades, we measured a suite of biogeochemical parameters during synoptic spatial surveys across two reefs in Mo’orea, French Polynesia conducted day and night at both low and high tide in two different seasons. These data were used to build a model that demonstrates how inputs of nutrients to coral reefs via submarine groundwater discharge directly alter reef metabolism with cascading effects on the cycling of dissolved organic and inorganic carbon that regulate productivity, calcification, and the microbial loop.
Submarine groundwater discharge via coastal aquifers is an important pathway for land‐derived chemicals to enter the oceans. While previous studies have mostly focused on natural coastal aquifers, little attention has been paid to those modified by land reclamation. This study used laboratory experiments and numerical simulations to investigate the effects of land reclamation on flow and mixing processes in a nearshore subterranean estuary. The results show that, regardless of the permeability, the addition of reclamation soil will shift the tide‐driven upper saline plume (USP) seaward and reduce the USP size. When the aquifer is reclaimed by low‐permeability soil, the seawater intrusion length is reduced for small tidal amplitudes but extended as tidal amplitude further increases. Depending on the tidal amplitude, high‐ and low‐permeability reclamation soils prolong or shorten the average particle travel time in the USP, the freshwater discharge zone, and the saltwater wedge. Compared to the case without reclamation, high‐/low‐permeability reclamation soil enhances/weakens the water discharge rate across the aquifer‐ocean interface, narrows/expands the freshwater discharge zone, and increases the tide‐/density‐driven recirculation percent. Findings from this study highlight the complexity of nearshore groundwater systems subject to human activities, and have significant implications for better understanding the fate of terrestrially sourced chemicals in subterranean estuaries before discharging into the coastal water.
The south shore of Oʻahu, Hawaiʻi is one of the most visited coastal tourism areas in the United States with some of the highest instances of recreational waterborne disease. A population of the pathogenic bacterium Vibrio vulnificus lives in the estuarine Ala Wai Canal in Honolulu which surrounds the heavily populated tourism center of Waikīkī. We developed a statistical model to predict V. vulnificus dynamics in this system using environmental measurements from moored oceanographic or atmospheric sensors in real time. Over a year, water from nine sampling events at three depths and 8 sites along the canal (n = 213) were analyzed for 36 biogeochemical variables and V. vulnificus concentration using quantitative polymerase chain reaction (qPCR) of the hemolysin A gene (vvhA). The best multiple linear regression model of V. vulnificus concentration, explaining 80% of variation, included only six predictors: the five-day average rainfall preceding sampling, daily maximum air temperature, water temperature, nitrate plus nitrite, and two metrics of humic dissolved organic matter (DOM). We show how real time predictions of V. vulnificus concentration can be made using these models applied to the time series of water quality measurements from the Pacific Islands Ocean Observing System (PacIOOS) as well as the PacIOOS plume model based on the Waikīkī Regional Ocean Modeling System (ROMS) products. These applications highlight the importance of including DOM variables in predictive modeling of V. vulnificus and the influence of rain events in elevating nearshore concentrations of V. vulnificus. Long-term climate model projections of locally downscaled monthly rainfall and air temperature were used to predict an overall increase in V. vulnificus density of approximately 2- to 3-fold by 2100. Improving these predictive models of microbial populations is critical for management of waterborne pathogen risk exposure, particularly in the wake of a changing global climate.
Cesspools are widely used as a sewage disposal method across the Hawaiian Islands. However, cesspools lack adequate treatment of waste to protect the surrounding environment from dangerous pathogens and nutrients. Each day, approximately 88,000 cesspools across Hawai‘i release nearly 55 million gallons of sewage into the ground, much of it reaching groundwater. This report assembled a comprehensive list and analysis of research studies pertaining to the status of wastewater pollution —with an emphasis on cesspools— and associated impacts on water resources, nearshore ecosystems, and human health. Many of the studies discussed within this white paper provided valid scientific evidence to support the creation of a long-range statewide cesspool conversion plan. Knowledge gaps in areas such as hydraulic/hydrologic modeling and technological methods were identified, with additional limitations in identifying and tracking specific wastewater sources. Many of these limitations, however, can be overcome. For example, limitations in identifying particular sources of wastewater pollution using %N and δ15N can be supplemented with available land-use information and source tracking information to clarify concentration and isotopic data of nitrogen. Resource management presents many challenges, including the recognition of diverse societal views and values. To overcome discrepancies in available data and varying societal values, the use of transparent, adaptable framework methods such as “structured decision making” offers approaches for problem-solving. Such frameworks are consistent with a holistic management approach to onsite wastewater management that incorporates the natural and social sciences to identify and address barriers as a method to reduce negative impacts.
It is critical to evaluate the in situ effects of multiple stressors on coastal community dynamics, especially those communities harboring high diversity such as coral reefs, in order to understand the resilience of these ecosystems, prepare coastal management for future scenarios, and aid in prioritizing restoration efforts. In this in situ study, at 2 sites with gradients of submarine groundwater discharge (SGD), a suite of physical parameters (wave exposure index, wind exposure index, and depth) and an all-encompassing SGD chemical parameter (average nitrate + nitrite daily load) were measured along spatially cohesive and temporally relevant scales and used to model macroalgal growth, biomass, and diversity in Maunalua Bay, Hawai‘i. We showed that (1) species-specific macroalgal biomass is significantly related to SGD and one of the 2 exposure indices (i.e. wind exposure or wave exposure), (2) SGD and wave exposure play key roles in species-specific growth rates, and (3) SGD supports low diversity and increased biomass of species that can tolerate the biogeochemistry associated with SGD. Our work suggests that SGD and local hydrodynamics predict local variation in macroalgal growth, biomass, and diversity in tropical reefs.
Cambridge Core - Oceanography and Marine Science - The Urban Ocean - by Alan F. Blumberg
The long term goal of this research is to develop a swarm of autonomous underwater vehicles (AUVs), which can be used to locate submarine sources of interest, like dumped radioactive waste or ammunition. The overall search strategy of the swarm is based on particle swarm optimisation (PSO). Standard PSO relies on correct localisation and timely communication in order to be able to converge towards the global optimum. However, underwater communication is slow and unreliable and the exact localisation of an AUV is difficult. Therefore, this paper presents an empirical study of the effect of communication and localisation error on the convergence capabilities of PSO. A simulation based on cellular automata is presented and a model of communication and localisation error is incorporated into the PSO. It is shown that both types of errors have a negative effect on the performance of the search, with localisation error having the greater contribution.
The importance of submarine groundwater discharge (SGD) is becoming increasingly recognized because of its potential significance as a source of dissolved species. To explore the probable coral geochemical signal of SGD and verify the validity of potential reliable proxies, multiple geochemical proxies over the last 137 years were identified from a Porites coral near a subterranean estuary in the northern South China Sea, where the SGD was reported to be the predominant flux of terrestrial waters to the coastal ocean. Results indicated that the SGD in the coastal zone was the dominant source of trace elements, especially REE and Ba, due to the various dissolution reactions occurring during groundwater flow in the karst terrain. The time- and frequency-domain comparison between the coral geochemical proxy and the local/regional precipitation indicated that coral REE/Ca ratios are predominantly impacted by the SGD associated with local precipitation, while coral Ba/Ca ratios are also affected by the primary productivity and allochthonous seawater Ba from surrounding areas. The REE signal from coral allows us to reconstruct the coastal surface seawater REE concentrations and the SGD rates on the coast of Sanya during 1870–2006. In a novel approach to developing a proxy for historic SGD to coastal waters, this study provides evidence that the coral REE/Ca record from the karst coast with large SGD has potentials to be a promising paleohydrological indicator.
This chapter summarizes fieldwork conducted to derive new estimates of coastal groundwater discharge and associated nutrient loadings at select coastal sites in Hawai’i, USA. Locations for this work were typically identified based on pronounced, recent ecosystem degradation that may at least partially be attributable to sustained coastal groundwater discharge. Our suite of tools used to evaluate groundwater discharge included select U/Th series radionuclides, a broad spectrum of geochemical analytes, multi-channel electrical resistivity, and in situ oceanographic observations. Based on the submarine groundwater discharge tracer 222Rn, coastal groundwater discharge rates ranged from about 22–50 cm per day at Kahekili, a site in the Ka’anapali region north of Lahaina in west Maui, while at Black Point in Maunalua Bay along southern O’ahu, coastal groundwater discharge rates ranged up to 700 cm per day, although the mean discharge rate at this site was 60 cm per day. The water chemistry of the discharging groundwater can be dramatically different than ambient seawater at both coastal sites. For example, at Kahekili the average concentrations of dissolved inorganic nitrogen (DIN), dissolved silicate (DSi) and total dissolved phosphorus (TDP) were roughly 188-, 36-, and 106-times higher in the discharging groundwater relative to ambient seawater, respectively. Such data extend our basic understanding of the physical controls on coastal groundwater discharge and provide an estimate of the magnitude and physical forcings of submarine groundwater discharge and associated trace metal and nutrient loads conveyed by this submarine route.
Submarine groundwater discharge (SGD) has become increasingly recognized as an important source of freshwater and nutrients to coastal waters worldwide. Although groundwater nutrients have been found to cause algal blooms in many temperate coastal waters, little is known about the biological response to these nutrients in the tropics. On the leeward coast of Hawaii Island, SGD is the dominant freshwater and nutrient source to coastal waters. Kiholo Bay, HI and Kaloko-Honokohau National Historical Park, HI are two nearshore regions with well-documented SGD with high nutrient concentrations; however, little is known about how biological processes within the surface waters respond to these inputs. This study examined how potential gross primary production (pGPP), respiration (RESP), and potential metabolism (pMET) within surface waters differed inside and outside of groundwater plumes at these two sites and between wet and dry seasons. pGPP and RESP were both significantly higher within groundwater plumes, suggesting that SGD stimulated these biological processes; however, RESP responded to a much greater extent than pGPP, resulting in heterotrophic surface waters. RESP also varied seasonally, with greater rates during the dry season compared to the wet one; pGPP did not vary seasonally. Autotrophic conditions were found within groundwater plumes at Kiholo Bay, while heterotrophic conditions were found within them at Kaloko-Honokohau and were greater during the dry season. Overall, our results show that coastal biological processes respond to SGD and that their responses vary over short spatial and temporal scales.
This chapter summarizes fieldwork conducted to derive new estimates of coastal groundwater discharge and associated nutrient loadings at select coastal sites in Hawai‘i, USA. Locations for this work were typically identified based on pronounced, recent ecosys-tem degradation that may at least partially be attributable to sus-tained coastal groundwater discharge. Our suite of tools used to evaluate groundwater discharge included select U/The series radio-nuclides, a broad spectrum of geochemical analytes, multi-channel electrical resistivity, and in situ oceanographic observations.
Based on the submarine groundwater discharge tracer 222Rn, coastal groundwater discharge rates ranged from about 22 to 50 cm per day at Kahekili, a site in the Ka‘anapali region north of Lahaina in west Maui, while at Black Point in Maunalua Bay along southern O‘ahu, coastal groundwater discharge rates ranged close to 400 cm per day, although the mean discharge rate at this site was 33.1 cm per day. The water chemistry of the discharging groundwater can be dramatically different than ambient seawater at both coastal sites. For example, at Kahekili the average concentrations of dissolved in-organic nitrogen (DIN), dissolved silicate (DSi) and total dissolved phosphorus (TDP) were roughly 188-, 36-, and 106-times higher in the discharging groundwater relative to ambient seawater, respec-tively. Such data extend our basic understanding of the physical controls on coastal groundwater discharge and provide an estimate of the magnitude and physical forcings of submarine groundwater dis-charge and associated trace metal and nutrient loads conveyed by this submarine route.
Introduction
The fluorescence of dissolved organic matter (DOM) is suppressed by a phenomenon of self-quenching known as the inner filter effect (IFE). Despite widespread use of fluorescence to characterize DOM in surface waters, the advantages and constraints of IFE correction are poorly defined. We assessed the effectiveness of a commonly used absorbance-based approach (ABA), and a recently proposed controlled dilution approach (CDA) to correct for IFE. Linearity between corrected fluorescence and total absorbance (A(Total); the sum of absorbance at excitation and emission wavelengths) across the full excitation-emission matrix (EEM) in dilution series of four samples indicated both ABA and CDA were effective to an absorbance of at least 1.5 in a 1 cm cell, regardless of wavelength positioning. In regions of the EEMs where signal to background noise (S/N) was low, CDA correction resulted in more variability than ABA correction. From the ABA algorithm, the onset of significant IFE (>5%) occurs when absorbance exceeds 0.042. In these cases, IFE correction is required, which was the case for the vast majority (97%) of lakes in a nationwide survey (n = 554). For highly absorbing samples, undesirably large dilution factors would be necessary to reduce absorbance below 0.042. For rare EEMs with A(Total) > 1.5 (3.0% of the lakes in the Swedish survey), a 2-fold dilution is recommended followed by ABA or CDA correction. This study shows that for the vast majority of natural DOM samples the most commonly applied ABA algorithm provides adequate correction without prior dilution.
Simultaneous measurement of the gross and net primary production of the benthic community and the net input of organic carbon delivered by currents to the coral reef flat of the north shore of Moorea, French Polynesia, confirms that carbon fixation by bottom-dwelling autotrophs, including algal symbionts in corals and benthic free-living algae, is the major source of new organic matter to this system. However, particulate organic carbon, including plankton, detritus, and macroalgal fragments originating primarily on the fore reef and reef crest and carried onto the reef flat by currents contributes up to 13% of the net organic carbon input to the zone nearest the reef crest. These results are typical of the many reef flats worldwide that experience unidirectional flow.
Multichannel electrical resistivity (ER) measurements were conducted at
two contrasting coastal sites in Hawaii to obtain new information on the
spatial scales and dynamics of the fresh water-seawater interface and
rates of coastal groundwater exchange. At Kiholo Bay (located on the
dry, Kona side of the Big Island) and at a site in Maunalua Bay (Oahu),
there is an evidence for abundant submarine groundwater discharge (SGD).
However, the hydrologic and geologic controls on coastal groundwater
discharge are likely to be different at these two sites. While at Kiholo
Bay SGD is predominantly through lava tubes, at the Maunalua Bay site
exchange occurs mostly through nearshore submarine springs. In order to
calculate SGD fluxes, it is important to understand the spatial and
temporal scales of coastal groundwater exchange. From ER time series
data, subsurface salinity distributions were calculated using
site-specific formation factors. A salinity mass balance box model was
then used to calculate rates of point source (i.e., spatially discreet)
and total fresh water discharge. From these data, mean SGD rates were
calculated for Kiholo Bay (˜9,200 m3/d) and for the
Maunalua Bay site (˜5,900 m3/d). While such results are
on the same order of magnitude to geochemical tracer-derived SGD rates,
the ER SGD rates provide enhanced details of coastal groundwater
exchange that can enable a more cohesive whole watershed perspective.
We present the results of a mesocosm experiment investigating the production and utilization of autochthonous dissolved organic matter (DOM) by the plankton community under different inorganic nutrient regimes. Fluorescence spectroscopy combined with parallel factor analysis was applied to study the dynamics of autochthonous DOM. Seven independent fluorescent fractions were identified, differing in their spectral characteristics, production rates, and sensitivity to photochemical and microbial degradation processes. Five different humic fractions, a marine protein, and a peptide fluorescence were found. The five humic fractions were produced microbially, with the greatest production occurring under combined Si- and P-limiting conditions. The two proteinaceous fractions were produced during exponential growth of phytoplankton, irrespective of biomass composition. Photodegradation was an important sink for the microbially derived humic material, and the marine protein material was susceptible to both photo- and microbial degradation. © 2005, by the American Society of Limnology and Oceanography, Inc.
Regional high-resolution (0.1°C, 0.5 m) low-altitude thermal
infrared imagery (TIR) reveals the exact input locations and fine-scale
mixing structure of massive, cool groundwaters that discharge into the
coastal zone as both diffuse flows and as >30 large point-sourced
nutrient-rich plumes along the dry western half of the large volcanic
island of Hawaii. These inputs are the sole source of new nutrient
delivery to coastal waters in this oligotrophic setting. Water column
profiling and nutrient sampling show that the plumes are cold, buoyant,
nutrient-rich brackish mixtures of groundwater and seawater. By way of
example, we illustrate in detail one of the larger plumes, which
discharges ca. 12,000 m3 d-1 (ca. 8,600
m3 d-1 freshwater), rates comparable in volume to
high-flux groundwater outputs in better-known tropical karst terrains.
We further show how nutrient mixing trends may be integrated into TIR
sea surface temperatures to produce surface water nutrient maps of
regional extent.
A sub-fraction of dissolved organic matter fluoresces when excited with ultraviolet light. This property is used to quantify and characterize changes in dissolved organic matter (DOM) in aquatic environments. Detailed mapping of the fluorescence properties of DOM produces excitation emission matrices (EEM), which are well suited to multi-way data analysis techniques (chemometrics). Techniques such as parallel factor analysis (PARAFAC) are increasingly being applied to characterize DOM fluorescence properties. Here, an introduction to the technique and description of the advantages and pitfalls of its application to DOM fluorescence is presented. Additionally a MATLAB based tutorial and toolbox specific to PARAFAC analysis of DOM fluorescence is presented.
Fluorescence spectroscopy has been extensively employed to characterize the source, age, and reactivity of aquatic dissolved organic matter (DOM). However, there is no consensus on the protocols for collecting and correcting DOM fluorescence spectra for the instrument-specific response associated with each component on a fluorometer involved in the excitation of DOM and the resulting detection of DOM emission. The central objective of this study was to evaluate the removal of instrument-specific response from DOM fluorescence spectra collected on three different fluorometers using manufacturer-provided emission and excitation correction files. We evaluated criteria and protocols for comparison of removal of instrument response, using quinine sulfate, a well-characterized fluorophore, as well as the International Humic Substance Society's microbially and terrestrially derived reference end-member fulvic acids: Pony Lake and Suwannee River fulvic acids, respectively. Our results demonstrate that sample spectra collected on different fluorometers differed significantly before correction. Although the effectiveness of manufacturer-provided correction factors in removal of instrument response from sample spectra varied by instrument, spectral overlap of the same sample on multiple instruments improved after correction. Our results suggest that conclusions based on analysis of trends within a dataset of DOM fluorescence spectra should be expected to be independent of the fluorometer employed. © 2010, by the American Society of Limnology and Oceanography, Inc.
We studied the fluorescence properties of fulvic acids isolated from streams and rivers receiving predominantly terrestrial sources of organic material and from lakes with microbial sources of organic material. Microbially derived fulvic acids have fluorophores with a more sharply defined emission peak occurring at lower wavelengths than fluorophores in terrestrially derived fulvic acids. We show that the ratio of the emission intensity at a wavelength of 450 nm to that at 500 nm, obtained with an excitation of 370 nm, can serve as a simple index to distinguish sources of isolated aquatic fulvic acids. In our study, this index has a value of ;1.9 for microbially derived fulvic acids and a value of ;1.4 for terrestrially derived fulvic acids. Fulvic acids isolated from four large rivers in the United States have fluorescence index values of 1.4-1.5, consistent with predominantly terrestrial sources. For fulvic acid samples isolated from a river, lakes, and groundwaters in a forested watershed, the fluorescence index varied in a manner suggesting different sources for the seepage and streamfed lakes. Furthermore, we identified these distinctive fluorophores in filtered whole water samples from lakes in a desert oasis in Antarctica and in filtered whole water samples collected during snowmelt from a Rocky Mountain stream. The fluorescence index measure- ment in filtered whole water samples in field studies may augment the interpretation of dissolved organic carbon sources for understanding carbon cycling in aquatic ecosystems.
Recently developed techniques for estimating bacterial biomass and productivity indicate that bacterial biomass in the sea is related to phytoplankton concentration and that bacteria utilise 10 to 50 % of carbon fixed by photosynthesis. Evidence is presented to suggest that numbers of free bacteria are controlled by nanoplankton~c heterotrophic flagellates which are ubiquitous in the marine water column. The flagellates in turn are preyed upon by microzooplankton. Heterotrophic flagellates and microzooplankton cover the same size range as the phytoplankton, thus providing the means for returning some energy from the 'microbial loop' to the conventional planktonic food chain.
This is the first comprehensive text on the theory and practice of aquatic organic matter fluorescence analysis, written by the experts who pioneered the research area. This book covers the topic in the broadest possible terms, providing a common reference for making measurements that are comparable across disciplines, and allowing consistent interpretation of data and results. The book includes the fundamental physics and chemistry of organic matter fluorescence, as well as the effects of environmental factors. All aspects of sample handling, data processing, and the operation of both field and laboratory instrumentation are included, providing the practical advice required for successful fluorescence analyses. Advanced methods for data interpretation and modeling, including parallel factor analysis, are also discussed. The book will interest those establishing field, laboratory, or industrial applications of fluorescence, including advanced students and researchers in environmental chemistry, marine science, environmental geosciences, environmental engineering, soil science, and physical geography.
We measured short-lived radium isotopes (223Ra and 224Ra), dissolved inorganic and organic nutrients, and photosynthetic pigments during the summers of 2006 and 2007 in the southern sea of Korea, where harmful dinoflagellate blooms occur every year. The Ra tracer measurements reveal that coastal groundwater, rather than other sources previous suggested (i.e., Yangtze River diluted water or Kuroshio currents), is the main source of nutrients that fuel red tides in this region. Although inorganic-nutrient levels are different for different regions and different years, either dissolved inorganic nitrogen or phosphorus is depleted in the red-tide region. This depletion is accompanied by highly elevated levels of dissolved organic nutrients, transformed from groundwaterborne dissolved inorganic nutrients either inside Yeoja Bay or in offshore red-tide areas, thereby creating favorable conditions for the growth of dinoflagellates in competition with diatoms. The intensity of red tides correlates well with the activity of 224Ra (half life = 3.66 d) in seawater over daily or yearly time scales. Because the chemically conservative 224Ra can trace groundwater-borne nutrients, which are utilized by marine biota in this red-tide region, the intensity of red tides seems to be related to the amount of nutrient-enriched groundwater supplied to the offshore red-tide region. © 2010, by the American Society of Limnology and Oceanography, Inc.
The borosilicate glass bottle sealed with a ground stopper and vacuum grease is a high quality container for preserving seawater samples for total alkalinity (TA) and dissolved inorganic carbon (DIC); it is recommended in the standard methods, even though this bottle is expensive and hard-to-handle. As there is an increased demand for sample storage and transportation by laboratories involved in biological and ocean acidification research, we explore alternative sample storage techniques by testing four types of containers. The results demonstrated that over a period of 47 d, TA values from seawater samples stored in polypropylene (PP) bottles and high density polyethylene (HDPE) bottles were not statistically different from those stored in the benchmark borosilicate glass bottles, both at room and refrigerated temperatures. In addition, DIC concentrations from a seawater sample stored in soda-lime glass bottles and small volume borosilicate glass vials with screw caps were not statistically different from those stored in the borosilicate glass bottles over at least 148 d. However, the TA value of seawater stored in soda-lime glass bottles increased significantly with increasing storage time, indicating that this type of soft glass bottle is not suitable for TA sample storage. Therefore, we suggest that PP or HDPE bottles can be used for TA sample storage and small volume borosilicate glass vials with screw caps can be used for DIC sample storage for a period of at least 1.5 months. These storage containers provide economical and easy-to-transport alternatives to the recommended high-quality borosilicate glass bottles.
Marine dissolved organic matter (DOM) is a complex mixture of molecules found throughout the worlds oceans. It plays a key role in the export, distribution, and sequestration of carbon in the oceanic water column, posited to be a source of atmospheric climate regulation. Biogeochemistry of Marine Dissolved Organic Matter, Second Edition, focuses on the chemical constituents of DOM and its biogeochemical, biological, and ecological significance in the global ocean, and provides a single, unique source for the references, information, and informed judgments of the community of marine biogeochemists. Presented by some of the worlds leading scientists, this revised edition reports on the major advances in this area and includes new chapters covering the role of DOM in ancient ocean carbon cycles, the long term stability of marine DOM, the biophysical dynamics of DOM, fluvial DOM qualities and fate, and the Mediterranean Sea. Biogeochemistry of Marine Dissolved Organic Matter, Second Edition, is an extremely useful resource that helps people interested in the largest pool of active carbon on the planet (DOC) get a firm grounding on the general paradigms and many of the relevant references on this topic. Features up-to-date knowledge of DOM, including five new chapters The only published work to synthesize recent research on dissolved organic carbon in the Mediterranean Sea Includes chapters that address inputs from freshwater terrestrial DOM.
The impact of groundwater on pCO2 variability was assessed in two coral reef lagoons with distinct drivers of submarine groundwater discharge (SGD). Diel variability of pCO2 in the two ecosystems was explained by a combination of biological drivers and SGD inputs. In Rarotonga, a South Pacific volcanic island, SGD was driven primarily by a steep terrestrial hydraulic gradient, and the water column was influenced by the high pCO2 (5,501 μatm) of the fresh groundwater. In Heron Island, a Great Barrier Reef coral cay, SGD was
dominated by seawater recirculation in permeable sediments (i.e. tidal pumping) and pCO2 was mainly impacted through the stimulation of biological processes. The Rarotonga water column had a relatively higher average pCO2 (549 μatm) than Heron Island (471 μatm), however, pCO2 exhibited a greater diel range in Heron Island (778 μatm) than in Rarotonga (507 μatm). SGD flux rates were quantified using a radon (222Rn) mass balance. The Rarotonga water column received 29.0 ± 8.2 mmol free-CO2 m-2 d-1 from SGD, while the Heron Island water column received 12.1 ± 4.2 mmol free-CO2 m-2 d-1. Both systems were sources of carbon dioxide to the atmosphere (averaging 8.8 ± 3.4 and 2.5 ± 2.1 mmol CO2 m-
2 d-1 in Rarotonga and Heron Island, respectively), with SGD-derived free-CO2 most likely contributing to the outgassing of CO2. Studies measuring the metabolism of coral reefs via changes in carbonate chemistry (e.g. photosynthesis, respiration, calcification, and calcium carbonate (CaCO3) dissolution rates) may need to consider the effects of groundwater seepage on water column carbonate chemistry and greenhouse gas evasion. Local drivers of coral reef carbonate chemistry such as SGD may offer more approachable management solutions to mitigating the effects of ocean acidification (OA) on coral reefs.
h i g h l i g h t s Determination of PAH and pesticide fluorescence properties using EEMs. Discrimination of PAH and pesticide fluorescence signatures using PARAFAC analysis. Good correlation between PAH concentrations and fluorescence intensities in marine waters. a b s t r a c t Polycyclic aromatic hydrocarbons (PAHs) and pesticides are among the most widespread organic contam-inants in aquatic environments. Because of their aromatic structure, PAHs and pesticides have intrinsic fluorescence properties in the ultraviolet/blue spectral range. In this study, excitation–emission matrix (EEM) fluorescence spectroscopy and parallel factor (PARAFAC) analysis were used to characterise and discriminate fluorescence signatures of nine PAHs and three pesticides at the lg L À1 level in the presence of humic substances (0.1–10 mg C L À1). These contaminants displayed a diversity of fluorescence signa-tures regarding spectral position (k Ex : 220–335 nm, k Em : 310–414 nm), Stokes shift (39–169 nm) and number of peaks (1–8), with detection limits ranging from 0.02 to 1.29 lg L À1 . The EEM/PARAFAC method applied to mixtures of PAHs with humic substances validated a seven-component model that included one humic-like fluorophore and six PAH-like fluorophores. The EEM/PARAFAC method applied to mixtures of pesticides with humic substances validated a six-component model that included one humic-like fluorophore and three pesticide-like fluorophores. The EEM/PARAFAC method adequately quantified most of the contaminants for humic substance concentrations not exceeding 2.5 mg C L À1 . The application of this method to natural (marine) samples was demonstrated through (1) the match between the Ex and Em spectra of PARAFAC components and the Ex and Em spectra of standard PAHs, and (2) the good linear correlations between the fluorescence intensities of PARAFAC components and the PAH concentrations determined by GC–MS.
The distributions of dissolved organic carbon (DOC), total dissolved hydrolysable amino acids (THAA), and colored dissolved organic matter (CDOM) were studied in pore-water/groundwater samples (including seeping water) from a subterranean estuary (STE) of a large tidal flat in Hampyeong Bay, Korea, in July 2011. The relatively low alanine D/L ratios and high THAA concentrations in the pore-water closest to the sediment surface (0–10 cm) indicate the active production of dissolved organic matter (DOM) from benthic algae, and the relatively low THAA concentrations and high D/L ratios in the subsurface pore-water (10–35 cm) indicate a relatively large presence of bio-degraded DOM. In the deep pore-water (35–75 cm), relatively low D/L ratios, high DOC concentrations, and intense humic-like fluorescence were observed, suggesting a net accumulation of less-reactive DOC in this layer. Overall, this STE appears to have net DOM sources because the concentrations of DOC (60–1700 μM) in the pore-water decreased toward the land, the surface, and the low-salinity waters. The concentrations of DOC in the seeping water (185 ± 52 μM) were higher than those in the overlying seawater (144 ± 9 μM), resulting in net DOC fluxes of 2–5 × 109 g·C·yr− 1 through submarine groundwater discharge (SGD) into Hampyeong Bay. The organic matter compositions in the seeping water indicated that SGD introduced DOM from both the surface and subsurface layers. Our results highlight that tidal flats are important sources for DOM, implying that SGD-driven DOM plays an important role in coastal carbon cycles and biogeochemistry.
During the past two decades coral reefs in the greater Caribbean area have been altered by phase shifts away from corals and toward macroalgae or algal turfs. This study tested the hypothesis that because the phase shift on reefs in Jamaica and southeast Florida involved frondose macroalgae, bottom-up control via nutrient enrichment must be a causal factor. The approach was multifaceted and included measurement of near-bottom nutrient concen- trations, salinity, nutrient enrichment bioassays, alkaline phosphatase assays, tissue C : N : P ratios, and tissue 15N : 14N (6"N) ratios. In both locations, concentrations of dissolved inorganic nitrogen (DIN) and soluble reactive phosphorus (SRP) exceeded nutrient thresholds (- 1 .O PM DIN, 0.1 PM SRP) noted to sustain macroalgal blooms on Caribbean coral reefs. High seawater DIN : SRP ratios, alkaline phosphatase activity, and tissue C : P and N : P ratios of macroalgae on the carbonate-rich Jamaican reef suggested SRP limitation of productivity compared to lower values of these variables on siliciclastic reefs in Florida that suggested DIN limitation. This pattern was corroborated experimentally when SRP enrichment increased P,,,, (photosynthetic capacity at light saturation) of the chlorophyte Chaetomorpha Zinum in Jamaica compared to DIN enrichment that increased (x (the photosynthetic efficiency under low irradiance) of the deeper growing chlorophyte Codium isthmocladum in southeast Florida. Increased DIN concentrations were associated with reduced salinity on both reefs, indicating submarine groundwatcr discharge was a significant source of DIN. Elevated S15N values of C. isthmocladum tissue further pointed to wastewater DIN as a source of nitrogen contributing to the blooms in southeast Florida.
A number of U- and Th-series isotopes have become popular tools for quantifying submarine groundwater discharge (SGD). These isotopic techniques enable large-scale estimates of various components of SGD, allowing detailed studies on the processes involved. Radium isotopes have proven to be useful tracers of total SGD in many environments on both small- and large scales. The existence of four naturally occurring radium isotopes makes Ra particularly useful for quantifying multiple sources of SGD. The utility of ²²²Rn as a tracer of SGD has been demonstrated in a wide range of environments from coastal embayments to the coastal ocean. The approach for quantifying SGD using ²²²Rn is similar to radium (²²⁶Ra), except for a few key differences. The ²²²Rn signature of SGD is best observed near the source because of its short half-life. In contrast, ²²⁶Ra has the ability to integrate the SGD signal over much wider spatial scales. Finally, of the intercomparison studies conducted till date, there appears to be no systematic difference in the two techniques based on the coastal hydrogeologic setting, though a wider range of estimates has been typically observed in areas with fractured crystalline rock aquifers and where springs have been a major conduit for SGD. More recently, the geochemical budget for uranium has been shown to be impacted by SGD. As uranium is enriched in seawater relative to most coastal groundwater, the observed depletion of uranium in estuarine and coastal waters may provide an estimate of the seawater component of SGD that recharges coastal aquifers, or submarine groundwater recharge (SGR). Additionally, new isotope techniques for in situ study are under development including underwater g-spectrometry and continuous radon monitoring. These techniques may be more useful for location of SGD discharges rather than estimation of their magnitudes.
Macroalgal blooms of Hypnea musciformis and Ulvafasciata in coastal waters of Maui only occur in areas of substantial anthropogenic nutrient input, sources of which include wastewater effluent via injection wells, leaking cesspools and agricultural fertilizers. Algal δ15N signatures were used to map anthropogenic nitrogen through coastal surveys (island-wide and fine-scale) and algal deployments along nearshore and offshore gradients. Algal δ15N values of 9.8‰ and 2.0–3.5‰ in Waiehu and across the north-central coast, respectively, suggest that cesspool and agricultural nitrogen reached the respective adjacent coastlines. Effluent was detected in areas proximal to the Wastewater Reclamation Facilities (WWRF) operating Class V injection wells in Lahaina, Kihei and Kahului through elevated algal δ15N values (17.8–50.1‰). From 1997 to 2008, the three WWRFs injected an estimated total volume of 193 million cubic meters (51 billion gallons) of effluent with a nitrogen mass of 1.74 million kilograms (3.84 million pounds).
Human activities in the watersheds surrounding Maunalua Bay, Oahu, Hawaii, have lead to the degradation of coastal coral reefs affecting populations of marine organisms of ecological, economic and cultural value. Urbanization, stream channelization, breaching of a peninsula, seawalls, and dredging on the east side of the bay have resulted in increased volumes and residence time of polluted runoff waters, eutrophication, trapping of terrigenous sediments, and the formation of a permanent nepheloid layer. The ecosystem collapse on the east side of the bay and the prevailing westward longshore current have resulted in the collapse of the coral and coralline algae population on the west side of the bay. In turn this has lead to a decrease in carbonate sediment production through bio-erosion as well as a disintegration of the dead coral and coralline algae, leading to sediment starvation and increased wave breaking on the coast and thus increased coastal erosion. The field data and resulting coral reef ecohydrology model presented in this paper demonstrate and quantify the importance of biophysical processes leading to coral reef degradation as the result of urbanization. Coral restoration in Maunalua Bay will require an integrated ecosystem approach.
A fluorometric method is described which provides sensitive measurements of extracted chlorophyll a free from the errors associated with conventional acidification techniques. Fluorometric optical configurations were optimized to produce maximum sensitivity to Chl a while maintaining desensitized responses from both Chl b and pheopigments. Under the most extreme Chl b :Chl a ratio likely to occur in nature (1 : 1 molar), the new method results in only a 10% overestimate of the true Chl a value, while estimates from older acidification methods are 2.5‐fold low. Under conditions of high pheopigment concentrations (pheo a: Chl a = 1 : 1 molar), the new method provides Chl a estimates that are equivalent to those determined from the acidification technique. The new simple method requires a single fluorescence determination and provides adequate sensitivity for small sample sizes (<200 ml) even in the most oligotrophic marine and freshwater environments.
The isolation, characterization and study of the properties of aquatic dissolved organic matter (DOM) still represent a challenge because of the heterogeneity, complexity and low concentration of organic material in natural waters. Based on its ability to interact with contaminants and thus to modify their transport and bioavailability, DOM is of interest for environmental purposes. The objective of this work was to better characterize DOM in the Gironde Estuary (southwestern France). The estuary represents an exchange zone between the continent and the Atlantic Ocean and conditions the transfer of organic and inorganic substances from the continental to the oceanic environment. Several samples were collected along the estuary during three cruises in 2002 and 2006. They were analysed using excitation–emission matrix (EEM) spectroscopy, a sensitive technique that allows direct analysis of water samples. Fluorescent DOM and dissolved organic carbon (DOC) did not behave conservatively in this estuarine system, i.e. the organic material did not undergo simple dilution from the upstream to the downstream part of the estuary. A seasonal variability in DOC content was pointed out, whereas few seasonal variations in DOM fluorescence were observed. DOM sources and processing in the estuary were further evaluated by determining two fluorescence indices – the humification index (HIX) and the index of recent autochthonous contribution (BIX). By applying these indices, the relative degree of humification (HIX) and autotrophic productivity (BIX) could be assessed. Based on the fluorescence and DOC results, the estuary was divided into three zones depending on salinity (S) and characterized by specific DOM: (i) A turbid zone of low salinity (S
Using radium (Ra) isotopes and nutrient analy- ses, we found that submarine groundwater discharge (SGD) is an important source of ‘‘new’’ nutrients, particularly nitrogen, to coral reefs around the world. Nitrogen input estimates as- sociated with SGD range from 3 to 800 mmol h1 per meter of shoreline. The use of Ra isotopes allows us to quantify the inorganic nitrogen input from this source of nutrients. Increas- ing coastal population and land use practices may enhance anthropogenic nutrient loading from submarine groundwater contributing to reef degradation.
Multiple tracers of groundwater input (salinity, Si, 223 Ra, 224 Ra, and 226 Ra) were used together to determine the magnitude, character (meteoric versus seawater), and nutrient contribution associated with submarine groundwater discharge across the leeward shores of the Hawai'ian Islands Maui, Moloka'i, and Hawai'i. Tracer abundances were elevated in the unconfined coastal aquifer and the nearshore zone, decreasing to low levels offshore, indicative of groundwater discharge (near-fresh, brackish, or saline) at all locations. At several sites, we detected evidence of fresh and saline SGD occurring simultaneously. Conservative estimates of SGD fluxes ranged widely, from 0.02–0.65 m 3 m − 2 d − 1 at the various sites. Groundwater nutrient fluxes of 0.04–40 mmol N m − 2 d − 1 and 0.01–1.6 mmol P m − 2 d − 1 represent a major source of new nutrients to coastal ecosystems along these coasts. Nutrient additions were typically greatest at locations with a substantial meteoric component in groundwater, but the recirculation of seawater through the aquifer may provide a means of transferring terrestrially-derived nutrients to the coastal zone at several sites.
A fraction of dissolved organic matter (DOM) is able to fluoresce. This ability has been used in the present study to investigate the characteristics and distribution of different DOM fractions. A unique global dataset revealed seven different fluorescent fractions of DOM: two humic-like, four amino acid-like and one chemically uncharacterized. The spectral characteristics of the amino acid-like fractions resemble those of pure tryptophan, tyrosine and phenylalanine. The fluorescence intensities of these amino acid-like fractions are highest in the surface waters, rapidly decreasing with depth and at a constant low level in the bathypelagic layer. Such profiles indicate that amino acid-like DOM is linked to surface water production. From calibration curves we estimated the mean surface water concentrations of tryptophan and tyrosine to be 12.5 and similar to 15.9 nmol L-1, respectively, while mean deep water concentrations are 5.1 and similar to 6.5 nmol L-1. The fluorescence intensities of the humic-like DOM fractions were low in the surface layer, rapidly increasing in the mesopelagic layer and at constant high level in the bathypelagic layer. The low fluorescence intensities observed in the surface layer indicate the quantitative importance of photochemical degradation as a sink of the humic-like compounds. In the dark ocean (below 200 m), significant linear relationships between humic-like DOM fluorescence and microbial activity (apparent oxygen utilization. NO3- and PO43-) were found. These observations imply a link to dark ocean microbial remineralization and indicate that the major source of humic-like compounds is microbial turnover of organic matter. The results of the present study show that the distribution of the humic-like DOM fractions is a balance between supply from continental run off, net microbial production and photochemical removal in surface waters.
We investigated submarine groundwater discharge (SGD)-associated nutrient fluxes and budgets in two coastal embayments, Hwasun Bay and Bangdu Bay, off the volcanic island of Jeju, Korea. SGD in Hwasun Bay is a composite of marine and meteoric groundwater, while that in Bangdu Bay mainly includes marine groundwater. The submarine inputs of groundwater into Hwasun and Bangdu Bays were approximately 0.12 and 0.27 m3 m−2 d−1, respectively, on the basis of the 222Rn mass balance models. The nitrogen:phosphorus ratios in coastal groundwater (85 ± 96) were considerably larger than those in the seawater (3.8 ± 1.6) of both bays. Fluxes of dissolved inorganic nitrogen (DIN) through SGD were more than 90% of the net DIN input into both bays; approximately 93% and 39% of SGD-driven DIN was consumed inside Hwasun and Bangdu Bays, respectively. The discharge of DIN through SGD from the entire island was approximately 2.1 × 109 mol yr−1, which is equivalent to that of some large rivers, potentially supporting approximately 1.6 × 1011 g carbon yr−1 of new primary production. Because Jeju accounts for less than 1% of the total land mass of the volcanic islands, SGD-driven nutrient fluxes from highly permeable islands standing in oligotrophic oceans could be very important for global nutrient budgets.
The seasonal dynamics of dissolved organic carbon (DOC) in a subterranean estuary were examined in a coastal water-table aquifer extending across a forest-marsh interface into an adjacent tidal creek that leads to North Inlet (SC). The aquifer is characterized by groundwater flow from the forest recharge area towards the creek. DOC concentrations range from 50 to 140 mg L-1 in the shallow portions of the aquifer below the forest and undergo seasonal changes that are inversely related to temperature and precipitation conditions. Markedly lower DOC concentrations (-1) in the deep portion of the aquifer are consistent with the loss of a large fraction of the original DOC along the groundwater flow paths. Mass balance estimates indicate that over 60% of the DOC losses are due to sorption reactions whereas the rest appear to be caused by heterotrophic decay. Groundwater DOC discharge from the forest, which occurs in a restricted zone of the high marsh, is 5.5 mg carbon m-2 d-1 and accounts for a minor component of the annual carbon export from North Inlet. In contrast, moderately saline (2–12 ppt) ground waters below the marsh display elevated DOC concentrations (20 mg L-1) that appear to be the result of mixing of fresh ground waters and surface seawater during tidal seepage and concentration during evapotranspiration. The flux of DOC associated with the discharge of these saline ground waters is 600 mg carbon m-2 d-1, which represents a significant fraction of the annual DOC budget for North Inlet.
Degradation of coral reefs often involves a “phase shift” from abundant coral to abundant macroalgae. This paper critically
reviews the roles of nutrient increases in such phase shifts. I conclude that nutrient overloads can contribute to reef degradation,
but that they are unlikely to lead to phase shifts simply by enhancing algal growth rates and hence allowing overgrowth of
corals, unless herbivory is unusually or artificially low. Concentrations of dissolved inorganic nutrients are poor indicators
of reef status, and the concept of a simple threshold concentration that indicates eutrophication has little validity. I discuss
the significance and consequences of these assessments for reef management, focusing on the Great Barrier Reef, and conclude
with some specific recommendations, including protection of herbivorous fishes, minimisation of terrestrial runoff, and protection
of coastal reefs.
Nonconservative fluxes of dissolved nutrients (Si, N and P) in groundwater and nearshore ocean water off two golf courses (Keauhou and Waikoloa) on the west coast of Hawaii were estimated using a one-dimensional mixing model. Groundwater flowing under the Keauhou course discharges into a semi-enclosed embayment with restricted circulation. Leaching of 10% of the N from golf course fertilizer accounted for an increase in groundwater N flux to the bay of 116%; leaching of 1% of the P from fertilizer accounted for an increase in groundwater P flux of 22%. Uptake of Si, N and P was not measurable within the bay; a buoyant surface layer (1 m thick) did not come into contact with benthic communities, and nutrient uptake by plankton was limited to no greater than 2% of the N and P delivered by groundwater flux.At Waikoloa, leaching of N and P from golf course fertilizers accounted for an increase in groundwater nutrient flux to brackish ponds near the coastline of 229% and 400%, respectively. Waikoloa is located on an exposed coastline with substantial turbulent mixing throughout the water column. Nutrient subsidies from golf course fertilizers were not distinguishable from natural groundwater input, and uptake of N in the nearshore zone equaled 80% of the groundwater flux at the shoreline.On a mass delivery basis, natural and anthropogenic nutrient fluxes to the nearshore ocean appear sufficient to cause alteration to community function and structure. Such effects, however, are not solely responses to nutrient loading. Consideration of physical processes, along with use of conservative tracers can provide a practical method for quantifying these effects of man's activities on the ocean.
Water samples were collected from rivers and estuarine environments within the Florida Coastal Everglades (FCE) ecosystem, USA, and ultrafiltered dissolved organic matter (UDOM; <0.7 μm, >1 kDa) was isolated for characterization of its source, bioavailability and diagenetic state. A combination of techniques, including 15N cross-polarization magic angle spinning nuclear magnetic resonance (15N CPMAS NMR) and X-ray photoelectron spectroscopy (XPS), were used to analyze the N components of UDOM. The concentrations and compositions of total hydrolysable amino acids (HAAs) were analyzed to estimate UDOM bioavailability and diagenetic state. Optical properties (UV–visible and fluorescence) and the stable isotope ratios of C and N were measured to assess the source and dynamics of UDOM. Spectroscopic analyses consistently showed that the major N species of UDOM are in amide form, but significant contributions of aromatic-N were also observed. XPS showed a very high pyridinic-N concentration in the FCE–UDOM (21.7 ± 2.7%) compared with those in other environments. The sources of this aromatic-N are unclear, but could include soot and charred materials from wild fires. Relatively high total HAA concentrations (4 ± 2% UDOC or 27 ± 4% UDON) are indicative of bioavailable components, and HAA compositions suggest FCE–UDOM has not undergone extensive diagenetic processing. These observations can be attributed to the low microbial activity and a continuous supply of fresh UDOM in this oligotrophic ecosystem. Marsh plants appear to be the dominant source of UDOM in freshwater regions of the FCE, whereas seagrasses and algae are the dominant sources of UDOM in Florida Bay. This study demonstrates the utility of a multi-technique and multi-proxy approach to advance our understanding of DON biogeochemistry.
Fluorescent dissolved organic matter (FDOM) in sediment pore waters from contrasting sites in the Chesapeake Bay and along the mid-Atlantic shelf/slope break was studied using three-dimensional fluorescence spectroscopy. Benthic fluxes of FDOM were also examined at the Chesapeake Bay sites. The major fluorescence peaks observed in these pore waters corresponded to those observed in the water column. These included peaks ascribed to the fluorescence of humic-like material (peaks A, C and M), as well as protein-like peaks that appear to result from the fluorescence of the aromatic amino acids tryptophan and tyrosine. In these pore waters we also observed a fourth humic-like fluorescence peak (A′). These four humic-like peaks appeared to occur in pairs (peaks A and M in one pair and peaks A′ and C in another pair) with near identical emission maxima but different excitation maxima. Peaks A′ and C were red shifted relative to peaks A and M.
We hypothesize that nutrient cycling in a Gulf of Mexico subterranean estuary (STE) is fueled by oxygen and labile organic matter supplied by tidal pumping of seawater into the coastal aquifer. We estimate nutrient production rates using the standard estuarine model and a non-steady-state box model, separate nutrient fluxes associated with fresh and saline submarine groundwater discharge (SGD), and estimate offshore fluxes from radium isotope distributions. The results indicate a large variability in nutrient concentrations over tidal and seasonal time scales. At high tide, nutrient concentrations in shallow beach groundwater were low as a result of dilution caused by seawater recirculation. During ebb tide, the concentrations increased until they reached a maximum just before the next high tide. The dominant form of nitrogen was dissolved organic nitrogen (DON) in freshwater, nitrate in brackish waters, and ammonium in saline waters. Dissolved organic carbon (DOC) production was two-fold higher in the summer than in the winter, while nitrate and DON production were one order of magnitude higher. Oxic remineralization and denitrification most likely explain these patterns. Even though fresh SGD accounted for only ∼5% of total volumetric additions, it was an important pathway of nutrients as a result of biogeochemical inputs in the mixing zone. Fresh SGD transported ∼25% of DOC and ∼50% of total dissolved nitrogen inputs into the coastal ocean, with the remainder associated with a one-dimensional vertical seawater exchange process. While SGD volumetric inputs are similar seasonally, changes in the biogeochemical conditions of this coastal plain STE led to higher summertime SGD nutrient fluxes (40% higher for DOC and 60% higher for nitrogen in the summer compared to the winter). We suggest that coastal primary production and nutrient dynamics in the STE are linked.
Ultraviolet (UV) excitation and emission spectra of cultures of marine bacteria and phytoplankton were investigated. The intensity of the emission band at 340 nm wavelength (λex=230 nm) is compared with cell number and protein content, and specific efficiencies are derived. Another weak emission band at 305 nm is found with most phytoplankton species. The same fluorescence signatures are observed with protein fragments and proteins of known composition and are caused by their tryptophan and tyrosine content. The UV fluorescence of marine particles is related to these aromatic amino acids bound to proteins.
Seasonal (Spring and Summer 2002) concentrations of dissolved (<0.22 μm) trace metals (Ag, Al, Co, Cu, Mn, Ni, Pb), inorganic nutrients (NO3, PO4, Si), and DOC were determined in groundwater samples from 5 wells aligned along a 30 m shore-normal transect in West Neck Bay, Long Island, NY. Results show that significant, systematic changes in groundwater trace metal and nutrient composition occur along the flowpath from land to sea. While conservative mixing between West Neck Bay water and the groundwaters explains the behavior of Si and DOC, non-conservative inputs for Co and Ni were observed (concentration increases of 10- and 2-fold, respectively) and removal of PO4 and NO3 (decreases to about half) along the transport pathway. Groundwater-associated chemical fluxes from the aquifer to the embayment calculated for constituents not exhibiting conservative behavior can vary by orders of magnitude depending on sampling location and season (e.g. Co, 3.4 × 102– 8.2 × 103 μmol d−1). Using measured values from different wells as being representative of the true groundwater endmember chemical composition also results in calculation of very different fluxes (e.g., Cu, 6.3 × 103 μmol d−1 (inland, freshwater well) vs. 2.1 × 105 μmol d−1(seaward well, S = 17 ppt)). This study suggests that seasonal variability and chemical changes occurring within the subterranean estuary must be taken into account when determining the groundwater flux of dissolved trace metals and nutrients to the coastal ocean.
High-resolution fluorescence spectroscopy was used to characterize dissolved organic matter (DOM) in concentrated and unconcentrated water samples from a wide variety of freshwater, coastal and marine environments. Several types of fluorescent signals were observed, including humic-like, tyrosine-like, and tryptophan-like. Humic-like fluorescence consisted of two peaks, one stimulated by UV excitation (peak A) and one by visible excitation (peak C). For all samples, the positions of both excitation and emission maxima for peak C were dependent upon wavelength of observation, with a shift towards longer wavelength emission maximum at longer excitation wavelength and longer wavelength excitation maximum at longer emission wavelength. A trend was observed in the position of wavelength-independent maximum fluorescence () for peak C, with maximum at shorter excitation and emission wavelengths for marine samples than for freshwater samples. Mean positions of these maxima were: rivers = nm; coastal water = nm; marine shallow transitional = nm; marine shallow eutrophic = nm; and marine deep = nm. Differences suggest that the humic material in marine surface waters is chemically different from humic material in the other environments sampled. These results explain previous conflicting reports regarding fluorescence properties of DOM from natural waters and also provide a means of distinguishing between water mass sources in the ocean.
Chromophoric dissolved organic matter (CDOM) was examined using fluorescence and absorbance spectra from sulfidic cave and thermal and non-thermal surface-discharging spring waters. Many of the sites have a limited allochthonous supply of organic matter (OM) and contain ecosystems that are dependent on chemolithoautotrophic microbial communities. Water-extracted OM from microbial mats at the sites had fluorescence signatures consistent with the fluorescent amino acids. Based on fluorescence-derived indices and absorbance spectral characteristics, the origin of the cave and spring CDOM appeared to be from microbially-derived material, and the degree of OM humification was low. Little of the CDOM pool was represented by terrestrial humic fluorescence signatures, which are typically observed in surface waters, as well as soil and sediment porewaters. Comparison of the cave and spring waters with a wide array of reference humic substances and OM from other environments showed a continuum of spectral properties constrained by origin and degree of humification.
Radon is useful as a tracer of certain geophysical processes in marine and aquatic environments. Recent applications include detection of groundwater discharges into surface waters and assessment of air/sea gas piston velocities. Much of the research performed in the past decade has relied on continuous measurements made in the field using a radon stripping unit connected to a radon-in-air detection system. This approach assumes that chemical equilibrium is attained between the water and gas phases and that the resulting air activity can be multiplied by a partition coefficient to obtain the corresponding radon-in-water activity. We report here the results of a series of laboratory experiments that describes the dependence of the partition coefficient upon both water temperature and salinity. Our results show that the temperature dependence for freshwater closely matches results that were previously available. The salinity effect, however, has largely been ignored and our results show that this can result in an overestimation of radon concentrations, especially in cooler, more saline waters. Related overestimates in typical situations range between 10 (warmer less saline waters) and 20% (cooler, more saline waters).
Fluorescence spectroscopy coupled with parallel factor analysis (PARAFAC) has been widely used to characterize dissolved organic matter (DOM). Characterization is based on the intensity and location of independent fluorescent components identified in models constructed from excitation-emission matrices (EEMs). Similar fluorescent components have been identified in PARAFAC studies across a wide range of systems; however, there is a lack of discussion regarding the consistency with which these similar components behave. The overall goal of this critical review is to compare results for PARAFAC studies published since the year 2000 which include one or more of three reoccurring humic-like components. Components are compared and characterized based on EEM location, characteristic ecosystems, and behavior in natural and engineered systems. This synthesis allows PARAFAC users to more confidently infer DOM characteristics based on identified components. Additionally, behavioral inconsistencies between similar components help elucidate DOM properties for which fluorescence spectroscopy with PARAFAC may be a weak predictive tool.
Systematic water sampling for characterization of chromophoric dissolved organic matter (CDOM) in the coastal South Atlantic Bight, was conducted as part of the long term Coastal Ocean Research and Monitoring Program (CORMP). Water samples were collected during a 3.5 year period, from October 2001 until March 2005, in the vicinity of the Cape Fear River (CFR) outlet and in adjacent Onslow Bay (OB). During this study there were two divergent hydrological and meteorological conditions in the CFR drainage area: a severe drought in 2002, followed by the very wet year of 2003. CDOM was characterized optically by the absorption coefficient at 350 nm, the spectral slope coefficient (S), and by Excitation Emission Matrix (EEM) fluorescence. Parallel Factor Analysis (PARAFAC) was used to assess CDOM composition from EEM spectra and six components were identified: three terrestrial humic-like components, one marine humic-like component and two protein-like components. Terrestrial humic-like components contributed most to dissolved organic matter (DOM) fluorescence in the low salinity plume of the CFR. The contribution of terrestrial humic-like components to DOM fluorescence in OB was much smaller than in the CFR plume area. Protein-like components contributed significantly to DOM fluorescence in the coastal ocean of OB and they dominated DOM fluorescence in the Gulf Stream waters. Hydrological conditions during the observation period significantly impacted both concentration and composition of CDOM found in the estuary and coastal ocean. In the CFR plume, there was an order of magnitude difference in CDOM absorption and fluorescence intensity between samples collected during the drought compared to the wet period. During the drought, CDOM in the CFR plume was composed of equal proportions of terrestrial humic-like components (ca. 60% of the total fluorescence intensity) with a significant contribution of proteinaceous substances (ca. 20% of the total fluorescence). During high river flow, CDOM was composed mostly of humic substances (nearly 75% of total fluorescence) with minor contributions by proteinaceous substances. The impact of changes in fresh water discharge patterns on CDOM concentration and composition was also observed in OB, though to a lesser degree.