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Submarine groundwater discharge (SGD) as a main nutrient source for benthic and water-column primary production in a large intertidal environment of the Yellow Sea
... Lee et al. (2009) estimated the Ra 226 flux into the Masan Bay and further used the radium data in the mass balance to calculate the SGD flux. Further the radium (Ra 226 ) mass balance is used in the Gamak bay and Hampyeong Bay (Waska and Kim, 2011) in order to estimate the SGD flux. Radon mass balance model is used for estimating the SGD flux of Geoje Bay by Hwang et al., 2016. ...
... There are numerous studies which are used the radium mass balance approach in the evaluation of SGD flux which include;Burnett et al. (2008),Schubert et al. (2014),Yuan et al. (2014),Chawla et al. (2014). The radium mass balance is used in the following studies;Yuan et al. (2014),Waska and Kim (2011), Hwang et al. (2010),Lee et al. (2009), andKim et al. (2008).. ...
Submarine groundwater discharge (SGD) is the combination of fresh and saline groundwater flux to marine system through continental boundaries regardless of its chemical composition and factors influencing the flow. We have discussed the SGD studies in the Asian context; SGD has been studied in various parts of Asia, including China, Japan, South Korea, and Southeast Asia. In China, SGD has been studied in several coastal regions, including the Yellow Sea, the East China Sea, and the South China Sea. In Japan, SGD has been studied in the Pacific coast, where it has been found to be an important source of fresh water to the coastal ocean. In South Korea, SGD has been studied in the Yellow Sea, where it has been found to be an important source of fresh water to the coastal ocean. In Southeast Asia, SGD has been studied in several countries, including Thailand, Vietnam, and Indonesia. Recently the SGD studies acquired much development India, the research on SGD in India is limited, and more studies are needed to understand the SGD process, its impact on the coastal environment, and the management strategies, Groundwater extraction for irrigation, industry, and domestic use is increasing in India, which can affect the SGD process in coastal aquifers. Overall, the studies suggest that SGD is an important process in Asian coastal regions, playing a role in the supply of fresh water and the transport of pollutants and nutrients.
... Despite individual studies that show impacts of fresh SGD on coral reefs (Lubarsky, Silbiger, & Donahue, 2018;Richardson, Dulai, Popp, Ruttenberg, & Fackrell, 2017), the implications of fresh SGD-derived nutrient influx to coral reefs are as of yet not fully assessed. Still, multiple studies in temperate lagoons have shown the process' significant ecological impact and provided evidence for a positive relationship between fresh SGD-derived nutrients and primary production (Andrisoa, Stieglitz, Rodellas, & Raimbault, 2019;Gobler & Sañudo-Wilhelmy, 2001;Johannes, 1980;Miller & Ullman, 2004;Sugimoto et al., 2017;Utsunomiya et al., 2017;Waska & Kim, 2011). ...
... Although globally, fresh SGD amounts to only a small percentage of river discharge (Luijendijk, Gleeson, & Moosdorf, 2020;Taniguchi, Burnett, Cable, & Turner, 2002) tropical coasts export more than 56% of all fresh SGD (Zhou, Sawyer, David, & Famiglietti, 2019). In coastal ecosystems nutrient-rich fresh SGD increases primary production and sustains higher primary and secondary consumer biomass (Dale & Miller, 2008;Encarnação et al., 2015;Hata et al., 2016;Lecher & Mackey, 2018;Piló et al., 2018;Utsunomiya et al., 2017;Waska & Kim, 2011). Further, physiologically beneficial environmental conditions brought about by the submarine influx of terrestrial nutrientrich cold acidic freshwater elevate the fitness of reef fish, potentially resulting in increased population sizes (Fujita et al., 2019;Lilkendey et al., 2019). ...
The inflow of terrestrial groundwater into the ocean is increasingly recognized as an important local source of nutrients and pollutants to coastal ecosystems. Although there is evidence of a link between fresh submarine groundwater discharge (SGD)-derived nutrients and primary producer and primary consumer abundances, the effects of fresh SGD on the productivity of higher trophic levels such as ichthyofaunal communities remain unclear. To further investigate this relationship, we sampled three sites inside a coral reef lagoon in Mauritius: One site entailing six distinct groundwater springs, a site highly influenced by freshwater influx through the springs, and a strictly marine control site. Using remote underwater video surveys, we found that fish abundances were significantly higher at the groundwater springs than at the other two sampling sites. Principal component analyses showed that the springs and the spring-influenced part of the lagoon were best described by elevated water nutrient loadings, whereas the control site was characterized by higher water salinity and pH. Macroalgae cover was highest at the control site and the springs. Herbivores and invertivores dominated the fish community at the springs, in contrast to generalists at the control site. At the spring-influenced site, we mainly encountered high coral/turf algae cover and high abundances of associated fish feeding groups (territorial farmers, corallivores). Our results provide evidence of a fresh SGD-driven relationship between altered hydrography and distinct fish communities with elevated abundances at groundwater springs in a coral reef lagoon. These findings suggest that the management and assessment of secondary consumer productivity in tropical lagoons should take into account the effects of groundwater springs.
... In coastal regions, SGD is a widespread phenomenon (Kwon et al., 2015;Sawyer et al., 2016) and is an important source of freshwater Moosdorf and Oehler, 2017), nutrients (Waska and Kim, 2011;Lecher et al., 2015;Wang et al., 2017), carbon (Atkins et al., 2013;Yong et al., 2017), and trace elements (Porubsky et al., 2014;Prouty et al., 2016) to coastal and open ocean. As a component of the hydrological cycle, SGD has significant impacts on conservation of local water resources and coastal ecological environments (Waska and Kim, 2011;Atkins et al., 2013). ...
... In coastal regions, SGD is a widespread phenomenon (Kwon et al., 2015;Sawyer et al., 2016) and is an important source of freshwater Moosdorf and Oehler, 2017), nutrients (Waska and Kim, 2011;Lecher et al., 2015;Wang et al., 2017), carbon (Atkins et al., 2013;Yong et al., 2017), and trace elements (Porubsky et al., 2014;Prouty et al., 2016) to coastal and open ocean. As a component of the hydrological cycle, SGD has significant impacts on conservation of local water resources and coastal ecological environments (Waska and Kim, 2011;Atkins et al., 2013). The continued occurrence of SFGD is critical for conservation of potable water as it can limit salt-water intrusion into aquifers. ...
Submarine groundwater discharge (SGD) is an important source of freshwater, nutrients, and other chemicals to the coastal water, and has significant impacts and implications for the coastal environment and ecology. Here, we combined geochemical tracers and hydrologic modeling to investigate the spatial patterns and quantities of SGD and submarine fresh groundwater discharge (SFGD) at a small island of western Japan. The results reveal large spatial variability in SGD and SFGD, significant discharge in areas with steep topography, and much lower discharge from low-lying areas. Topographic influences are likely to be a major driver of spatial variability in SFGD. The ²²²Rn mass balance model and two-end-member mixing model were used to estimate the SGD and SFGD rates. The values were ranged from 5.43 to 25.4 cm·d⁻¹ with an average value of 16.2 cm·d⁻¹, and 0.39 to 1.98 cm·d⁻¹ with a mean value of 1.17 cm·d⁻¹, for SGD and SFGD, respectively. The total flux of SFGD was calculated to be 8.49 × 10⁶ m³·yr⁻¹, with 4.37 × 10⁶ and 4.12 × 10⁶ m³·yr⁻¹ in the northern and southern regions of the island, respectively. The results were consistent with the values estimated by the topographic-based model. The ratio of the average annual SFGD to average annual precipitation (3.69 × 10⁷ m³) reached up to 23%, and the magnitude of annual SFGD was found to be similar to the total discharge from nine large rivers. Since the island faces a risk of water shortage, the results of the study can provide a useful insight into developing appropriate groundwater management strategies in this island's habitats.
... These findings encompass not only contaminants but also nutrients from terrestrial sources that are discharged to the sea as submarine groundwater discharge (SGD) (Santos et al., 2021;Taniguchi et al., 2019). Such nutrient supply via SGD affects marine biota by supporting primary productivity (Waska & Kim, 2011, 2010Blanco et al., 2011;Adolf et al., 2019), whereas excess nutrient loadings can lead to eutrophication (Hwang et al., 2005;Lee et al., 2009;Kwon et al., 2017;Cho et al., 2019). In light of these studies, our findings imply the reduced transport of nutrients from terrestrial sources to the sea via SGD, which might have multifaceted effects on marine ecosystems, potentially limiting primary productivity or alleviating eutrophication. ...
The combined effect of tidal forcing and aquifer heterogeneity leads to intricate transport patterns in coastal aquifers that impact both on solute residence times and mixing dynamics. We study these patterns through detailed numerical simulations of density-dependent flow and transport in a three-dimensional heterogeneous coastal aquifer under tidal forcing. Advective particle tracking from both the freshwater and seawater domains reveals the formation of chaotic and periodic orbits in the freshwater-saltwater transition zone that may persistently trap contaminants. We find that increasing heterogeneity results in increased trapping, but also increased mixing entropy, which suggests that the chaotic orbits enhance mixing between contaminants from the freshwater and seawater domains. These findings highlight on the one hand, the long-term contamination risks of coastal aquifers through trapping, and on the other hand, the creation of hotspots for chemical and biological reactions through chaotic mixing in the transition zone.
... Submarine groundwater discharge (SGD) has been identified as a source for nutrient fluxes in tidal-flat ecosystems in previous studies (e.g., Billerbeck et al., 2006;Røy et al., 2008;Santos et al., 2021;Waska and Kim, 2011;Wu et al., 2013). As we observed relatively constant marine salinities, we suspect that deep pore water flow (e.g., Røy et al., 2008) which is enriched with respect to nutrients acts as a source for our observed increasing TA and nutrient parameters. ...
Total alkalinity (TA) is an important chemical property that plays a decisive role in the oceanic buffering capacity with respect to CO2. TA is mainly generated by weathering on land as well as by various anaerobic metabolic processes in the water and sediments. The Wadden Sea, located in the southern North Sea, is hypothesized to be a source of TA for the North Sea, but quantifications are scarce. This study shows observations of TA, dissolved inorganic carbon (DIC), and nutrients in the Dutch Wadden Sea in May 2019. Surface samples were taken along several transects in order to investigate spatial distribution patterns and compare them with data from the late 1980s. A tidal cycle was sampled to further shed light on TA generation and potential TA sources. We identified the Dutch Wadden Sea as a source of TA and estimated an export of 6.6 Mmol TA per tide to the North Sea. TA was generated in the sediments, with deep pore water flow during low tide enriching the surface water. A combination of anaerobic processes and CaCO3 dissolution were potential TA sources in the sediments. We deduce that seasonality and the associated nitrate availability specifically influence TA generation by denitrification, which is low in spring and summer.
... SGD volumes can be calculated with simple mass balance models using activities of the respective endmembers (offshore ocean and groundwater), the inventory of the impacted water body, and the local water residence time (which in turn can be determined using 223/224 Ra decay models). The SGD-driven input signals of radionuclides are integrated over the water columns of semi-enclosed embayments and even marginal seas, allowing estimates on scales from several (Waska and Kim 2011) to thousands of kilometres (Moore et al. 2008;Rodellas 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).
... Studies have shown that abundance of cyanobacteria, phytoplankton, and macroalgae usually increases with highnutrient SGD input (Gobler and Boneillo, 2003;Troccoli-Ghinaglia et al., 2010;Waska and Kim, 2011;Peterson et al., 2012;Lee and Kim, 2015;Amato et al., 2016;Lecher and Mackey, 2018;La Valle et al., 2021;Dulai et al., 2023). Specifically, in Hawai'i studies have found that SGD increases invasive macroalgal abundance, thus altering reef community composition (Amato et al., 2016;La Valle et al., 2019;Dulai et al., 2023). ...
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 NO3-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.
... Burnett et al. (2007) calculate the SGD of 6.20E + 03 L/m day in the Gulf of Thailand. Waska and Kim (2011) used the tracers of radium ( 226 Ra), chlorophyll (Chl), and dissolved silicate (DSi) to quantify discharge rates between 0.14 and 0.35 m 3 m −2 day −1 in Yellow Sea. In Volcanic Jeju Island (Korea), the tracers of 222 Rn, 224 Ra, 226 Ra, Si, and nutrients quantified the discharge rate of 159 m 3 m −2 year −1 (Hwang et al., 2005). ...
Submarine groundwater discharge (SGD) is the groundwater flow from land to the sea across the seabed, and it includes both terrane freshwater and recirculated seawater in the sub-surface. This review (i) systematically evaluates findings of various quantification methodologies, (ii) examines the estimated SGD in scientific publications between 2000 and 2020, and (iii) quantitatively evaluates current situation of coastal zone management through the bibliometric analysis of research papers. Apart from enhancing the shortage of groundwater resources in coastal area, the SGD brings nutrients (nitrate and phosphate), toxic heavy metals, and organic compounds, and thus contaminate the seawater. Therefore, the improved understanding about location and quantity of global SGD is essential to conserve the coastal and ocean ecosystems.
... Ra and Rn; Kim and Hwang, 2002), inorganic 210 nutrients (nitrate, phosphate and silicate; e.g. Waska et al., 2011;Szymczycha et al., 2012), trace metals (e.g. Knee and Paytan, 2011), climate-relevant trace gases (carbon dioxide, nitrous oxide, methane and carbon monoxide; e.g. ...
For millennia humans have gravitated towards coastlines for their resource potential and as geopolitical centres for global trade. A basic requirement ensuring water security for coastal communities relies on a delicate balance between the supply and demand of potable water. The interaction between freshwater and saltwater in coastal settings is, therefore, complicated by both natural and human-driven environmental changes at the land-sea interface. In particular, ongoing sea level rise, warming and deoxygenation might exacerbate such perturbations. In this context, an improved understanding of the nature and variability of groundwater fluxes across the land-sea continuum is timely, yet remains out of reach. The flow of terrestrial groundwater across the coastal transition zone as well as the extent of freshened groundwater below the present-day seafloor are receiving increased attention in marine and coastal sciences because they likely represent a significant, yet highly uncertain component of (bio)geochemical budgets, and because of the emerging interest in the potential use of offshore freshened groundwater as a resource. At the same time, “reverse” groundwater flux from offshore to onshore is of prevalent socio-economic interest as terrestrial groundwater resources are continuously pressured by overpumping and seawater intrusion in many coastal regions worldwide. An accurate assessment of the land-ocean connectivity through groundwater and its potential responses to future anthropogenic activities and climate change will require a multidisciplinary approach combining the expertise of geophysicists, hydrogeologists, (bio)geochemists and modellers. Such joint activities will lay the scientific basis for better understanding the role of groundwater in societal-relevant issues such as climate change, pollution and the environmental status of the coastal oceans within the framework of the United Nations Sustainable Development Goals. Here, we present our perspectives on future research directions to better understand land-ocean connectivity through groundwater, including the spatial distributions of the essential hydrogeological parameters, highlighting technical and scientific developments, and briefly discussing its societal relevance in rapidly changing coastal oceans.
... Furthermore, inland lateral flows are the main source of groundwater recharge in the coastal region and discharge to the Yellow Sea (8.4-360 L/m 2 . d) (Peterson et al., 2008;Waska and Kim, 2011;Shi et al., 2015). ...
The relationship between groundwater and seawater is sensitive to groundwater exploitation, thus, not a simple result of groundwater and seawater lateral flow. The upward recharge of deep groundwater leads to a more complex relationship between groundwater and seawater in coastal regions with developed faults. The Quaternary confined groundwater level is significantly decreased in the case of groundwater overexploitation in the coastal region of the Subei Basin, where active faults are developed. The water level and hydrochemistry data for confined groundwater during periods of groundwater overexploitation was analyzed in the Subei Basin. Results show that confined groundwater is negligibly affected by seawater intrusion, while groundwater desalination is apparent during this period. High confined groundwater levels have been observed for many years in the area with intense groundwater exploitation. Additionally, the chemical and stable isotopic characteristics of confined groundwater, phreatic water, and surface water in the study area were investigated to reveal the pathways and origin of deep groundwater. The hydrogeochemical results demonstrate that the confined groundwater originates from basalt groundwater and is related to deep circulation. The active faults in the study area serve as pathways for upward recharge of basalt groundwater. Our findings provide new insights into the relationship between groundwater and seawater, and indicate that the upward recharge of deep groundwater should be considered in coastal regions with developed faults.
... However, it has several environmental consequences as the assortment of heavy metals, nutrients, organic compounds and radionuclides has polluted the near-shore groundwater (Moore, 1996;Burnett et al., 2003aBurnett et al., , 2003b. Similarly, it has also impacted local water possessions in the coastal settings (Waska and Kim, 2011;Mejías et al., 2012;Atkins et al., 2013). ...
Application of natural tracers such as radon isotope mass balance has been useful in estimating the submarine groundwater discharge (SGD). This study used 222Rn and evaluated the magnitude of SGD at Tiruchendur coast of southeast India in the Gulf of Mannar (Indian Ocean). Higher magnitudes of 222 Rn in the porewater and seawa-
ter in comparison with the groundwater suggest simultaneous SGD with fluxes of 0.1–0.25 m 3m −2 d−1at off-shore and 0.4–0.20 m3m−2d−1at the near shore. These baseline data would contribute to the management and protection of the Gulf of Mannar region in near future.
... Conversely, excess nutrient input via SGD may lead to harmful algal blooms in coastal waters (Hwang et al., 2005;Lee et al., 2010) and other ecological problems (Boesch, 2002). Site-specific studies have found that SGD is a major source of a variety of chemical constituents to tidal flats, estuaries, coral reefs, and other communities near the shoreline and on the continental shelf (Hwang et al., 2005;Waska and Kim, 2011;Charette et al., 2013;Niencheski et al., 2014). For instance, Hwang et al. (2005) found that SGD represents as much as 90% of the total dissolved inorganic nitrogen load to Bangdu Bay in Jeju, Korea, and such excess submarine input is a major nutrient source for benthic eutrophication. ...
Coastal beach aquifers are biogeochemically active systems that mediate chemical and material fluxes across the land-sea interface. This paper provides a review of major physical stressors and geologic features that influence flow and solute fate and transport in coastal beach aquifers. We outline current understanding of the interactions between these factors and their associated impacts on water and geochemical fluxes within and across these aquifers. The physical processes that control flow, transport, and the formation and distribution of reactive zones in beach aquifers (e.g., tides, waves, density gradients, precipitation, episodic ocean events, and evaporation) operate across overlapping temporal and spatial scales, and present challenges for measuring and modeling physical flow and biogeochemical processes in coastal groundwater systems. Geologic heterogeneity introduces further complexity by modifying flowpaths, mixing patterns, and rates of biotransformation. Interactions between these physical stressors and geological controls are likely to evolve with changes in sea level, climate variability, human settlement, coastal erosion, and other natural and anthropogenic stresses, providing avenues for scientific exploration into the future role of beach aquifers as chemical mediators between the land and ocean.
... Ra and Rn isotopes can be combined with other tracers, such as temperature (Cranswick et al., 2014), salinity (Example in Figure 3), or dissolved silicon (DSi) (Waska and Kim, 2011;Oehler et al., 2019b). Since DSi behaves rather conservatively during transport through the STE, it can be used to trace processes that change other solutes in groundwater in STE, such as N or P (Oehler et al., 2019b). ...
Subterranean estuaries the, subsurface mixing zones of terrestrial groundwater and seawater, substantially influence solute fluxes to the oceans. Solutes brought by groundwater from land and solutes brought from the sea can undergo biogeochemical reactions. These are often mediated by microbes and controlled by reactions with coastal sediments, and determine the composition of fluids discharging from STEs (i.e., submarine groundwater discharge), which may have consequences showing in coastal ecosystems. While at the local scale (meters), processes have been intensively studied, the impact of subterranean estuary processes on solute fluxes to the coastal ocean remains poorly constrained at the regional scale (kilometers). In the present communication, we review the processes that occur in STEs, focusing mainly on fluid flow and biogeochemical transformations of nitrogen, phosphorus, carbon, sulfur and trace metals. We highlight the spatio-temporal dynamics and measurable manifestations of those processes. The objective of this contribution is to provide a perspective on how tracer studies, geophysical methods, remote sensing and hydrogeological modeling could exploit such manifestations to estimate the regional-scale impact of processes in STEs on solute fluxes to the coastal ocean.
... Implications for primary production SGD can enhance primary production (PP) in coastal ecosystems (e.g., Waska and Kim 2011;Sugimoto et al. 2017). In Moune Bay, negative differences between nutrient inflows and outflows (Table 1) suggest the biological assimilation of nutrients, possibly due to PP. ...
External nutrient supply from the land and ocean is crucial for sustaining high primary productivity in coastal seas. Submarine groundwater discharge (SGD) is recognized as one of the most important sources of terrestrial nutrients. However, the relative importance of SGD-derived nutrients from different sources in coastal ecosystems controlled by offshore exchange has not been well quantified. Here, we assessed water and nutrient budgets in the semi-enclosed bay along the Sanriku ria coast, where the intrusion of nutrient-enriched oceanic water is substantial. We conducted seasonal sampling campaigns and monitored the groundwater level throughout the year. Water and nutrient fluxes from fresh groundwater, saline groundwater, river water, and oceanic water were estimated using a hydrological method and radium (Ra) mass balance model. The results indicated that oceanic water was a dominant source, accounting for 99.5%, 86%, 97%, and 84% of the total influx of water, dissolved inorganic nitrogen, dissolved inorganic phosphorus, and dissolved silica, respectively. Although the mean fluxes of land-derived nutrients were small, the contribution increased to 28-59% in October, when nutrient fluxes of oceanic water weakened. Of the terrestrial sources, SGD dominated (41-94%), particularly saline SGD (>99% of total SGD). We concluded that an efficient supply of the primary limiting nutrient from land to the coastal ecosystem can accelerate coastal primary production during certain seasons, even if oceanic nutrients are typically the dominant source.
... ). However, most of the SGDs are recirculated saline groundwater, supplying no "new" nutrients to the nearby coastal ocean(Santos et al., 2009;Waska and Kim, ...
The Bohai Sea is a shallow-water, semi-enclosed marginal sea of the Northwest Pacific. Since the late 1990s, it has suffered from nutrient over-enrichment. To better understand the eutrophication characteristics of this important coastal sea, we examined four survey datasets from summer (June 2011), late autumn (November 2011), winter (January 2016), and early spring (April 2018). Nutrient conditions in the Bohai Sea were subject to seasonal and regional variations. Survey-averaged N/P ratios in estuarine and nearshore areas were 20−133. In contrast, the central Bohai Sea had mean N/P ratios of 16.9 ± 3.4 in late autumn, 16.1 ± 3.0 in winter and 13.5 ± 5.8 in early spring, which are close to the traditional N:P Redfield ratio of 16. In summer, both dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphate (DIP) were used up in the surface waters of the central Bohai Sea, suggesting that the biological consumption of DIN and DIP may also follow the Redfield ratio. Wintertime nutrient budgets of the central Bohai Sea water were then established based on a mass balance study. Our results suggest that the adjacent North Yellow Sea supplied additional DIP to the central Bohai Sea via wintertime water intrusion, balancing terrigenous excess DIN that was introduced in summer. A water-mixing simulation combining these two nutrient sources with atmospheric nitrogen deposition suggests that eutrophication in the central Bohai Sea will likely be enhanced by the large-scale accumulation of anthropogenic nitrogen in adjacent open oceans. Such changes in nutrients may have fundamentally contributed to the recent development of algal blooms and seasonal hypoxia in the central Bohai Sea.
... In some cases, SGD influences the composition and abundance of marine biota in the receiving coastal water, e.g. fish [11,12], bacteria [13,14], macrophytes [15,16], or phytoplankton [17,18]. Subterranean estuaries (STEs), where SGD flows through before discharging to the coastal water, are active mixing zones and biogeochemical cycling hot spots [19]. ...
Mobile Bay, the fourth largest estuary in the USA located in the northern Gulf of Mexico, is known for extreme hypoxia in the water column during dry season caused by NH4⁺-rich and anoxic submarine groundwater discharge (SGD). Nutrient dynamics in the coastal ecosystem point to potentially elevated microbial activities; however, little is known about microbial community composition and their functional roles in this area. In this study, we investigated microbial community composition, distribution, and metabolic prediction along the coastal hydrological compartment of Mobile Bay using 16S rRNA gene sequencing. We collected microbial samples from surface (river and bay water) and subsurface water (groundwater and coastal pore water from two SGD sites with peat and sandy lithology, respectively). Salinity was identified as the primary factor affecting the distribution of microbial communities across surface water samples, while DON and PO4³⁻ were the major predictor of community shift within subsurface water samples. Higher microbial diversity was found in coastal pore water in comparison to surface water samples. Gammaproteobacteria, Bacteroidia, and Oxyphotobacteria dominated the bacterial community. Among the archaea, methanogens were prevalent in the peat-dominated SGD site, while the sandy SGD site was characterized by a higher proportion of ammonia-oxidizing archaea. Cyanobium PCC-6307 and unclassified Thermodesulfovibrionia were identified as dominant taxa strongly associated with trends in environmental parameters in surface and subsurface samples, respectively. Microbial communities found in the groundwater and peat layer consisted of taxa known for denitrification and dissimilatory nitrate reduction to ammonium (DNRA). This finding suggested that microbial communities might also play a significant role in mediating nitrogen transformation in the SGD flow path and in affecting the chemical composition of SGD discharging to the water column. Given the ecological importance of microorganisms, further studies at higher taxonomic and functional resolution are needed to accurately predict chemical biotransformation processes along the coastal hydrological continuum, which influence water quality and environmental condition in Mobile Bay.
... The process is responsible for habitat modifications and niche partitioning in benthic communities via local changes in water temperature, salinity, and pH (Amato et al., 2016;Foley, 2018). Fresh SGD can drive a coastal ecosystem's primary production via nutrient enrichment, which in turn leads to elevated primary and secondary consumer biomass (Dale and Miller, 2008;Waska and Kim, 2011;Encarnação et al., 2014;Hata et al., 2016;Utsunomiya et al., 2017;Lecher and Mackey, 2018;Piló et al., 2018). The assessment of factors influencing the abundance and growth of these consumers, in particular teleost fishes, is of ever-growing concern since this information is vital to predict consequences of anthropogenic actions on ecosystem functioning and productivity (Burnett et al., 2018;Shoji and Tominaga, 2018). ...
Fresh submarine groundwater discharge (fresh SGD), the efflux of terrestrial groundwater directly into the ocean, is a ubiquitous pathway for nutrient-rich freshwater to coastal ecosystems, altering their hydrography, hydrochemistry, and primary productivity. Yet only little is known about the effects of fresh SGD on the fitness of higher trophic levels such as teleost fish. Otolith analysis revealed that somatic growth rates were significantly higher and settlement to reef habitat took place significantly earlier in juvenile gray demoiselle Chrysiptera glauca exposed to fresh SGD as compared to strictly marine conditions. Contrary to expectations, feeding conditions were comparable in both habitats. We propose that physiologically beneficial environmental conditions brought about by the submarine influx of cold acidic freshwater enabled juvenile fish to exhibit elevated growth rates, thereby increasing their survival potential. This effect would directly link changes in groundwater on land to variations in marine primary and secondary consumer biomass at the coast.
... Primary production can be estimated by using a nutrient budget method (Waska and Kim, 2011), although estimates of SGD flux and nutrient loading commonly reported in SGD studies may not be appropriate for use as a comparison of nutrient availability for marine biota. In an urbanized embayment in Hong Kong, primary productivity was estimated at 1.5-15 × 10 6 g C d −1 , with 2-53% of the production being supported by SGD-driven PO 3− 4 flux (Luo et al., 2014). ...
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.
... nitrification (Rogers and Casciotti, 2010), denitrification (Santoro et al., 2006), or the dissimilatory nitrate reduction to ammonium (DNRA) (Song et al., 2014). Additionally, SGD impact on the benthic and planktonic communities at the receiving water bodies have also been observed (Adolf et al., 2019;Garcés et al., 2011;Waska and Kim, 2011). The composition of the microbial community at SGD sites can be highly variable due to groundwater level fluctuations, waves, and tidal cycles, which subsequently may modify the dominant biogeochemical reactions at the given sites (Grossart et al., 2004;Lee et al., 2017;Ye et al., 2016). ...
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.
... Seasonal sea level and groundwater table fluctuations, climate-driven sea level anomalies, wave conditions, storms, and precipitation patterns create variability in SGD flux (Gonneea et al., 2013a;Heiss & Michael, 2014;Michael et al., 2005;Moore & Wilson, 2005;Wilson et al., 2011;Xin et al., 2014;Yu et al., 2017), and consequently, the fluxes of nutrients, trace elements, and radioactive tracers to the adjacent ocean (Beck et al., 2007;Gonneea et al. (2013b); Jeong et al., 2012;Kelly & Moran, 2002;Roy et al., 2013;Seidel et al., 2015;Santos et al., 2009;Waska & Kim, 2011). The circulation cell is spatiotemporally dynamic, responding primarily to seasonal freshwater gradient fluctuations, and also to spring-neap cycles, and tidal stage to a lesser extent (Heiss & Michael, 2014). ...
Biogeochemical reactions within intertidal zones of coastal aquifers have been shown to alter the concentrations of terrestrial solutes prior to their discharge to surface waters. In organic‐poor sandy aquifers, the input of marine organic matter from infiltrating seawater supports active biogeochemical reactions within the sediments. However, while the seasonality of surface water organic carbon concentrations (primary production) and groundwater mixing have been documented, there is limited understanding of the transience of various organic carbon pools (pore water particulate, dissolved, sedimentary) within the aquifer and how these relate to the location and magnitudes of biogeochemical reactions over time. To understand the relationship between changes in groundwater flow and the seasonal migration of geochemical patterns, beach pore water and sediment samples were collected and analyzed from six field sampling events spanning 2 years. While the seasonally dynamic patterns of aerobic respiration closely followed those of salinity, redox conditions and nutrient characteristics (distributions of N and P, denitrification rates) were unrelated to contemporaneous salinity patterns. This divergence was attributed to the spatial variations of reactive particulate organic carbon distributions, unrelated to salinity patterns, likely due to filtration, retardation, and immobilization dynamics during transport within the sediments. Results support a “carbon memory” effect within the beach, with the evolution and migration of reaction patterns relating to the distribution of these scattered carbon pools as more mobile solutes move over less mobile pools during changes in hydrologic conditions. This holds important implications for the prediction and quantification of biogeochemical reactions within beach systems.
... Many studies published on this subject were performed at the scale of long coastlines of tens or hundreds of kilometers where porous aquifers are discharging (e.g., Moore et al., 2008;Peterson et al., 2009;Waska and Kim, 2011;Lamontagne et al., 2015) (e.g. Rapaglia et al., 2010;Baudron et al., 2015). ...
Submarine Groundwater Discharge (SGD) has received increased attention in recent years since it was recognized that it may be both volumetrically and chemically important. Around the Mediterranean Sea, 60% of the coastline is composed of karstic aquifers, and to properly estimate the hydrological budget of the Mediterranean Sea it is therefore necessary to better assess the karstic submarine groundwater discharges (KSGD). However, quantifying KSGD is still challenging. Among the methods recently developed to detect and quantify SGD, the mass balance method of the radium quartet 223Ra, 224Ra, 226Ra and 228Ra has proved to be a powerful technique. This approach requires characterizing all the contributing terms and sinks in the coastal water volume affected by SGD, the residence time of coastal waters, as well as a representative concentration of the tracers for both surface water and discharging groundwater. In this study we combine several approaches (223Ra, 224Ra, salinity profiles and Acoustic Doppler Current Profiler (ADCP) measurements) to examine both the accuracy and sensitivity of the radium mass balance method in the case of the cove of Port-Miou (Mediterranean Sea, France) where the main karstic spring discharges locally at 10 m depth. This study benefits from the inland in-situ access to the main karst conduit discharging to the sea which provides a long time series to characterize the brackish submarine groundwater end-member. We show that the composition of the cove water is stratified, with two water bodies: a surface brackish layer and a deeper layer. The mean KSGD value obtained with 223Ra and 224Ra mass balances in the surface water body is precise but significantly lower (0.6 ± 0.1 m3/s) than the karstic spring discharge (4 ± 1 m3/s) estimated within the karst conduit with pressure sensors. The residence time of the cove water estimated using both 224Ra and 223Ra isotopes is very low (1 ± 1 day). Our study shows that the water residence time that we calculated using the Ra mass balance is the key parameter that may impact KSGD. In addition, based on ADCP transects, we suggest that the shape and geometry of the cove, as well as the location of the discharge point of the spring play a key role in explaining these discrepant results. We therefore recommend that in such stratified coves, estimations of KSGD based on short-lived radium isotopes require accurate and independent estimates of the water residence time as well as a good knowledge of the shallow and deep circulation patterns of the cove water.
... Through the microbial remineralization driven by supply of electron donors and acceptors via advective ground-and porewater flow, the STE can change the concentration and speciation of terrestrially-and marine-derived chemical constituents before submarine groundwater is discharged into the coastal ocean (SGD, Moore, 1996). During passage through the STE, pore water may become enriched compared to the overlying water column, with chemical constituents such as nitrate and silicate (Waska and Kim, 2011), dissolved organic carbon (DOC; Kim et al., 2012), barium and radium (Charette and Sholkovitz, 2006), rare earth elements (REE; Johannesson et al., 2011) and other trace elements, such as copper and iron (Cu and Fe, Montluçon andSañ udo-Wilhelmy, 2001, Windom et al., 2006). SGD occurs ubiquitously worldwide (Zhang and Mandal, 2012), and on a global scale, its volumetric and constituent fluxes are estimated to be in the same order of magnitude as that of rivers (Burnett et al., 2003, Kwon et al., 2014, Cho and Kim, 2016, Cho et al., 2018. ...
Subterranean estuaries (STEs) are land-ocean interfaces where meteoric fresh groundwater mixes with intruding seawater in a coastal aquifer, before discharging into the adjacent water column. In contrast to surface estuaries, STEs have the potential to amplify concentrations of constituents such as copper (Cu) and iron (Fe) due to long residence times and reductive dissolution of mineral phases along the groundwater flowpaths. However, oxidative precipitation of Fe and Mn at the sediment-water interface may scavenge many constituents again before they reach the coastal water column. Hence, the geochemical impact of the suboxic to anoxic submarine groundwater discharge (SGD) on the oxygenated coastal ocean relies on the capability of constituents such as Cu and Fe to stay in solution across redox boundaries. Here, we propose that dissolved organic matter (DOM) in the STE plays a pivotal role in the speciation of Cu and Fe through (i) fueling reductive dissolution and (ii) providing ligands to form stable metal-DOM complexes, increasing their transfer from the STE into the coastal ocean. We investigated the concentrations and speciation of Cu and Fe, and DOM chemical characteristics, in two beach STEs of a barrier island. By combining well-established techniques with novel quantification and speciation approaches from both the inorganic and organic geochemical realm (size-fractionation filtration, ferrozine detection, voltammetry, sequential DOM extraction, and ultra-high resolution mass spectrometry) we characterized metal-DOM associations down to the molecular level. Overall, pore water from both STEs was enriched with Cu and Fe compared to seawater, which indicated transfer potential for both trace metals across the sediment-water interface. However, Fe gradients from pore water to surface were steeper than those for Cu, indicating a larger net transfer of the latter compared to the former. Our voltammetry data showed that Cu was exclusively organically bound in both STEs and the water column, mostly in soluble form (<20 nm). The majority of >60 newly identified Cu-containing complexes had primarily aliphatic character and N and S in their molecular formulae resembling labile marine DOM, while two Cu-DOM complexes had polyphenol (“humic-like”) molecular formulae indicative of terrestrial vascular plant-derived material. In contrast to Cu, the Fe pool consisted of either reduced, soluble (<20 nm), likely free Fe(II) in the anoxic STE, or of larger colloids (<200 nm and >20 nm) in the fresh groundwater and seawater endmembers, likely as Fe(III)(hydr)oxides stabilized by DOM. Furthermore, while Fe and humic-like DOM seemed to share common sources, all directly identified mobile Fe-DOM complexes appeared to have marine origins. Therefore, organic forms of Fe in the STE may primarily consist of immobile humic-Fe coagulates, partially mobile Fe-nanocolloids, and mobile, N-containing, marine aliphatic Fe-complexes. Our study indicates that aliphatic, N-containing ligands may play an important role in the organic complexation and stabilization of Fe and particularly Cu in the STE, and enable them to cross redox boundaries at the sediment-water interface.
... nitrification (Rogers and Casciotti, 2010), denitrification (Santoro et al., 2006), or dissimilatory nitrate reduction to ammonium (DNRA) (Bernard et al., 2015). Additionally, impacts of SGD on the benthic and planktonic communities at the receiving water bodies have also been observed (Adolf et al., 2019;Garcés et al., 2011;Waska and Kim, 2011). The composition of the microbial community at SGD sites can be highly variable due to groundwater level fluctuations, waves, and tidal cycles, which subsequently may modify the dominant biogeochemical reactions at given sites (Grossart et al., 2004;Lee et al., 2017;Ye et al., 2016). ...
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.
... Generally, the DIN:DIP ratios in groundwater are greater than the widely accepted average requirements of phytoplankton growth (16:1; Slomp & Van Cappellen, 2004). It was repeatedly observed that SGD-derived DIN:DIP ratios were much higher than DIN:DIP ratios in the rivers and other sources in coastal waters (Hwang et al., 2005;Lee et al., 2009;Waska & Kim, 2011). In this study, the average DIN:DIP ratio of net SGD-derived nutrients was approximately 320, which was much higher than those found in the KRE surface layer water (~17) and wastewater from the city of Šibenik (~12; Figure 7). ...
Submarine groundwater discharge (SGD) is a significant source of biogenic elements in estuaries, and relevant studies in karstic estuaries are scarce. Krka River Estuary (KRE), located on the eastern Adriatic Sea (Croatia), is a typical oligotrophic stratified karstic estuary. In this study, based on ²²⁶Ra and ²²⁸Ra, the total SGD flux into the KRE surface layer was estimated to be (12.8–16.2) × 10⁵ m³/day. A conservative estimation of the fresh groundwater flux was (5.0–8.3) × 10⁵ m³/day, which accounts for 10–17% of the Krka River discharge into the estuary. By establishing water and nutrient budgets in the KRE surface layer, we found that SGD dominated the nutrient sources, although it accounted for a small portion of the total inflow water. Specifically, net SGD‐derived dissolved inorganic nitrogen (DIN) and silicates contributed 58–90% and 24–64%, respectively, to the total input fluxes. These results indicate that SGD was a major external nutrient source, in which net SGD‐derived high DIN flux and high DIN to dissolved inorganic phosphorus ratio may affect productivity in the KRE ecosystem and nearby Adriatic Sea. Additionally, net SGD‐derived dissolved inorganic carbon (DIC) flux in the KRE (1.53 mol · m² · day) was much higher than those in most estuaries worldwide, suggesting that the DIC‐enriched karst aquifers are important sources for global carbon cycle. Therefore, the impact of net SGD‐derived DIC from karst aquifers on coastal seas will likely become more evident and substantial with further development of global climate change, such as sea level rise.
... Conversely, excess nutrient input via SGD may lead to harmful algal blooms in coastal waters (Hwang et al., 2005;Lee et al., 2010) and other ecological problems (Boesch, 2002). Site-specific studies have found that SGD is a major source of a variety of chemical constituents to tidal flats, estuaries, coral reefs, and other communities near the shoreline and on the continental shelf (Hwang et al., 2005;Waska and Kim, 2011;Charette et al., 2013;Niencheski et al., 2014). For instance, Hwang et al. (2005) found that SGD represents as much as 90% of the total dissolved inorganic nitrogen load to Bangdu Bay in Jeju, Korea, and such excess submarine input is a major nutrient source for benthic eutrophication. ...
Extreme storms can cause rapid morphological changes that pose high risk to society (Sallenger 2000). Semiempirical and process-based models often are used to simulate storm-induced coastal processes (Roelvink et al. 2009, Palmsten & Holman 2012, Stockdon et al. 2014, Overbeck et al. 2017). However, there are few observations of surfzone waves and currents during extreme storms. Therefore, parameterizations often are calibrated by minimizing model-data errors for pre- to post-storm bathymetric and topographic changes, and the accuracy of the simulated processes during the storm is unknown. Here, surf, swash, and dune observations collected near Duck, NC, USA, will be used to investigate wave processes and dune erosion during the passage of recent (2015-2017) Hurricanes.
... Data from multiple studies of the same geographic region were averaged together to avoid any geographical bias in determining the mean DSi endmember concentration. The lithological endmembers for which we summarized studies were: "extrusive igneous" (Burnett et al., 2006;Dollar and Atkinson, 1992;Georg et al., 2009;Holleman, 2011;Hwang et al., 2005a;Johnson, 2008.; Knee et al., 2010Knee et al., , 2016Mandal et al., 2011;Schopka and Derry, 2012;Street et al., 2008;Zavialov et al., 2012); "carbonate" (El-Gamal et al., 2012;Garcia-Solsona et al., 2010a,b;Gonneea et al., 2014;Hernández-Terrones et al., 2011;Herrera-Silveira, 1994;Null et al., 2014;Rocha et al., 2015;Tamborski et al., 2018;Tovar-Sánchez et al., 2014;Young et al., 2008); "sandstone" (Sugimoto et al., 2017;Weinstein et al., 2011); "shield/ granite" (Lecher et al., 2016;Onodera and Saito, 2007;Rengarajan and Sarma, 2015;Wang et al., 2015); "shale" (Kim et al., 2005;Lee et al., 2012;Luo et al., 2014;Ye et al., 2016); and "complex" (Anschutz et al., 2016;Boehm et al., 2004;Burnett et al., 2007;Charette et al., 2013;Charette and Sholkovitz, 2006;Ehlert et al., 2016a,b;Godoy et al., 2013;Hwang et al., 2005a,b;Johannes and Hearn, 1985;Kim et al., 2008;Lecher et al., 2015;Lee et al., 2010;Lee et al., 2009;Li et al., 2014;Liu et al., 2011;Niencheski et al., 2007;Null et al., 2012;Su et al., 2011;Swarzenski et al., 2007b;Swarzenski et al., 2007a;Tamborski et al., 2017;Ullman et al., 2003;Urquidi-Gaume et al., 2016;Waska and Kim, 2011). Only samples with a salinity ≤6 were considered for the terrestrial endmember to reduce the influence of mixing between dissimilar water masses. ...
Groundwater's role in the global marine budget of dissolved silica (DSi), an essential nutrient, is constrained using DSi groundwater concentrations from multiple endmember lithologies and a global terrestrial submarine groundwater discharge (SGD) model. We report new DSi concentrations in nine subterranean estuaries throughout the world, including Panama, Mauritius, Guam, Yucatan (Mexico), Chile, Argentina, Southwest Florida (USA), Long Island Sound (USA) and Waquoit Bay (USA). These new data are augmented with a literature survey of DSi endmember concentrations in the subterranean estuary to determine the global DSi endmember in SGD, classified by the regional lithology (carbonate, shale, sandstone, extrusive igneous, shield and “complex”). DSi fluxes to the ocean from terrestrial (fresh) SGD equal 0.7 ± 0.1 Tmol y−1, more than half of which enters the Pacific Ocean. Non-conservative DSi enrichment was observed in marine groundwaters circulated through extrusive igneous and complex lithology sediments for twenty different study sites. Dissolution rate calculations indicate that non-conservative DSi enrichments in marine groundwaters can be supported, in part, by lithogenic dissolution of the coastal sediment, rather than biogenic silica dissolution. We make preliminary estimates of DSi inputs via marine SGD in shallow coastal aquifers of ~3 Tmol y−1. Considering recent revisions to the marine silica budget (e.g., increased estimates of biogenic silica storage via reverse weathering reactions, increased estimates of standing stocks of biogenic silica, and changing estimates of silica burial efficiency), sources and sinks in the marine Si budget can be balanced when taking into account estimates of new DSi inputs from total SGD (terrestrial and marine flow paths). These findings impact the residence time of oceanic Si and mass balances of the stable Si isotopes, as well as associated silicate weathering products, including Li and Ge.
... There are no SGD data in the western NYS, but according to the result of Peterson et al. (2008), the SGD flux was 40.4-46.7 km 3 /a in the Yellow River Delta from 2006 to 2007. DIN concentration of SGD in the Bohai Sea was calculated to be 410 ± 270 μmol/L in the summer of 2008 and 2009 (Liu et al., 2011), and PO 4 -P concentration of SGD was 0.49 ± 0.39 μmol/L in the summer of 2008 (Waska and Kim, 2011). It was estimated that SGD contributed an annual DIN load of 17. 65 Gmol, 198865 Gmol, 199065 Gmol, 199265 Gmol, 199465 Gmol, 199665 Gmol, 199865 Gmol, 200065 Gmol, 200265 Gmol, 2004 ...
... Error bars represent standard errors (n = 3) among the multiple abiotic factors affecting intertidal areas, runoff events and groundwater discharge have a significant influence in the nutrient recycling (Page et al., 1995;Uchiyama et al., 2000;Hays & Ullman, 2007b;Rocha et al., 2009;Waska & Kim, 2011). However, at the studied sites, the freshwater influence seems to be low, as the salinity of the pore and SZW throughout the year range from 34.9 ± 0.6 to 31.5 ± 0.6. ...
... Error bars represent standard errors (n = 3) among the multiple abiotic factors affecting intertidal areas, runoff events and groundwater discharge have a significant influence in the nutrient recycling (Page et al., 1995;Uchiyama et al., 2000;Hays & Ullman, 2007b;Rocha et al., 2009;Waska & Kim, 2011). However, at the studied sites, the freshwater influence seems to be low, as the salinity of the pore and SZW throughout the year range from 34.9 ± 0.6 to 31.5 ± 0.6. ...
... Thus, SGD is very important to the marine geochemical cycle of elements such as nutrients and metals, and it can lead to various environmental problems in coastal zones (Moore, 1997(Moore, , 2006Beck et al., 2007;Wudtisin and Boyd, 2005). Numerous studies have shown that the nutrient inputs to coastal water through SGD may trigger harmful algal blooms, which have negative impacts on the marine environment and economy (Lee and Kim, 2007;Lee et al., 2009;Waska and Kim, 2011). ...
Daya Bay, a semi-enclosed bay in the South China Sea, is well known for its aquaculture, agriculture, and tourism. In recent years, many environmental problems have emerged, such as the frequent (almost yearly) occurrence of harmful algal blooms and red tides. Therefore, investigations of submarine groundwater discharge (SGD) and associated nutrient inputs to this bay have important theoretical and practical significance to the protection of the ecological system. Such a study was conducted using short-lived radium isotopes ²²³Ra and ²²⁴Ra. The estimated SGD fluxes were 2.89 × 10⁷ m³/d and 3.05 × 10⁷ m³/d based on ²²³Ra and ²²⁴Ra, respectively. The average SGD flux was about 35 times greater than that of all the local rivers. The SGD-associated dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) fluxes ranged from 1.95 × 10⁶ to 2.06 × 10⁶ mol/d and from 5.72 × 10⁴ to 6.04 × 10⁴ mol/d, respectively. The average ratio of DIN to DIP fluxes in SGD was 34, much higher than that in local rivers (about 6.46), and about twice as large as the Redfield ratio (16). Our results indicate that SGD is a significant source of nutrients to the bay and may cause frequent occurrence of harmful algal blooms. This study provides baseline data for evaluating potential environmental effects due to urbanization and economic growth in this region.
... (1)). The exchange rate of this study was within the range of exchange rates measured in other studies (Table 1), including those using numerical simulation (e.g., Xia and Li, 2012) and radioisotope tracers (e.g., Boehm et al., 2004;Gleeson et al., 2013;Krest et al., 2000;Waska and Kim, 2011). Readers interested in more comparisons of seawater- Fig. 5. ...
Submarine groundwater discharge (SGD) is a hydrological phenomenon that involves the transport of fresh and brackish groundwater from land into the sea through permeable sediments. In addition to surface water discharges, SGD also acts as a channel for directly transferring inorganic nutrients, metals and contaminants into the coastal systems. Research evidence shows that SGD can alter various abiotic properties of coastal waters including salinity, temperature, pH and nutrient concentrations which influence the life of local marine biota at different trophic levels. However, little is known about how it affects marine organisms since each group responds uniquely. Here, we elaborate on a comprehensive review of SGD in the context of biota represented by phytoplankton, meiofauna, macrofauna, corals and fishes. A case study that investigates the response of interstitial fauna over SGD addition along the subterranean estuaries of the Kerala coast is also discussed in this chapter.
Groundwater is defined as water stored underground, in rock and soil pore and fracture space. About 99% of all fresh liquid water is groundwater. It covers ~30% of human freshwater needs, with ~70% used in agriculture. However, groundwater quality is declining worldwide. Contaminated groundwater is shown to be an important stressor in marine habitats, with multiple negative consequences to ecology, ecosystem function and the provision of societal goods and benefits. To assess the need to restore groundwater quality to preserve the productivity of coastal ecosystems, we review the contribution of submarine groundwater discharge (SGD) to coastal nutrient budgets and discuss the impact of SGD on eutrophication and eutrophication mitigation plans. We find that roughly 14 % of the total nitrogen (N) and 3.9 % of the phosphorus (P) annual inputs into agroecosystems arrives at sea via SGD. This transfer is modulated by subterranean estuaries (STEs), underground zones named after their surface analogs where saline and fresh groundwater mix. Total (fresh + saline) SGD contributes an average of 2.3, 0.06 and 3.8 Tmol yr−1 of total dissolved N, P and Silicate (Si), respectively to the coastal ocean (N:P:Si ratio of 38:1:63). This flux is comparable to that of global riverine input of 2.32, 0.08 and 6.42 Tmol yr−1 of total dissolved N, P and Si into the ocean (N:P:Si ratio of 29:1:80). The flux of groundwater nitrogen closes the gap between sources and sinks in the coastal N cycle to between −0.96 and −3.91 Tmol N yr−1, helping SGD support ~2% (15.2 Tmol C yr−1) of coastal Net Primary Production (NPP). These fluxes make groundwater borne nutrients an important load pressure on coastal ecosystems. Subterranean estuaries function as particulate filters and further uncouple N from P and Si loading, making groundwater contribute to significant enrichment of coastal ecosystems in N by comparison to P and Si. The impact of nutrient enrichment of coastal systems by SGD will be amplified in both time and space by the relatively slow movement of water through soils and permeable rock, compared to open water discharge. Indeed, the relatively slow pace of transfer of mass through the sub-surface pathway results in a transient retention of nutrients in transit to sea in continental aquifers and soils. As a result, the effect of groundwater discharge on marine habitats lags behind the contamination of groundwater, sometimes by decades. This ‘time lag’ depends on the hydrogeology and biogeochemistry of aquifer systems and is thus regionally distinct. Time lags are poorly constrained worldwide, and yet cause the visible effects of groundwater nutrient pollution mitigation programs to lag implementation by years to decades. Furthermore, because SGD is ignored in current assessments of eutrophication status in coastal ecosystems, feedbacks between legacy pressures (i.e. / e.g., the nutrients accumulated in coastal groundwater bodies as a result of decades of intensive agriculture and already in transit to sea), SGD, surface runoff and coastal eutrophication are not understood. Long term strategies for nutrient (N, P) reduction, coupled to monitoring of water quality of both surface and subterranean estuaries and their microbial populations, are necessary to preserve coastal productivity and ecosystem health for future generations. The prioritization of these strategies should be informed by groundwater age profiling of coastal aquifers. This information will lead to a better management of lag times between the implementation of catchment nutrient management strategies and their outcomes in terms of coastal eutrophication reduction.
As the major global CO2 sink, the oceanic buffering capacity total alkalinity (TA) is of growing scientific interest. TA is mainly generated by weathering, and further by various anaerobic metabolic processes. The Wadden Sea, located in the southern North Sea is thought to be a source of TA for the carbonate system of the North Sea, but quantifications are scarce. Here, we observed TA, dissolved inorganic carbon (DIC), and nutrients in the Dutch Wadden Sea in May 2019. We sampled transect surface waters to detect spatial distributions and compared it with earlier data. A tidal cycle was sampled to further shed light on TA generation and potential TA sources. We identified the Wadden Sea as a source of TA with an average TA generation of 7.6 µmol kg-1 h-1 during ebb tide in the Ameland Inlet. TA was generated in the sediments and washed out with off running water. A combination of anaerobic processes and CaCO3 dissolution were potential sources of TA in the sediments. We assume that seasonality and the associated nitrate availability in particular influence TA generation by denitrification, which we assume is low in spring and summer.
For millennia, humans have gravitated towards coastlines for their resource potential and as geopolitical centres for global trade. A basic requirement ensuring water security for coastal communities relies on a delicate balance between the supply and demand of potable water. The interaction between freshwater and saltwater in coastal settings is, therefore, complicated by both natural and human-driven environmental changes at the land–sea interface. In particular, ongoing sea-level rise, warming and deoxygenation might exacerbate such perturbations. In this context, an improved understanding of the nature and variability of groundwater fluxes across the land–sea continuum is timely yet remains out of reach. The flow of terrestrial groundwater across the coastal transition zone and the extent of freshened groundwater below the present-day seafloor are receiving increased attention in marine and coastal sciences because they likely represent a significant yet highly uncertain component of (bio)geochemical budgets and because of the emerging interest in the potential use of offshore freshened groundwater as a resource. At the same time, “reverse” groundwater flux from offshore to onshore is of prevalent socio-economic interest, as terrestrial groundwater resources are continuously pressured by over-pumping and seawater intrusion in many coastal regions worldwide. An accurate assessment of the land–ocean connectivity through groundwater and its potential responses to future anthropogenic activities and climate change will require a multidisciplinary approach combining the expertise of geophysicists, hydrogeologists, (bio)geochemists and modellers. Such joint activities will lay the scientific basis for better understanding the role of groundwater in societally relevant issues such as climate change, pollution and the environmental status of the coastal oceans within the framework of the United Nations Sustainable Development Goals. Here, we present our perspectives on future research directions to better understand land–ocean connectivity through groundwater, including the spatial distributions of the essential hydrogeological parameters, highlighting technical and scientific developments and briefly discussing the societal relevance of that connectivity in rapidly changing coastal oceans.
Judicial use of submarine groundwater discharge (SGD) can be a potential water resource for countries facing water scarcity. Very few studies report the fluxes of SGD into the oceans, especially from those countries that are located in the tropics and facing water scarcity. Another dimension of the SGD is its potential to control the biogeochemical cycles of nutrients and trace metals and the anthropogenic impact on the oceans. This work attempts to give an overview of the challenges and limitations involved in achieving the above. We have reviewed 1628 published literature that reported SGD in different contexts in the last 21 years (the Year, 2000 to this date). Several studies exist in bits and pieces across the world's coastline, with different methodologies adopted for identifying and quantifying the SGD. This compilation has attempted to extract these findings and listed the challenges and limitations in estimating the SGD fluxes. Significant challenges in quantifying the discharge include inconsistent sampling strategies adopted by researchers, uncertainties in modeling, spatio-temporal variations in discharge, extreme weather conditions, and difficulty in quantifying discharge at inaccessible areas (mangroves, large tidal flats, etc.). Some limitations discussed in this work include insufficient knowledge of coastal aquifer data, geology, and lack of historical hydrological data. Based on the critical analysis of the published literature, we recommend a few solutions that can provide a better resolution in the quantification of SGD. Decision makers and water conservation professionals will benefit from this work as they can suitably plan the water management, pollution control, and sustainable extraction of the SGD. We suggest strategies calling for assessments of SGD in areas of potentially significant discharge and developing new monitoring networks and strict policies for groundwater usage.
In intertidal zones, groundwater is often present as seepage that provides freshwater and nutrients to marine ecosystems. Point discharge or springs in intertidal zones have been observed in many locations, often in the form of sand boils. The spatial extent, temporal variability and source of intertidal springs are rarely documented and typically, not well understood. This study examined four intertidal groundwater springs at Sellicks Beach, South Australia, during May 2017, November 2019 and September 2020 using a combination of hydrogeophysical methods. A thermal infrared survey undertaken in 2017 showed springs as groupings of closely spaced sand boils that were warmer (15°C) than the surrounding saturated beach sediments (7°C). The four springs ranged in diameter from 0.20 to 0.45 m. Electromagnetic geophysical surveys identified a resistive anomaly (3.5 to 5.0 ohm.m), assumed to represent freshwater upwelling at the location of a spring, that extended 10 m horizontally and at least 6.7 m vertically. The average electrical conductivity of water discharging from the springs was 18.4 mS/cm, while seawater was 54.8 mS/cm. δ¹⁸O and δ²H data from the springs showed a variation between winter and spring, likely caused by variations in mixing ratios between seawater and groundwater. The springs are proximal to major regional fault systems that likely create preferential flow paths that control spring location and flow rates. The observations of spring characteristics highlight the critical role of seawater-groundwater mixing ratios, preferential flow paths and tidal variations in creating temporal variability in spring discharge and salinity.
Direct release of fresh/recirculated groundwater to the ocean is of greater significance due to its material transfer pathways along the coastal zones. To assess the significance of submarine groundwater discharge (SGD) as a cause of chemical and dissolved fluxes to the ocean, estimation of submarine groundwater discharge, and associated dissolved nutrients and rare earth elements fluxes to the Bay of Bengal from the Sankarabarani river basin, India has endeavored. A total of 180 groundwater samples were collected from the three different locations for 30 days by considering high and low tides. The submarine groundwater discharge through the aquifer ranges from 0.75 to 2.90 m d− 1 calculated by the radon mass balance model. The SGD rate is increasing toward the coast, which is higher in location C followed by locations A and B suggesting the influence of recirculated seawater resulting in brackish SGD. From the nutrient mass balance, DIN’s average net invention was 192.00 μ mol d− 1, DIP is 4.00 μ mol d− 1, and DSi is 97.00 μ mol d− 1 suggesting SGD-derived nutrients were higher and responsible for algal blooms influencing the biodiversity of the study area. From the REE mass balance, the SGD supported REE flux was 25.70 m mol d− 1 for Nd, 5.04 m mol d− 1 for Gd, and 2.92 m mol d− 1 for Yb, respectively. The removal of LREEs in a more significant quantum in comparison with MREE and HREEs. The more excellent removal of LREEs is mainly due to increasing salinity and release of MREEs and HREEs due to its particle reactivity. The SGD-derived fluxes were influenced by tidal fluctuations, hydraulic gradient, the conductivity of the formation, groundwater extraction, and other biogeochemical influences. The sources for nutrients and REEs fluxes seem to be influenced by interaction between the fresh and bay water end members. The other dominant sources are the availability of elements, speciation, presence or absence of colloidal fragments, and various sources and sinks.
This book chapter includes the study of submarine groundwater discharge (SGD) across various coastlines and related nutrient fluxes to the coastal oceans. SGD represents an essential pathway of materials like carbon, nitrate, phosphate, silicate, and trace metals between land and sea. Global DOC and DIC Fluxes through SGD studied across different mangrove climatic zones indicate that the area of tropics accounted for 71% and 81% of DIC and DOC fluxes, respectively. Subtropics contributes 29% and 19% respectively, while the contribution of temperate region is even less than 1% for both the parameters. An increase of about 38% in SGD nitrogen input between 1950 and 2000 has been observed in global coastal water and is estimated to increase by 22% between 2000 and 2050. The rise in nitrogen is not followed by an equivalent increase in phosphate and silicate, suggesting that nitrogen is the primary nutrient affected by SGD. The increase in nitrogen is mainly due to the anthropogenic contamination of the certain aquifer, which in turn modify the Redfield ratio where the N/P ratio exceeds the river system revealing that human activities can change N-limited coastal primary production to P-limited one. The Indian coastline is mainly composed of red, yellow, and alluvial soils, which are poor in nutrients. Hence Indian farmers use excessive fertilizers to obtain a better yield. Which plays a significant role in controlling nutrient dynamics in groundwater. SGD-derived trace metals are also equal to or higher than the river fluxes and contribute approximately 10% of total trace elements to the oceans.
Submarine groundwater discharge (SGD) is being increasingly recognized as a significant source of nutrient into coastal waters, and generally comprises of two components, submarine fresh groundwater discharge (SFGD) and recirculated saline groundwater discharge (RSGD). The separate evaluation of SFGD and RSGD is extremely limited as compared to the conventional estimation of total SGD and associated nutrient fluxes, especially in marginal-scale regions. Here, new high-resolution radium isotopes data in seawater and coastal groundwater enabled an estimation of SGD flux in a typical marginal sea of the Yellow Sea. By establishing 226Ra and 228Ra mass balance models, we obtained the SGD-derived radium fluxes, and then estimated the SFGD and RSGD fluxes through a two end-member model. The results showed that the total SGD flux into the Yellow Sea was equivalent to approximately 6.6 times the total freshwater discharge of surrounding rivers, and the SFGD flux accounted for only 5.2-8.8% of the total SGD. Considering the nutrient concentrations in coastal fresh and saline groundwater, we obtained the dissolved inorganic nutrient fluxes (mmol m-2 yr-1) and to be 52-353 for nitrogen (DIN), 0.21-1.4 for phosphorus (DIP), 34-226 for silicon (DSi) via SFGD, and 69-262 for DIN, 1.0-3.9 for DIP, 70-368 for DSi via RSGD, with the sum of nutrient fluxes equaling to (1.8-9.3)-, (1.3-5.6)- and (2.0-9.5)-fold of the riverine inputs. Compared to the conventional estimation of the total SGD flux, the nutrient fluxes derived from the separation of SFGD and RSGD were (1.6-2.1), (1.6-1.8) and (4.0-4.9) times lower for DIN, DIP and DSi, respectively, indicating that the estimates by separating SFGD and RSGD could be conservative and representative results of the Yellow Sea. Furthermore, we suggested that SGD played an important role in nutrient sources among all the traditional nutrient inputs sources, providing 15-48%, 33-68% and 14-43% of the total DIN, DIP and DSi input fluxes into the Yellow Sea, and the high nutrient stoichiometric ratios (i.e., DIN/DIP) in SGD probably contributed to the increasing ratios in the Yellow Sea. In addition delivering large amounts of nutrient into the Yellow Sea, SGD would create primary productivity of 10-49, 1.6-6.8 and 8.8-42 g C m-2 yr-1 based on N, P and Si, which were equivalent to 5.2-27%, 0.9-3.7% and 4.7-23% of the total primary productivity, respectively. In particular, the SFGD-derived DIN flux can be converted to primary productivity of 4.2-28 g C m-2 yr-1 thus demonstrating the disproportionately large role of SFGD in ecological environment of the Yellow Sea relative to its flux. Therefore, we conclude that SGD, particularly SFGD, serves an important role as a nutrient source for the Yellow Sea, and not only affects nutrient budgets and structures but also enhances the primary productivity.
The flux of water, nutrients, carbon and salt through the subsurface at the land-sea interface is an important control on coastal nutrient processes, salinization of coastal aquifers and carbon balances of the coastal zone. However, these fluxes are often spatially and temporally complex and difficult to quantify, especially in high-energy mesotidal systems. Here we use vertical temperature profiles along a morphologically complex mesotidal high-energy beachface to map and quantify water infiltration and exfiltration on the island of Spiekeroog, Germany. Water fluxes were quantified using heat transport calculations from three solutions to the 1D heat transport equation, and include 1) a steady state analytical solution, 2) a non-steady state numerical model and 3) a non-steady state analytical solution. The temperature profiles could clearly map areas of upwelling warm (up to 10 °C) groundwater during the winters of 2018 and 2019. These upwelling zones were focused on an intertidal runnel system and at the low water line, consistent with the current understanding of the site based visual observations and hydrogeological models. The steady state model provided good fits to the measured data in the winter when the seawater temperatures were not changing significantly, but was less able to reproduce the measured profiles in spring when seawater was warming. The steady state flux rates ranged from −110 to −43 mm d−1 in the runnel and low water line to +43 mm d−1 towards the high water line. The dynamic numerical model successfully captured the propagation of the seawater temperature signal into the subsurface and was able to reproduce the temperature profiles during both seasons. The flux estimates tended to be larger with the numerical model, with up to −150 mm d−1 in the runnel and +110 mm d−1 towards the high water line. The non-steady state analytical solution could only be applied to a limited time series due to the difficulty of logging temperatures in the subsurface at this highly dynamic site. Up to 1.5 days of data suggested fluxes that were considerably higher than the other two methods with best-estimates of −400 to −900 mm d−1. Thermal Peclet numbers ranged from 0.2 to 2 suggesting that both conduction and advection of heat is important. This study demonstrates that the morphology of the beach face is an important control on spatial distribution of down-welling and upwelling zones along the beach and that temperature measurement combined with heat modelling are potentially useful methods for understanding the interactions between groundwater and the sea.
We measured dissolved organic carbon (DOC), δ¹³C-DOC, fluorescent dissolved organic matter (FDOM), dissolved inorganic and organic nitrogen (DIN; DON), dissolved inorganic phosphate (DIP), and radium isotopes (²²³Ra, ²²⁴Ra, and ²²⁶Ra) in three different bays: Gwangyang Bay (GB), Suyoung Bay (SB), and Ulsan Bay (UB), Korea. The water residence times based on Ra isotopes were approximately 15 (±7) for GB and 1–3 days for SB and UB. UB and SB showed clear two (terrestrial and marine) endmember mixing trends for DOC, DIN, and DIP. In addition, significant “excess” DOC (10–50% of outer bay) was observed for SB under depleted nutrient conditions. GB, which had the longest water-residence time, showed depleted DIN and DIP, with large “excess” DOC (40–60% of the outer bay) and DON (10–40% of the outer bay) concentrations. The excess DOC observed in SB and GB was found to be marine in origin, although there was a slight influence of terrestrial DOC in the low-salinity waters of SB based on δ¹³C-DOC values. Terrestrial humic-like FDOM was conservatively mixed in the three bays. The net fluxes of these components, estimated using the water residence times, suggest that GB is a significant source of DOM (DOC, DON, and humic FDOM) but a sink of DIN and DIP. SB and UB are the sources of inorganic nutrients as well as DOM to the open ocean. Therefore, our study reveals that the residence time of coastal embayment play an important role in the biogeochemical production and alteration of nutrients and DOM.
Nutrient budgets in semi-arid estuaries, with ephemeral freshwater inflows and limited nutrient sources, are likely incomplete if contributions from submarine groundwater discharge (SGD) are not included. Here, the relative importance of saline/recirculated SGD-derived nutrient fluxes spatiotemporal variability to the overall nutrient budget is quantified for Nueces Bay, Texas, U.S.A., across hydroclimatic conditions ranging from drought to normal, to flood. On average, 67% of the variance in water quality is due to temporal differences while 16% is explained by spatial differences. Principal component analysis (PCA) reveals three principal components: freshwater inflow (PC1 28.8%), saline/recirculated SGD and recycled nitrogen (PC2 15.6%), and total SGD and “new” nitrogen (PC3 11.2%). Total SGD porewater fluxes ranged from 29.9-690.3 mmol∙m-2d-1 for ammonium, 0.21-18.7 mmol∙m-2d-1 for nitrite+nitrate, 3.1-51.3 mmol∙m-2d-1 for phosphate, 57.1-719.7 mmol∙m-2d-1 for silicate, and 95.9-36,838.5 mmol∙m-2d-1 for dissolved organic carbon. Total and saline/recirculated SGD fluxes were on average 150 - 26,000 and 5.8 - 466 times, respectively, greater than surface runoff fluxes across all seasons. Nitrogen (N) enrichment in porewater occurs near the agricultural fields because of soil N flushing and percolation to groundwater, which facilitates N-rich groundwater fluxes. There were substantial “new” N inputs from terrestrial groundwater following precipitation while saline/recirculated SGD of recycled N accounts for only <4 % of total SGD inputs. The “new” N inputs occur in the river and river mouth during flooding, and near the north shore where topography and hydraulic gradients are steeper during drought. Thus, while significant inputs of N may be associated with atmospheric deposition, or remineralization in the porewater, groundwater is the highest contributor to the nutrient budget in Nueces Bay. This result implies that nutrient management strategies should focus on land-use practices to reduce N contamination of shallow groundwater and subsequent contamination of estuaries.
Submarine groundwater discharge (SGD) is an important process driven by marine and terrestrial forces. Low tide affects SGD the most, therefore the ideal time to detect SGD is the low tide, especially during spring tide. Techniques to detect and quantify SGD along with the understanding of the related aquifer characteristics is discussed in this study. Scientific community across the world is realizing the importance of studying and mapping SGD because in the scenario of climate change, this part of the global hydrological cycle is an important process and is known to have a significant effect on the marine ecosystem due to nutrient and metal inputs around the region of discharge. Therefore, understanding the processes governing SGD becomes very important. In this review , various components and processes related to SGD (e.g. Submarine Groundwater Recharge, Deep Porewater Upwelling, Recirculated Saline Groundwater Discharge), along with detailed discussion on impacts of SGD for marine ecosystem is presented. Also, it highlights the future research direction and emphasis is put on more research to be done keeping in mind the changing climate and its impacts on SGD.
Submarine groundwater discharge (SGD) is a global and well-studied geological process by which groundwater of varying salinities enters coastal waters. SGD is known to transport bioactive solutes, including but not limited to nutrients (nitrogen, phosphorous, silica), gases (methane, carbon dioxide), and trace metals (iron, nickel, zinc). In addition, physical changes to the water column, such as changes in temperature and mixing can be caused by SGD. Therefore SGD influences both autotrophic and heterotrophic marine biota across all kingdoms of life. This paper synthesizes the current literature in which the impacts of SGD on marine biota were measured and observed by field, modeling, or laboratory studies. The review is grouped by organismal complexity: bacteria and phytoplankton, macrophytes (macroalgae and marine plants), animals, and ecosystem studies. Directions for future research about the impacts of SGD on marine life, including increasing the number of ecosystem assessment studies and including biological parameters in SGD flux studies, are also discussed.
Dissolved organic carbon (DOC) is the largest pool of reduced carbon in the oceans, with a reservoir equivalent to atmospheric CO2. In nearshore marine regions, DOC sources include primary production, terrestrial DOC delivered by river discharge, and/or terrestrial and marine DOC delivered via submarine groundwater discharge (SGD). While the importance of SGD to coastal carbon cycling has been implicated, the actual influence of this process on nearshore carbon dynamics and offshore export has not been explicitly identified. This study, conducted at a predominantly marine-influenced intertidal beach-nearshore ocean system along the Santa Barbara, California coastline, aimed to address this knowledge gap. Dark, temperature-controlled laboratory incubations, radioisotopic (radon-222) SGD estimates, and an SGD-DOC mixing-reaction box model were coupled to identify the influence of pore water mixing with seawater on nearshore DOC reactivity, concentration dynamics, and offshore export. Even with a relatively low volumetric contribution, SGD pore water mixing altered nearshore DOC reactivity, and elevated the nearshore DOC concentration by 0.4–6.6 µmol L⁻¹ over nearshore seawater residence times spanning 1–6 days. These elevated DOC concentrations were equivalent to 0.5% to 9% of the mean offshore DOC concentration for the summer months in the Santa Barbara Channel, when the coastal water column is highly thermally stratified. Despite the challenge of assessing carbon dynamics in physically and biogeochemically complex nearshore marine regions, this study demonstrates the need for future investigations to assess and account for SGD as a non-trivial component of coastal marine carbon cycles.
Freshwater submarine groundwater discharge (SGD) frequently occurs in areas of karst coastline where karst aquifers are hydraulically connected to the sea. Though up to 25% of the world's coastline is karst, little work has been done to determine the ecological impacts of karst-channelled freshwater SGD. Reported herein are the effects of karst-channelled freshwater SGD on the rock pool sessile assemblage in the locale of Olhos de Agua on the south Portuguese coast. Two quadrat surveys were conducted in eight intertidal rock pools, which ranged in freshwater SGD content from 0 to 57.3%. Salinity and nitrogen (NO3⁻ and NO2⁻) concentrations were measured in the eight surveyed rock pools, as well as a further three rock pools on the same plateau, three inland groundwater boreholes and the adjacent coastal marine environment. There was a strong negative correlation between salinity (used as a proxy for freshwater SGD) and nitrogen concentration (r² = 0.98; p < 0.01), with rock pool nitrogen concentrations ranging from 15.3 μM at salinity 36 (0% freshwater SGD) to ∼515 μM at salinity 4.27 (88% freshwater SGD). The concentration of nitrogen in the inland boreholes was elevated (∼258–∼450 μM), though lower than that of the rock pool with the highest freshwater SGD content. Freshwater SGD was associated with altered percentage cover of sessile organisms (p = 0.03), decreased organism cover (n.s.), and decreased species number (n.s). There was a negative correlation between freshwater SGD and the percentage cover of the coralline red algae Ellisolandia elongata (r² = 0.811; t = 4.88; df = 8; p < 0.01). The most abundant algae, Ulva spp., was found in all rock pools except the rock pool with the highest fraction of SGD. These results provide evidence for a direct influence of karst-channelled freshwater SGD over the composition of the rock pool sessile assemblage. These results suggest a greater role of salinity than nitrogen additions in conditioning the form of the rock pool sessile assemblage.
In order to understand the circulation of nutrient between muddy tidal flat and the surrounding coastal area, tidal time-scale variations in nutrient concentrations were seasonally investigated at the entrance of Hampyeong Bay. The results show that the temperature was higher in ebb tide and lower in flood tide during the summer, but it was lower in ebb tide and higher in flood tide during the autumn/winter. The salinity was higher in flood tide and lower in ebb tide during the summer/winter because of the inflow of external sea water resulting from the increase in the tide level. By contrast, the salinity was lower in flood tide and higher in ebb tide during the autumn. Salinity difference was lower than 0.3 psu between flood tide and ebb tide during survey period. Meanwhile, all nutrient concentrations observed in Hampyeong Bay was lower in flood tide and higher in ebb tide during the summer, and by contrast, it was higher in flood tide and lower in ebb tide during the winter. Characteristically, no clear variation of concentrations was found depending on the tide level during the autumn. This tidal variations imply that the muddy tidal flat of Hampyeong Bay supplies nutrients to the seawater in summer and removes nutrient from the seawater in winter. During tidal cycle, seasonal variation of nutrient concentration in seawater is considered as the result of complex interactions between the mud flat and external sea water.
We have previously shown that primary productivity in San Francisco Bay, USA, is highly correlated with phytoplankton biomass B (chlorophyll a concentration) and an index of light availabil - ity in the photic zone, Z,I, (photic depth times surface irradiance). To test the generality of this relation, we compiled data from San Francisco Bay and 5 other USA estuarine systems (Neuse and South Rivers, Puget Sound, Delaware Bay and Hudson hver Plume), and regressed daily productivity (P (mg C m-2 d-') against the composite parameter B Z, I,. Regressions for each estuary were signifi- cant and typically over 80 % of the variation in 5 P was correlated with variations in B Z, I,. Moreover, the pooled data (n - 211) from 4 estuaries where methodologies were comparable fell along one regression line (r 2 = 0.82), indicating that primary productivity can be estimated in a diversity of estuarine waters from simple measures of phytoplankton biomass and light availability. This implies that physiological variability (e, g. responses to variations in nutrient availability, temperature, salinity, photoperiod) is a secondary control on phytoplankton production in nutrient-rich estuaries, and that one empirical function can be used to estimate seasonal variations in productivity or to map productivity along estuarine gradients of phytoplankton biomass and turbidity.
To investigate the role of the seepage zone in transport, chemical speciation, and attenuation of nitrogen loads carried by submarine groundwater discharge, we collected nearshore groundwater samples (n 5 328) and examined the distribution and isotopic signature (d15N) of nitrate and ammonium. In addition, we estimated nutrient fluxes from terrestrial and marine groundwater sources. We discuss our results in the context of three aquifer zones: a fresh groundwater zone, a shallow salinity transition zone (STZ), and a deep STZ. Groundwater plumes containing nitrate and ammonium occurred in the freshwater zone, whereas the deep STZ carried almost exclusively ammonium. The distributions of redox-cycled elements were consistent with theoretical thermody- namic stability of chemical species, with sharp interfaces between water masses of distinct oxidation : reduction potential, suggesting that microbial transformations of nitrogen were rapid relative to dispersive mixing. In limited locations in which overlap occurs between distribution of nitrate with that of ammonium and dissolved Fe2+, changes in concentration and in d15N suggest loss of all species. Concurrent removal of NO { 3 and NH z 4 , both in freshwater and the deep STZ, might occur through a range of mechanisms, including heterotrophic or autotrophic denitrification, coupled nitrfication : denitrification, anammox, or Mn oxidation of NH z 4 . Loss of nitrogen was not apparent in the shallow STZ, perhaps because of short water residence time. Despite organic C- poor conditions, the nearshore aquifer and subterranean estuary are biogeochemically active zones, where attenuation of N loads can occur. Extent of attenuation is controlled by the degree of mixing of biogeochemically dissimilar water masses, highlighting the critical role of hydrogeology in N biogeochemistry. Mixing is related in part to thinning of the freshwater lens before discharge and to dispersion at the fresh : saline groundwater interface, features common to all submarine groundwater discharge zones.
The influence of fortnightly spring-neap tidal variability on submarine discharge of fresh and saline groundwater was examined at Stinson Beach, California. Stinson Beach is a residential community that utilizes on-site systems for wastewater disposal. Fresh, shallow groundwater at the site contains high concentrations of nutrients (dissolved inorganic nitrogen (DIN), soluble reactive phosphate (SRP), and silicate) and human fecal bacteria. A groundwater-derived freshening and nutrification of the surf zone during neap tides was observed, followed by a 4-d increase in chlorophyll a concentrations. Analytical models and a freshwater budget in the surf zone were used to estimate the saline and fresh discharge of submarine groundwater. We estimate fresh groundwater discharge between 1.2 and 4.7 L min21 m21 shoreline during neap tides compared with 0.1 and 0.5 L min21 m21 during spring tides. This compares with 15.9 and 22.0 L min21 m21 saline groundwater discharge (forced by waves and tides) during neap and spring tides, respectively. Despite the smaller total (fresh + saline) flux of groundwater during neap compared with spring tides, the larger fresh discharge component during neap tides raises surf zone silicate, DIN, and SRP by 14% ,3 5 %, and 27%, respectively, relative to spring tides. This observed fortnightly pulsing of fresh groundwater-derived nutrients was consistent with seaward hydraulic gradients across the fresh part of the beach aquifer, which varied due to aquifer overheight near the beach face. Darcy-Dupuit estimates of seaward fresh groundwater flow in this area agreed well with the fresh discharge results of the mass balance.
A physical model based on determining the fraction of the tidal prism that returns to the estuary on the next high tide is used to estimate the flushing time of the Okatee River estuary. The return flow factor (b) of 0.81 yields a flushing time of 2 days. A mass balance model of 228Ra and salinity is also used to estimate b. This model yields an average b = 0.79, virtually the same as the physical model. A third model based on the decay of 224Ra relative to 228Ra is used to determine the apparent age of water in the estuary. These ages range from 1.6 to 5 days, with an average of 3.4 days. These three independent estimates are in remarkably close agreement, certainly within the error of each estimate. We use these residence times to develop a mass balance model for the radium isotopes in the Okatee estuary. We consider decay, mixing, sedimentary input, river input, and submarine groundwater discharge (SGD). The major loss term for each isotope is mixing with water in Port Royal Sound; the major input for each isotope is SGD. At steady state these terms must balance. Knowing the water age and the radium isotope composition of groundwater entering the Okatee allows us to estimate an average SGD flux of 1 m3/s. The SGD flux is a factor of 3–4 greater during the summer relative to the winter. This SGD supplies a considerable quantity of nutrients and carbon into the Okatee system.
Blooms of Aureococcus anophagefferens, the alga responsible for brown tide in Long Island waters, have been hypothesized to occur during years in which groundwater discharge is low. The precise mechanism by which blooms are initiated, however, remains unknown. To better understand the influence of groundwater inputs on brown tide, a 2 yr sampling campaign was established at 'bloomprone' embayment, West Neck Bay (WNB), Long Island, New York. During 1997 and 1998, changes in water-column chemistry and phytoplankton dynamics were observed, along with groundwater composition and flow rates. Groundwater entering WNB was enriched in nitrate (> 250 muM), During 1997 and 1998, elevated levels of dissolved nitrate (1 to 25 muM) were measured in the water column of WNB during the annual peak in groundwater flow, Peak nitrate levels were followed by mixed-assemblage phytoplankton blooms that were succeeded by monospecific brown tide with densities >5 x 10(-5) cells ml(-1), Interannual differences in groundwater seepage were reflected in the magnitude of water-column nitrate concentrations and phytoplankton biomass. Fifty percent more groundwater recharge in spring of 1998 compared to spring of 1997 resulted in levels of nitrate and chlorophyll in 1998 (25 muM and 25 pg 1(-1), respectively) exceeding those of 1997 (1 muM and 15 mug 1(-1)). Phytoplankton blooms preceding brown tide may supply A. anophagefferens with organic nutrients, as annual bloom densities seemed dependent on the magnitude of dissolved organic nitrogen inputs prior to brown tide events. A multivariate regression model is presented which accounts for 72% of the variability in brown tide densities during the 2 yr study period at WNB. A highly significant correlation between groundwater seepage and A. anophagefferens densities in the model suggests that rather than repressing brown tide, groundwater inputs to WNB can stimulate A. anophagefferens growth by initiating phytoplankton blooms prior to the brown tide which supply remineralized organic nitrogen.
Acrylic fibers impregnated with manganese oxide-dioxide extract radium
and other trace elements from natural waters. This extraction technique
may be used to quantitatively extract radium from 20-liter water samples
for precise 226Ra analysis and to concentrate radium from
several-thousand-liter samples for 228Ra/226Ra
activity ratio determinations.
A model is developed for tidal flushing of a small, well-mixed embayment, based on the classic tidal prism flushing formation. Both a flushing expression and a steady-state concentration expression for a nonconservative tracer are derived. The return flow factor is identified as an important, but previously poorly constrained, input to the flushing model. A model for estimating the return flow factor is derived considering the fate of the plume of effluent water, which exits the embayment mouth on ebb tide, mixes with coastal waters, and partially returns to the embayment on flood tide. The predicted return flow factor depends primarily on the relative phases and speeds of the embayment channel currents and coastal currents and on the amount of mixing that occurs outside the embayment. The return flow factor model is solved for a range of representative cases, and the results are plotted for reference. The complete flushing model is compared to the results of a dye study performed in a marina basin in Indian River Bay, Del., with favorable results.
The mass balance and activity ratios of naturally occurring radium (Ra) isotopes (223Ra, 224Ra, 226Ra, 228Ra) were investigated in Cockburn Sound (Western Australia) to further understand submarine groundwater discharge (SGD) into these coastal waters. Water samples from 11 marine stations and 20 groundwater sites (encompassing three aquifer layers) were analysed for Ra at four times, at the end of winter (September 2003), early summer (December 2003), late summer (March 2004) and mid-winter (July 2004). Variable isotopic signatures of groundwater suggested that vertical mixing may occur between different aquifer layers and two isotopically different water sources were identified in marine waters both inside and outside of Cockburn Sound. A mass balance of the long lived radium isotopes (228Ra and 226Ra) produced a range of discharge estimates from 0.8 × 107 L day−1 in late summer to 2.7 × 107 L day−1 at the end of winter. Signatures of radium isotopes in the coastal waters suggested that groundwater discharge was not confined to the shoreline and may have occurred from a number of aquifer sources at a temporally variable scale.
The Yellow Sea is a shelf sea surrounded by the Korean peninsula and the eastern coast of China. The bordering countries derive a substantial share of their food from fishing in these coastal waters. Synoptic maps of water-column integrated primary production in May and September were derived using a primary production algorithm applied to ocean color satellite data from the Yellow Sea from 1998 to 2003. The middle of the Yellow Sea (MYS) had higher levels of primary production in May and September than the shallower (<50 m) areas off the coasts of Korea and China. Although the coastal areas had high phytoplankton biomass, lower levels of primary production were caused by high turbidity arising from strong tides and shallow depths. Lower turbidity in the central part of the Yellow Sea allows light necessary for primary production to penetrate deeper into the water column. The mean daily integrated primary production in the MYS was 947 mg C m(-2) d(-1) in May and 723 mg C m(-2) d(-1) in September. The mean values in Chinese and Korean coastal waters were 590 and 589 mg C m(-2) d(-1) in May, and 734 and 553 Mg C m(-2) d(-1) in September, respectively. Our computation of daily total primary production for the entire Yellow Sea was 19.7 x 10(4) t C d(-1) in May, and 15.8 X 10(4) t C d(-1) in September.
1] There has emerged a recognition that the submarine discharge of fresh, brackish, and marine groundwaters into the coastal ocean is comparable to the inputs via river discharge. However, the factors controlling submarine groundwater discharge (SGD) are poorly understood owing to a lack of SGD measurement tools in dynamic coastal waters. We observed bi-hourly variations of the natural SGD tracers, 222 Rn and CH 4 , at a coastal seawater station over two seasons using novel monitoring techniques. This unique data set suggests that SGD increases sharply from neap to spring tide during the wet season. The observed increase is much greater than what would be expected from the rainfall and tidal-height changes. We conclude from this that the temporal variation of SGD is regulated predominantly by a semi-monthly fluctuation of a tidal oscillating pumping force in this environment.
1] Naturally occurring chemical tracers were used to assess the magnitude of submarine groundwater discharge (SGD) during two different sampling periods at a coastal site south of the Yellow River delta, China. We used salinity and pH as indicators of the terrestrial and recirculated seawater components of discharging groundwater and radium isotopes to quantify offshore transport rates. We then used an hourly time series of multiple radium isotopes (224 Ra, 223 Ra, and 226 Ra) to quantify SGD rates and also used 222 Rn and seepage meters to independently quantify SGD rates as a comparison to the radium results. Offshore transport rates were found to range from 3.3 to 4.7 cm s À1 . Modeled time series radium activities indicated average SGD rates ranging from 4.5 to 13.9 cm d À1 in September 2006 and from 5.2 to 11.8 cm d À1 in July 2007. Temporal trends associated with the radium approach agree with SGD patterns revealed by automated seepage meters deployed nearby, but the absolute fluxes are about 70% lower than those determined by the seepage meters. Modeled SGD rates based on 222 Rn (mean = 13.8 cm d À1 in 2006 and 8.4 cm d À1 in 2007) agree with those determined by the radium analysis. Differences in derived SGD rates between the different radium isotopes (226 Ra highest; 224 Ra lowest) are likely results of uncertainties in the background activities and our limited selection of appropriate groundwater/pore water end-member values. Scaling our results to the entire Yellow River delta, we find SGD fluxes (and corresponding nitrate fluxes) 2–3 times that of the Yellow River.
In three intertidal sand bottom communities of the Knigshafen (Island of Sylt, North Sea), the biomass production and respiration of phytobenthos, phytoplankton, macrozoobenthos, and in situ community metabolism were measured monthly during 1980. The study sites were characterized by different communities (Nereis-Corophium-belt, seagrass-bed,Arenicola-flat) and by a high abundance of the molluscHydrobia ulvae. Benthic diatoms are the major constituents of plant biomass in theArenicola-flat. In this community, gross primary productivity amounts to 148 g C m–2 a–1. 82 % of this productivity is caused by microbenthos, whereas phytoplankton constitutes only 18 %. In the seagrass-bed, gross primary productivity amounts to 473 g C m–2 a–1. 79 % of this is generated by seagrass and its epiphytes, whereas microphytobenthos contributes 19 %. In theNereis-Corophium-belt, only microphytobenthos is important for biomass and primary productivity (gross: 152 g C m–2 a–1). Annual production of macrofauna proved to be similar in theArenicola-flat (30 g C m–2 a–1) to that in the seagrass-bed (29 g C m–2 a–1). Only one third of this amount is produced in theNereis-Corophium-belt (10 g C m–2 a–1). The main part of secondary production and animal respiration is contributed by grazingH. ulvae. In the seagrass-bed, 83 % of the energy used for production is obtained from the grazing food chain. In theArenicola-flat and theNereis-Corophium-belt, the importance of non-grazing species is greater. A synchrony of seasonal development of plant biomass and monthly secondary production was observed. In theArenicola-flat and the seagrass-bed, where density and production of macrofauna are high, a conspicuous decrease in biomass of microbenthos occurs during the warmer season, whereas in theNereis-Corophium-belt primary production causes an increase in microphytobenthic biomass in summer and autumn. Energy flow through the macrofauna amounts to 69 g C m–2 a–1 in theArenicola-flat, 85 g C m–2 a–1 in the seagrass-bed and 35 g C m–2 a–1 in theNereis-Corophium-belt. Based on the assumption that sources of food are used in proportion to their availability, 49 g C m–2 a–1 (Arenicola-flat), 72 g C m–2 a–1 (seagrass-bed) and 26 g C m–2 a–1 (Nereis-Corophium-belt) are estimated as taken up by the grazing food chain. All three subsystems are able to support the energy requirements from their own primary production and are not dependent on energy import from adjacent ecosystems.
Submarine Groundwater Discharge (SGD) has been frequently ignored as a nutrient source to marine ecosystems because it is
difficult to identify and quantify. However, recent studies show its ubiquity and ecological importance to the coastal zone,
particularly when associated with contaminated continental aquifers. The Ria Formosa is a coastal lagoon located in the south
of Portugal and surrounded by an intensely farmed area. Following a 12-month field study using seepage meters, we identified
groundwater discharge in the intertidal zone of the lagoon. The seeping fluid was a mixture of two water types: one with low
salinity and high nitrate concentration and another similar to local seawater. Based on the integration of monthly seepage
rate measurements throughout the year, we estimate the mean discharge of submarine groundwater into the lagoon to be 3.6m3day−1 per linear meter of coastline with freshwater contributions (per volume) ranging from 10% to 50%. The results of this study
suggest a continental origin for the freshwater component, thus linking the biogeochemical cycles in the lagoon to anthropogenic
activities taking place in the neighboring coastal plain. We further identify SGD as an important nutrient source to the Ria
Formosa, estimating annual loads of 36.2mol (0.507kg) of Nitrogen, 1.1mol (0.034kg) of Phosphorus and 18.6mol (0.522kg)
of Silicon per meter of coastline. Based on these results, we suggest that SGD is a potential contributor to the observed
nutrification status of the Ria Formosa lagoon.
This paper reviews previous work on groundwater behaviour in sandy beaches. We consider the variations in beach watertables and moisture conditions above the watertable in relation to tidal and wave frequencies, and the relationship between beach groundwater, swash/backwash hydrodynamics and sediment transport. We note that most work on beach groundwater processes has tended to be empirical and argue that if understanding of beach groundwater/swash zone sediment transport interactions is to be improved, better measurement and physical representation of the relevant processes are needed, together with the formulation of physically- based models. We identify the form that future field monitoring programmes and models of beach groundwater processes could take.
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.
Improvements in the analyses of 226Ra and 228Ra in seawater made possible by better extraction and processing techniques reduce significantly the errors associated with these measurements. These improvements and the extensive sampling for Ra isotopes conducted on the TTO North Atlantic Study should enable us to use the distribution of 228Ra to study mixing processes on a 3-15 year time scale in both the upper and deep North Atlantic. The 228Ra profiles already analyzed show a closer resemblance to GEOSECS tritium data than to TTO tritium data in the upper ocean. This is because the transient tracer tritium was responding on a 10-year time scale during GEOSECS and a 20-year time scale during TTO. The steady state tracer 228Ra should always respond on a time scale of 8 years. Thus the 228Ra data obtained on TTO should provide a means to extend the features of the GEOSECS tritium field to the regions of the TTO study. The 226Ra data are of high enough quality to identify features associated with different water masses. Changes in the positions of the deep-water masses since the GEOSECS cruise are revealed by the 226Radata.
We estimated the submarine discharge of groundwater (SGD) and associated nutrients into the semienclosed Bangdu Bay on a volcanic island, Jeju, Korea, by analyzing 222 Rn, Ra isotopes ( 224 Ra and 226 Ra), and nutrients in seawater, pore water, and coastal groundwater. The submarine inputs of groundwater into Bangdu Bay of 120-180 m 3 m 22 yr 21 (on the basis of 222 Rn, 224 Ra, 226 Ra, and Si mass balances) were much higher than those reported from typical continental margins. The nutrient fluxes from SGD were about 90%, 20%, and 80% of the total input (except from open ocean waters) for dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP), and dis- solved inorganic silicate (DSi), respectively. These excess nutrient inputs from SGD are the major sources of ''new nutrients'' in this bay. On the basis of photosynthetic pigments and benthic algal distributions, we suggest that the large fluxes of excess nutrients from SGD can cause benthic eutrophication in a semienclosed bay on this highly permeable volcanic island.
Total groundwater discharge from a sandy seepage site to the Delaware Bay at Cape Henlopen, Delaware, was determined together with the associated nutrient loads by trapping the discharge from a length of the shoreline in a tidal pond at low tide and measuring the discharge of the trapped water through a weir at steady state. Salinity was used to parse the total groundwater discharge from the beachface at low tide into a "recycled" estuarine component and a "new" fresh groundwater component. Based on 16 measurements over 18 months, average total discharge and average freshwater discharge at low tide were found to be 2.76 ± 1.08 and 0.87 ± 0.43 L min-1 m-1 of shoreline, respectively. The estuarine component of discharge varied with the maximum height of the immediately preceding high tide. Owing to the control of freshwater discharge by the average upland hydraulic gradient away from the beach, individual measurements of freshwater discharge at low tide represent a good estimate of this discharge component over time scales of hours to days. Based on the nutrient concentrations in the discharge waters, nutrient loads from the beach to the adjacent estuary at low tide were 148 ± 79, 7.5 ± 5.1, and 197 ± 101 μmol min-1 m-1 of shoreline for total dissolved N, P, and Si, respectively. Groundwater discharge and these nutrient fluxes contribute significantly to the unusually productive benthic communities that inhabit the seepage and nearby intertidal zones. © 2007, by the American Society of Limnology and Oceanography, Inc.
224Ra (half-life, t1/2=3.64 days) has been used widely as a tracer. Its activity in natural waters can be measured by the 220Rn emanation method [Rama, Todd, J.F., Butts J.L., and Moore, W.S., 1987. A new method for the rapid measurements of 224Ra in natural waters. Mar. Chem., 22, 43–54.]. The measurement efficiency of 220Rn is affected by Mn-fiber surface conditions and water content. We studied the effects of precipitated NaCl and water content on the emanation characteristics of 220Rn from the 224Ra adsorbed on Mn-coated fiber. The best emanation efficiency can be achieved when the Mn-fiber is covered by a water film with a thickness comparable to the recoil range of 220Rn in water. By controlling the water content of the Mn-fiber, the measurement efficiency may be increased 2–3× compared to the very wet or very dry condition.
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.
Multi-annual sedimentological observations on tidal-flat sediments were carried out in Hampyong Bay, southeastern Yellow Sea, to infer the budget of modern muddy sediments in the bay. Sedimentation rates over a four-year period show contrasting types of seasonal sedimentary cycles occurring in the tidal flats within the elongated bay. Both sides of the bay are largely sheltered from winter waves, resulting in surface mud deposition during winter and erosion during summer. In contrast, tidal flats along the head of the bay are influenced by winter waves, resulting in a reversed trend where erosion occurs during winter and deposition during summer. Tidal flats near the bay-mouth, however, show a sedimentary cycle disrupted by the construction of sea walls undergoing consistent erosion throughout the observational period. The shoreline artificially straightened seems to cause tidal currents to bypass the tidal flat and hence to be much stronger. These differences in sedimentary cycles suggest the critical importance of the orientation of tidal flats relative to the propagating direction (from N–NW) of the monsoon winter-storm waves on the tidal-flat sedimentation in the eastern coastal area of the Yellow Sea. The preliminary budget estimation for the tidal-flat mud suggests that the tidal flats in Hampyong Bay are subject to the slight but consistent erosion as a whole.
A seasonal study of radium-derived submarine groundwater discharge (SGD) and associated nitrogen fluxes was carried out in a salt marsh estuary between 2001 and 2003 (Pamet River Estuary, Massachusetts). Twelve- hour time series of salinity and radium at the estuary inlet were used to determine the relative importance of fresh versus saline SGD, respectively. The distinct radium (228Ra: 226Ra) isotopic signature of marsh peat pore water and aquifer-derived brackish groundwater was used to further partition the Ra-derived SGD estimate. Of these three groundwater sources, only the marsh-derived groundwater was constant across time. The ratio of brackish to fresh SGD was inversely correlated with water table elevation in the aquifer, suggesting that Ra-derived SGD was enhanced during dry periods. The various SGD fluxes were responsible for an average annual dissolved inorganic nitrogen (DIN) input of between 1.7 mol m22 yr21 and 7.1 mol m22 yr21 and a soluble reactive phosphate (SRP) flux of 0.13-0.54 mol m22 yr21. Approximately 30% of the SGD-derived DIN and SRP flux is exported to coastal waters (Cape Cod Bay), whereas 70% is retained by the salt marsh ecosystem.
Data on SGWD from sites in the Florida Keys and on the southeastern continental shelf of the US indicate that water movement across sediment/water interfaces is a common occcurrence at least to water depths of 30 to 35m. Discharge values from the Florida Keys were 8.9l m-2d-1 for depths <27m and 5.4 l m-2d-1 for depths of c27 to 39m. On the southeastern continental shelf, discharge ranged between c6 and 20 l m-2d-1. One site was found in 20m depth where there was a persistent negative hydraulic head and mean influx of seawater to the sediments (c10.8 l m-2d-1). Even though geohydrological models would predict coupling of SGWD with landbased hydraulic heads, definitive lower salinity SGWD could not be detected. The driving force seems to be subtidal pumping and much of the discharge measured was probably recycled seawater. SGWD serves to move dissolved solutes into the water column, and could be an important link in benthic-pelagic coupling in continental shelf ecosystems. -from Author
We describe a nuclear detector system for measuring low activities of 223Ra and 224Ra in natural waters based on an original design of Giffin et al. (1963). Samples are obtained by adsorbing 223Ra and 224Ra onto a column of MnO2 coated fiber (Mn fiber). The short-lived Rn daughters of 223Ra and 224Ra which recoil from the Mn fiber are swept into a scintillation detector where alpha decays of Rn and Po occur. Signals from the detector are sent to a delayed coincidence circuit which discriminates decays of the 224Ra daughters, 220Rn and 216Po, from decays of the 223Ra daughters, 219Rn and 215Po. The system is calibrated using 232Th and 227Ac standards with daughters in equilibrium adsorbed on Mn fiber. Results of samples from Tampa Bay, Florida, and the Atchafalaya and Mississippi Rivers mixing zones are reported. The method is extendible to measurements of 227Ac, 231Pa, 228Th, and 228Ra.
Measurements of the naturally occurring radioisotopes 223Ra (t1/2 = 11.4 days) and 224Ra (t1/2 = 3.66 days) in southern Rhode Island salt ponds were combined with a simple model to obtain independent estimates of the age of these coastal waters. Surface water and porewater samples were collected quarterly in Winnapaug, Quonochontaug, Ninigret, Green Hill, and Pt. Judith-Potter Ponds, as well as nearly monthly in the surface water of Rhode Island Sound, beginning January 2002 through August 2003. Surface water activities ranged from 1–78 dpm 100 L− 1 and 5–885 dpm 100 L− 1 for 223Ra and 224Ra, respectively. Porewater radium activities ranged from 3 to 715 dpm 100 L− 1 for 223Ra, and 57–4926 dpm 100 L− 1 for 224Ra. Results indicate seasonally varying water mass ages for Ninigret (5–12 days), Winnapaug (2–6 days) and Pt. Judith-Potter Ponds (1–9 days) and, in contrast, relatively constant ages for Green Hill (5–7 days) and Quonochontaug Ponds (3–6 days).
We compared near-bottom seston and surface sediment composition in Cape Cod estuaries receiving different N loads to determine whether eutrophic-driven changes in seston and sediment composition occur in tandem and what implications such coupling has for the quantity and quality of particles available as food for benthic consumers. δ15N signatures in seston and sediment increased with increasing N loads to estuaries and linked particles in nearshore seston and sediment to land-derived wastewater sources. δ13C signatures in seston and sediment reflected C inputs primarily from microalgae. Sediments, however, were consistently lighter in δ15N and heavier in δ13C compared with seston among our estuaries and in other estuaries worldwide, which suggests a seston-sediment biogeochemical coupling that may be independent of estuary-specific differences. In Cape Cod estuaries, N enrichment increased microalgal production in seston and sediment in nearshore areas, and higher N loads decreased C:N in sediments, but not in seston. The biogeochemical coupling reflected in isotopic signatures in seston and sed ment persisted despite these changes associated with N enrichment and differences in grazer abundance, salinity, and flushing times across estuaries. Differences in isotopic signatures in benthic algae compared with phytoplankton were consistent with isotopic differences in sediment compared with seston, and microalgal production was the only aspect of composition that responded similarly to N loading in both seston and sediment across estuaries. The consistent coupling between seston and sediment composition, therefore, was likely related to differences in microalgal composition and, in turn, the type and quantity of particles available as food to consumers at the sediment-water interface.
The variability of marine diatom Si:C and Si:N composition ratios was examined to assess their utility as ecological conversion factors. Twenty‐seven diatom species grown under an 18:6 h LD cycle and sampled at the end of the light period gave mean ratios, by atoms, of 0.13 ± 0.04 and 1.12 ± 0.33 for Si:C and Si:N ratios, respectively (95% C.I. reported). The mean ratios for 18 species grown under continuous illumination were 0.12 ± 0.03 for Si:C and 0.95 ± 0.23 for Si:N. The mean ratios of the clones grown under constant light were not statistically different from those calculated for the same species grown under an 18:6 h LD photoperiod. The overall mean Si:C and Si:N ratios for the 18:6 h LD and continuous light experiments taken together, weighted by the number of species in each experiment, are 0.13 and 1.05, respectively. The average ratios for the nine nanoplankton species (<20 μm) examined were 0.09 ± 0.03 for Si:C and 0.80 ± 0.35 for Si:N. The eighteen netplankton species (>20 μm) had higher mean ratios, Si:C = 0.15 ± 0.04 and Si:N = 1.20 ± 0.37.
Time course sampling throughout a 24 h period revealed twofold variations in both ratios for individual species grown on a 14:10 h LD cycle. Changes in irradiance can also produce factor of two variations, both ratios being higher under low light. Comparisons of these data with those from the literature regarding the effects of temperature and nutrient limitation on diatom elemental composition suggest that use of these ratios to convert field estimates of biogenic silica into nitrogen or carbon units, or to estimate silica production from ¹⁴ C data, should yield results accurate to within a factor of three under most circumstances.
We examined the importance of nitrogen inputs from groundwater and runoff in a small coastal marine cove on Cape Cod, MA, USA. We evaluated groundwater inputs by three different methods: a water budget, assuming discharge equals recharge; direct measurements of discharge using bell jars; and a budget of water and salt at the mouth of the Cove over several tidal cycles. The lowest estimates were obtained by using a water budget and the highest estimates were obtained using a budget of water and salt at the Cove mouth. Overall there was more than a five fold difference in the freshwater inputs calculated by using these methods.
Nitrogen in groundwater appears to be largely derived from on site septic systems. Average nitrate concentrations were highest in the region where building density was greatest. Nitrate in groundwater appeared to behave conservatively in sandy sediments where groundwater flow rates were high (> 11/m2/h), indicating that denitrification was not substantially reducing external nitrogen loading to the Cove.
Nitrogen inputs from groundwater were approximately 300 mmol-N/m3/y of Cove water. Road runoff contributed an additional 60 mmol/m3/y. Total nitrogen inputs from groundwater and road runoff to this cove were similar in magnitude to river dominated estuaries in urbanized areas in the United States.
Tomales Bay, a graben structure along the San Andreas Fault, was selected for modeling ecosystem nutrient dynamics because
of its linear, one-dimensional morphology and relatively pristine state. Groundwater is a potentially important term in the
nutrient budget. The geologic complexities created by the San Anreas Fault, however, complicate the hydrogeology and require
the area to be subdivided into three regions: granite to the west, Franciscan Formation to the east, and alluvial fill in
the trough. Nutrient concentrations in the groundwater were determined through extensive well sampling; groundwater discharge
was estimated using both Darcy's Law calculations and a soil moisture budget. Results indicate that groundwater discharge
is of the same order of magnitude as summer streamflow into the Bay, while being significantly less than other freshwater
inputs in winter. Dissolved nutrient (phosphate, nitrate + nitrite, ammonium, silica and DIC) concentrations in groundwater
were consistently higher (by as much as an order of magnitude) than in surface water discharges. During the summer months,
groundwater flow contributes about as much nutrient load to the bay as does streamflow. During the winter, the groundwater
contribution to nutrient loading is about 20% of the streamflow contribution. Our findings indicate that groundwater is a
significant component of terrestrial nutrient and freshwater loading to Tomales Bay, particularly so during the summer months.
However, neither groundwater nor streamflow nutrient fluxes are large in comparison to the mixing flux at the bay mouth or
the flux of N2 gas across the air-water interface.
This paper reports the results derived fromradium isotopes of a submarine groundwaterdischarge (SGD) intercomparison in thenortheast Gulf of Mexico. Radium isotopesamples were collected from seepage meters,piezometers, and surface and deep ocean waters.Samples collected within the near-shore SGDexperimental area were highly enriched in allfour radium isotopes; offshore samples wereselectively enriched. Samples collected fromseepage meters were about a factor of 2–3higher in radium activity compared to theoverlying waters. Samples from piezometers,which sampled 1–4 meters below the sea bed were1–2 orders of magnitude higher in radiumisotopes than surface waters. The twolong-lived Ra isotopes, 228Ra and226Ra, provide convincing evidence thatthere are two sources of SGD to the study area:shallow seepage from the surficial aquifer andinput from a deeper aquifer. A three end-membermixing model can describe the Ra distributionin these samples.The short-lived radium isotopes, 223Ra and224Ra, were used to establish mixing ratesfor the near-shore study area. Mixing wasretarded within 3 km of shore due to a strongsalinity gradient. The product of the mixingrate and the offshore 226Ra gradientestablished the 226Ra flux. This flux mustbe balanced by Ra input from SGD. The flux ofSGD within 200 m of shore based on the226Ra budget was 1.5 m3 min–1.This flux agreed well with other estimatesbased on seepage meters and 222Rn.
Anthropogenic activities on coastal watersheds increase nutrient concentrations of groundwater. As groundwater travels downslope
it transports these nutrients toward the adjoining coastal water. The resulting nutrient loading rates can be significant
because nutrient concentrations in coastal groundwaters may be several orders of magnitude greater than those of receiving
coastal waters. Groundwater-borne nutrients are most subject to active biogeochemical transformations as they course through
the upper 1 m or so of bottom sediments. There conditions favor anaerobic processes such as denitrification, as well as other
mechanisms that either sequester or release nutrients. The relative importance of advective vs. regenerative pathways of nutrient
supply may result in widely different rates of release of nutrients from sediments. The relative activity of denitrifiers
also may alter the ratio of N to P released to overlying waters, and hence affect which nutrient limits growth of producers.
The consequences of nutrient (particularly nitrate) loading include somewhat elevated nutrient concentrations in the watercolumn,
increased growth of macroalgae and phytoplankton, reduction of seagrass beds, and reductions of the associated fauna. The
decline in animals occurs because of habitat changes and because of the increased frequency of anoxic events prompted by the
characteristically high respiration rates found in enriched waters.
We have measured the concentrations of nutrients and radium isotopes (223Ra, 224Ra, and 226Ra) in surface seawater and coastal groundwater in Yeoja Bay (in the southern sea of Korea) to estimate submarine groundwater discharge (SGD) and associated nutrient fluxes. In general, the radium and nutrient concentrations in brackish groundwater were an order of magnitude higher than those in ambient bay water or stream water. We determined the water residence time and SGD in the bay using the simultaneous equations for 226Ra, 223Ra, and Si mass balances. The mean residence time of bay water was about 7 days in Yeoja Bay. The inputs of submarine groundwater to the surface layer (0–3 m) were estimated to be approximately 2.6×107 m3 day−1. The nutrient fluxes driven by SGD were approximately 26, 0.11, and 26 mmol m−2 day−1 for dissolved inorganic nitrogen (DIN), phosphorus (DIP), and silicate (DISi), respectively. These fluxes of nutrients through SGD were much higher than those through stream flow and/or diffusion from bottom sediments in this bay. This excess nutrient input from coastal groundwater is the most likely cause of harmful algae blooms occurring in the open sea areas out from the bay.
Nutrient input through submarine groundwater discharge (SGD) rivals river inputs in certain regions and may play a significant role in nutrient cycling and primary productivity in the coastal ocean. In this paper, we review the key factors determining the fluxes of nitrogen (N) and phosphorus (P) associated with SGD and present a compilation of measured rates. We show that, in particular, the water residence time and the redox conditions in coastal aquifers and sediments determine fluxes and ratios of N and P in SGD. In many coastal groundwater systems, and especially in contaminated aquifers, N/P ratios exceed those in river water and are higher than the Redfield ratio. Thus, anthropogenically driven increases in SGD of nutrients have the potential to drive the N-limited coastal primary production to P-limitation. River input of N and P to the coastal ocean has doubled over the past 50 yr. Results of a dynamic biogeochemical model for the C, N and P cycles of the global proximal coastal ocean (which includes large bays, the open water part of estuaries, deltas, inland seas and salt marshes), suggest that this has led to a factor 2 increase in primary production and biomass and a decline in water column N/P ratios, i.e. the system has become more N-limiting. With the same model, we show that an increase of SGD-N fluxes to ∼0.7–1.1 Tmol yr−1 (with a SGD N/P ratio of 100; equal to ∼45–70% of pre-human riverine N-inputs) is required to drive the coastal ocean to P-limitation within the next 50 yr.
In order to estimate submarine groundwater discharge (SGD) and SGD-driven nutrient fluxes, we measured the concentrations of nutrients, 224Ra, and 226Ra in seawater, river water, and coastal groundwater of Yeongil Bay (in the southeastern coast of Korea) in August 2004 and February 2005. The bottom sediments over the shallow areas of this bay are composed mainly of coarse sands. Large excess concentrations of 224Ra, 226Ra, and Si supplied from SGD were observed in August 2004, while these excess concentrations were not apparent in February 2005. Based on the mass balance for 224Ra, 226Ra, and Si, which showed conservative mixing behavior in seawater, SGD was estimated to be approximately 6 × 106 m3 day− 1 (seepage rate = 0.2 m day− 1) in shallow areas (< 9 m water depth) in August 2004, which is much higher than the SGD level typically found in other coastal regions worldwide. During the summer period, SGD-driven nutrients in this bay contributed approximately 98%, 12%, and 76% of the total inputs for dissolved inorganic nitrogen (DIN), phosphorus (DIP), and silicate (DSi), respectively. Our study implies that the ecosystem in this highly permeable bed coastal zone is influenced strongly by SGD during summer, while such influences are negligible in winter.
Over the past decade, radium isotopes have been frequently applied as tracers of submarine groundwater discharge (SGD). The unique radium signature of SGD is acquired within the subterranean estuary, a mixing zone between fresh groundwater and seawater in coastal aquifers, yet little is known about what controls Ra cycling in this system. The focus of this study was to examine controls on sediment and groundwater radium activities within permeable aquifer sands (Waquoit Bay, MA, USA) through a combination of field and laboratory studies. In the field, a series of sediment cores and corresponding groundwater profiles were collected for analysis of the four radium isotopes, as well as dissolved and sediment associated manganese, iron, and barium. We found that in addition to greater desorption at increasing salinity, radium was also closely tied to manganese and iron redox cycling within these sediments. A series of laboratory adsorption/desorption experiments helped elucidate the importance of 1) contact time between sediment and water, 2) salinity of water in contact with sediment, 3) redox conditions of water in contact with sediment, and 4) the chemical characteristics of sediment on radium adsorption/desorption. We found that these reactions are rapid (on the order of hours), desorption increases with increasing salinity and decreasing pH, and the presence of Fe and Mn (hydr)oxides on the sediment inhibit the release of radium. These sediments have a large capacity to sorb radium from fresh water. Combined with these experimental results, we present evidence from time series groundwater sampling that within this subterranean estuary there are cyclic periods of Ra accumulation and release controlled by changing salinity and redox conditions.
The coastal, unconfined aquifer at Huntington Beach, California contains saline groundwater (33 psu) adjacent to the water line, and a brackish groundwater (BGW) mixing zone (3–9 psu) approximately 50 m from the water line. According to salinity and water isotope analyses, the BGW composition varies spatially in the alongshore direction. Measurements obtained from two BGW wells indicate this water is a mixture of seawater and a freshwater end member (likely infiltrated runoff); results from a third more southerly well suggest the BGW is composed of seawater, freshwater, and water from the confined Talbert Aquifer. Saline groundwater, on the other hand, shares similar salinity and water isotopic composition with seawater. The saline groundwater is enriched with short- and long-lived radium isotopes relative to the surf zone, which in turn is enriched relative to waters further offshore. We derive eddy diffusion coefficients for the nearshore using 223Ra and 224Ra, and in conjunction with 226Ra activities, estimate a submarine groundwater discharge (SGD) rate between 4.2 and 8.9 L min−1m−1 of shoreline, respectively. Using analytical models, we estimate that between 74 and 100% of this discharge represents seawater than has been forced by tides and waves through the beach aquifer. Under one set of model assumptions, results indicate that a portion (up to 56%) of seawater pumped into the beach aquifer by waves and tides is not discharged suggesting salt water intrusion may be occurring. Because saline groundwater is enriched with dissolved inorganic nitrogen and soluble reactive phosphate relative to the coastal ocean, our results suggest that tidally and wave-driven seawater circulated through the beach aquifer represents an important mechanism for nutrient input to the nearshore environment in dry weather. Estimates of nutrient flux to the coastal ocean via SGD along 16–17 km of shoreline are equivalent to nutrient fluxes from two salt water wetlands near the study site during the dry season.
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.
We estimated the magnitude of submarine groundwater discharge (SGD) into the Yellow Sea, which is one of the largest continental shelves in the world, using 226Ra and 228Ra isotopes. On the basis of 228Ra and 226Ra mass-balance models, we estimated the advective flux of 226Ra through SGD to be ∼270 × 1012 dpm yr− 1 in the Yellow Sea. Using this 226Ra flux and the measured 226Ra activity in coastal groundwater, the submarine discharge of groundwater (mostly brackish groundwater) was calculated to be at least 40% of the river-water input (∼2.3 × 1011 m3 yr− 1). Then, on the basis of the SGD and the concentration of Si in coastal groundwater, we estimated the flux of Si through SGD to be 20–100% of that associated with river discharge (∼23 × 109 mol yr− 1). This large SGD is likely to be due to the high tidal range (up to 10 m) and the wide distribution of sandy sediments in the Yellow Sea, favorable for the recirculation of seawater through bottom sediments and rocks. This result from such a large area implies that the Si flux through SGD may be significant on a global scale.
We determined groundwater flow rates shortly after the wet season into an embayment near Ubatuba, Brazil as part of an international intercomparison experiment for submarine groundwater discharge (SGD) assessment techniques. Our estimated rates were determined by the combined use of continuous radon measurements and assessment of radium isotope patterns. The spatial distribution of the short-lived radium isotopes (223Ra and 224Ra) provided the means for independent evaluations of radon losses by mixing and atmospheric evasion. We were thus able to construct a well-constrained mass balance for radon that included a groundwater flux term. Our results showed that the groundwater discharge into this embayment from the fractured crystalline rock aquifer is not steady-state but varies with tidal modulation and rain-induced forcing. Tidally modulated and rain-induced flow rates were comparable during this period. The SGD rates estimated from radon ranged from 1 cm/day to 29 cm/day (cm3/cm2 day) with a mean and standard deviation of 13 ± 6 cm/day. These estimates were mostly similar to a dye-dilution automatic seepage meter (15 ± 19 cm/day) and were within the broad ranges estimated by manual and continuous heat seepage meters but lower than indicated by an artificial tracer test performed nearshore.