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Greenhouse Gases and Submarine Groundwater Discharge in a Sydney Harbour embayment (Australia)
Abstract
We investigated whether submarine groundwater discharge (SGD) traced by radon (222 Rn, a natural groundwater tracer) may drive carbon dioxide (CO 2), methane (CH 4) and nitrous oxide (N 2 O) in surface waters in Chowder Bay, a marine embayment in Sydney Harbour, Australia. A radon mass balance revealed significant groundwater discharge rates into the bay (8.7 ± 5.8 cm d-1). The average CO 2 , CH 4 , and N 2 O concentrations in the subterranean estuary were 3.5, 7.2, and 2.8 times higher than the average surface water concentrations, indicating the possibility of coastal groundwater as a source of greenhouse gases to the bay. SGD-derived fluxes of greenhouse gases were 5.02 ± 2.28 mmol m-2 d-1 , 5.63 ± 2.55 µmol m-2 d-1 , and 1.72 ± 0.78 µmol m-2 d-1 for CO 2 , CH 4 and N 2 O, respectively. The average CO 2 evasion rate from surface water was 2.29 ± 0.46 mmol m-2 d-1 while CH 4 and N 2 O evasion rates were 12.89 ± 3.05 and 1.23 ± 0.25 µmol m-2 d-1 respectively. Therefore, groundwater-derived greenhouse gas fluxes accounted for >100% CO 2 and N 2 O and ~43% of CH 4 surface water evasion, indicating SGD is likely an important source of greenhouse gases to surface waters. However, this may be due to observations being performed near the SGD source, which may overestimate its contribution to the wider Sydney Harbour. On a 20-year time frame the combined emissions of CH 4 and N 2 O from surface waters to the atmosphere accounted for 25% of the total CO 2-equivalent emissions. Although this study gives preliminary insight into SGD and greenhouse gas dynamics in Sydney Harbour, more spatial and temporal resolution sampling is required to fully constrain these processes.
... While several studies have now quantified the influence of groundwater to nutrient budgets in surface waters surrounding large cities (Xiao et al. 2019;Correa et al. 2020), few studies have focused on how this may influence N 2 O emissions ). The few investigations quantifying the specific contribution of groundwater discharge to coastal N 2 O dynamics indicate groundwater can be a major driver of coastal surface-water N 2 O dynamics (Georgia and Amélie 2009;O'Reilly et al. 2015;Tait et al. 2017b), especially in estuaries with high groundwater discharge rates Sadat-Noori et al. 2018). ...
... To date, few studies have assessed greenhouse gas dynamics in the Harbour. Sadat-Noori et al. (2018) measured CO 2 , CH 4 , and N 2 O in a small embayment (Chowder Bay) near the mouth of the harbor, and Tanner et al. (2017) performed harbor wide CO 2 surveys. SGD and related nutrient fluxes have been estimated using radium isotopes at the embayment and harbor scales (Correa et al. 2020). ...
... In contrast to the low SGD rates of the lower central harbor sections, the lower estuary embayments of Rose and Rozelle Bays displayed much higher SGD rates (5.0 AE 1.4 cm d −1 and 4.0 AE 0.8 cm d −1 , respectively). These rates were within the reported uncertainty range of another lower Sydney Harbour embayment (8.7 AE 5.8 cm d −1 , Chowder Bay) where high SGD rates were attributed to the steep subcatchment morphology and prior rainfall driving fresh SGD (Sadat-Noori et al. 2018). The porous sandy sediment (Rose Bay) and exposed beaches in the lower harbor embayments likely accentuated tidal pumping which could explain higher local SGD than the Outer Harbour and Lower Central Harbour sections featuring both natural and constructed rocky shorelines. ...
Coastal waterways can be significant sources of the potent greenhouse gas nitrous oxide (N 2 O) due to nitrogen inputs and eutrophication. Here, we quantify groundwater derived N 2 O inputs and atmospheric emissions within a modified urban embayment (Sydney Harbour, Australia). Overall, we found low N 2 O saturation (91-171%) and air-water fluxes (−2.2 to 24.6 μmol m −2 d −1). Concentrations were highest in upstream brackish areas and a commercial/industrial subembayment. Dissolved inorganic nitrogen concentrations were low and inversely correlated to N 2 O throughout the harbor. N 2 O surface water dynamics were apparently driven by saline submarine groundwater discharge, as quantified by the radioisotope tracer radon-222. Groundwater discharge was highest within the embayments and mangrove-lined upper estuary. While groundwater was a net N 2 O source to surface waters, two upstream sub-embayments featured groundwater N 2 O concentrations lower than surface water, suggesting a sink driven by surface waters recirculating in intertidal sediments. Surface-water N 2 O was undersaturated within one upstream embayment, likely due to N 2 O consumption within sediments. Contrastingly, the downstream embayments featured higher groundwater N 2 O and accounted for 45% AE 21% of the groundwater N 2 O flux. Sydney Harbour was a net source of N 2 O to the atmosphere (mean 0.6 AE 0.3 μmol m −2 d −1) with larger N 2 O fluxes occurring from relatively small areas. N 2 O emissions (expressed in CO 2 equilivents) were equivalent to 17% of CO 2 emission estimates from previous studies. The low N 2 O emissions in Sydney Harbour contrast with other modified estuaries which often emit higher N 2 O fluxes due to larger nitrogen inputs.
... The development of isotopic (radon and radium isotopes) techniques in the 1990s (Burnett et al., 2001;Moore, 1997;Moore and Arnold, 1996) has led to an increase in SGD investigations in the coastal ocean and estuaries. SGD has been demonstrated to be a major source of nutrients (Anwar et al., 2014;Moore, 2010;Slomp and Van Cappellen, 2004), greenhouse gasses (Sadat-Noori et al., 2018), and heavy metals (Knee and Paytan, 2011) in several locations around the world. SGD has been assessed in a broad range of habitats such as saltmarshes (Krest et al., 2000), mangroves (Mcgowan and Martin, 2007;Sanders et al., 2012;Tait et al., 2016), river deltas , coral reefs (McMhon and Santos, 2017), lagoons , estuaries and the continental shelf (Moore et al., 2008). ...
... The difference of concentration between radium in salted samples and unsalted samples was assumed to be equivalent to desorption . Radium contamination resulting from the addition of salt was assumed to be zero as it has been found to be undetectable in our previous similar experiments (Sadat-Noori et al., 2018;Stewart et al., 2015). Water discharge for the creeks was obtained from the Water NSW service (https://realtimedata.waternsw.com.au/water.stm). ...
... The major source of uncertainty in the estimation of saline SGD is usually related to the groundwater endmember (Moore and Arnold, 1996;Sadat-Noori et al., 2018;Santos et al., 2009). ...
The effects of urbanization and scales on submarine groundwater discharge (SGD) remain poorly understood. Here, we used radium isotopes to quantify SGD-derived fluxes of N, P and C into Sydney Harbour estuary, Australia. Sydney is the most populated city of Oceania, with several localised cases of historical groundwater pollution. We sampled top and bottom waters at the harbour scale (∼20 km) and also at four small scale embayments (∼2 km). A decreasing gradient in radium isotope concentrations from upstream to downstream was observed. Mass balances constructed with different radium isotopes revealed that total SGD ranged from 42 to 121 x104 m3 d-1 depending on assumptions and isotope. These fluxes were related mostly to saline SGD (recirculated seawater), and are equivalent to >60 times the mean annual freshwater river discharge into Sydney Harbour (0.68 x104 m3 d-1). The estimated SGD rates (2.2 ±1.5 cm d-1) were comparable to the global average radium-derived-SGD in other urban estuaries (∼3.1 cm d-1). No obvious relationships were observed between SGD and scale in Sydney Harbour. However, higher SGD rates estimated for embayments closer to the ocean indicate that a combination of waves, tides and urbanization control SGD. SGD derived fluxes exceeded maximum riverine nutrient fluxes by a factor of 2 for DOC, 6 for PO43-, 40 for NH4+ and 1.3 for NOX. Previous work has suggested that nutrients enter Sydney Harbour primarily through rivers or stormwater following episodic rain events. Our results imply that diffuse saline SGD can also be an important but overlooked source of nutrients, potentially sustaining primary productivity in times of no river flow.
... It has been recently shown that aquatic CO 2 emissions may be enhanced or driven by SGD 24,25 with large and often unaccounted implications on regional and global marine carbon budgets. 26 Higher CO 2 concentrations in seeping groundwater than in surface waters 24,26 can modify seawater− carbon chemistry around coral reefs 27 and change the sink or source function of the ecosystem. CO 2 inputs can impact coral reef function and health by decreasing pH and the aragonite saturation state and ultimately slowing down the accretion of CaCO 3 15,28,29 and changing biological communities. ...
... It has been recently shown that aquatic CO 2 emissions may be enhanced or driven by SGD 24,25 with large and often unaccounted implications on regional and global marine carbon budgets. 26 Higher CO 2 concentrations in seeping groundwater than in surface waters 24,26 can modify seawater− carbon chemistry around coral reefs 27 and change the sink or source function of the ecosystem. CO 2 inputs can impact coral reef function and health by decreasing pH and the aragonite saturation state and ultimately slowing down the accretion of CaCO 3 15,28,29 and changing biological communities. ...
... To calculate SGD rates, we simply divided F benthicRn and F benthicRa by the activities of 222 Rn and 224 Ra in shallow groundwater. We ignored diffusive flux, because flux from the seafloor is usually dominated by SGD [16,33]. 222 Rn-derived SGD rates were thus estimated by dividing 222 Rn advection by the 222 Rn activity of groundwater. ...
... In this study, we used the intercept of the mixing line between 222 Rn and salinity to determine the 222 Rn-derived SGD rate, which agreed well with the total SGD rate obtained using the seepage meter at site A (Table 2). This approach may be not common, because most of the similar studies used mean or median value [33,43,46]. Use of the mean value of 222 Rn activity in groundwater (20.7 ± 17.1 dpm L −1 ) would result in an 222 Rn-derived SGD rate approximately three times that determined using the seepage meter (349.4 ± 215.0 cm d −1 ). ...
Submarine groundwater discharge (SGD) consists of fresh submarine groundwater discharge (FSGD) and recirculated submarine groundwater discharge (RSGD). In this study, we conducted simultaneous 25-hour time-series measurements of short-lived 222Rn and 224Ra activities at two sites with differing SGD rates in the central Seto Inland Sea of Japan to evaluate SGD rates and their constituents. At both sites, we also quantified the total SGD, FSGD, and RSGD using a seepage meter to verify the water fluxes estimated with 222Rn and 224Ra. SGD rates estimated using 222Rn and 224Ra at the site with significant SGD approximated the total SGD and RSGD measured by the seepage meter. However, SGD rates derived using 222Rn at the site with minor SGD were overestimated, since 222Rn activity at the nearshore mooring site was lower than that in the offshore area. These results suggest that the coupling of short-lived 222Rn and 224Ra is a powerful tool for quantification of FSGD and RSGD, although it is important to confirm that tracer activities in coastal areas are higher than those in offshore.
... Methane (CH 4 ) and other dissolved gases such as carbon dioxide (CO 2 ) are ubiquitous in groundwater systems, however gas occurrences in drinking water aquifers can cause community and environmental concerns especially in regions that also host commercial oil or gas production and other anthropogenic activities (Larson et al., 2018;Loomer et al., 2018;Molofsky et al., 2021). In addition, methane is gaining increasing interest owing to its impact in fugitive emissions (Kulongoski and McMahon, 2019;Luhar et al., 2020;Sadat-Noori et al., 2018). ...
Understanding the occurrence and sources of methane in aquifers is an international issue, especially in regions that host oil and gas reservoirs. Multi-isotopic studies are important to understand sources and concentrations of dissolved gases in aquifers of basins that host oil and gas extraction; or future CO2, natural gas, or hydrogen storage, or compressed air energy storage. The largest artesian basin in the world, the Great Artesian Basin, extends across more than one fifth of Australia providing a fresh water source from various aquifers. The Precipice and Hutton sandstones, aquifers of the Surat Basin in the eastern Great Artesian Basin, are important water resources for town water supply, agriculture, mines, feedlots, and private landholders in Queensland. Overlying and underlying formations host gas extraction. The southern part of the basin in Queensland additionally hosts an oil field, and potential CO2 storage sites, therefore understanding the aquifer gas concentrations, processes and sources is important for fluid management. The dissolved concentrations of methane and ethane in the Hutton Sandstone were up to 96 mg/L and 1 mg/L respectively, and in the Precipice Sandstone methane ranged up to 2100 mg/L and ethane 6.5 mg/L in the southern Surat Basin. Dissolved gas concentrations were measured by an open and closed sampling method, with the concentrations lower from open sampling, and the gas wetness parameter systematically slightly lower. The Hutton Sandstone aquifer dissolved gas isotopic signatures are mainly consistent with primary microbial CO2 reduction generating methane in situ, with fractionation factors indicating acetate fermentation in a few cases. The Hutton Sandstone δ¹³C-DIC, δ¹³C-CO2, and δ¹³C-CH4 have an enrichment trend with increasing depth that indicate a microbial substrate depletion effect. Moonie Oil Field Precipice Sandstone and Evergreen Formation samples plot in the thermogenic region, with enriched δ¹³C-CO2 indicating microbial biodegradation is also occurring. The Precipice Sandstone aquifer samples have a mixed microbial CO2 reduction and early mature thermogenic gas signature in both isotope cross plots and gas wetness diagrams, with a subset of sampled δ¹³C-C2H6 and isotope factors consistent with early mature thermogenic gas. A potential historic source of thermogenic gas is the underlying gas reservoir of the Bowen Basin. Large volumes of water are abstracted from these aquifers, hence dissolved gases may contribute to greenhouse gas emissions.
... SGD can be composed of fresh or saline waters (Diggle et al. 2019), and its salinity can vary widely (from 0.3 to 34.8) (Sadat-Noori et al. 2018). SGD is known to play a crucial role in regulating the biogeochemistry of several elements and compounds, either through direct freshwater discharge or by recirculation of seawater via coastal aquifer systems (Burnett and Dulaiova 2003). ...
Estuaries and coastal waters are generally significant emitters of CO2 to the atmosphere. Globally, submarine groundwater discharge (SGD) is a significant driver of inorganic carbon dynamics and the partial pressure of CO2 in water [pCO2(water)] in estuaries and coastal waters. However, there are few studies of CO2 emission and SGD in large tropical estuaries. To investigate the drivers of pCO2(water) dynamics in two large tropical estuaries in India, the Hooghly and Matla estuaries, we conducted coupled radon and pCO2(water) surveys of groundwater and surface waters. We also collected high‐temporal‐resolution (1 min) data of related biogeochemical parameters. Using radon‐222 (222Rn) as a proxy for SGD, we found significant regulation of pCO2(water) by SGD in both studied estuaries. This observation can be considered direct evidence of the influence of SGD on pCO2(water) and consequent air–water CO2 fluxes in these two previously poorly studied estuaries. Our finding that total alkalinity (TAlk) and dissolved inorganic carbon (DIC) were also significantly higher in the groundwater than in the estuarine surface waters also supports a substantial contribution of SGD to the inorganic carbon dynamics of these estuaries. However, high groundwater TAlk to DIC ratios (> 1) indicate that SGD also enhances the carbonate buffering capacity in these estuaries. By contributing to an improved understanding of the role of SGD in inorganic carbon cycling in tropical estuaries and coastal oceans, this study will contribute to efforts to upscale regional‐scale coastal carbon budgets to a global scale.
... Hence, Southern Hemisphere temperate and tropical locations are widely under-represented in global estimates (Gerardo-Nieto et al., 2017;Maher and Eyre, 2012;Staehr et al., 2012). As such, it is important to provide estimates from data-limited regions which will improve global estimations of greenhouse gas fluxes from freshwater lakes and feed into global climate models (Li et al., 2018;Sadat-Noori et al., 2017). ...
Freshwater lakes can play a significant role in greenhouse gas budgets as they can be sources or sinks of carbon to the atmosphere. However, there is limited information on groundwater discharge being a source of carbon to freshwater lakes. Here, we measure CO2 and CH4 in the largest urban freshwater lake in the metropolitan area of Sydney (Australia) and quantify groundwater discharge rates into the lake using radon (²²²Rn, a natural groundwater tracer). We also assess the spatial variability of radon, CO2 and CH4 in the lake, in addition to surface water and groundwater nutrient and carbon concentrations. Results revealed that the lake system was a source of CO2 and CH4 to the atmosphere with fluxes of 113 ± 81 and 0.3 ± 0.1 mmol/m²/d, respectively. These calculated CO2 fluxes were larger than commonly observed lake fluxes and the global average flux from lakes. However, CH4 fluxes were lower than the average global value. Based on the radon mass balance model, groundwater discharge to the lake was 16 ± 10 cm/d, which resulted in groundwater-derived CO2 and CH4 fluxes contributing 25 and 13% to the overall greenhouse gas emissions from the lake, respectively. Radon, CO2 and CH4 maps showed similar spatial distribution trends in the lake and a strong relationship between radon, NO3 and NH4 suggested groundwater flow was also a driver of nitrogen into the lake from the western side of the lake, following the general regional groundwater flow. This work provides insights into groundwater and greenhouse gas dynamics in Sydney's largest urban freshwater lake with two implications for carbon budgets: to incorporate urban lakes in global carbon budgets and to account for, the often ignored, groundwater discharge as a source of carbon to lakes.
... This is a common trend observed by others in coastal settings (Davis et al., 2020;Webb et al., 2019). In such cases, a significant inverse relationship between groundwater exfiltration and tidal height is expected (Call et al., 2015;Sadat-Noori et al., 2017), as was also observed here ( Fig. 8) confirming that groundwater exfiltration was mostly driven by tidal pumping (Fig. 4). Fig. 5c showed the lowest net flow occurred during the 2nd diel cycle which was caused by a shorter peak in surface water radon concentrations. ...
Subsurface flow plays an important role in the functioning of wetlands and in the maintenance of their ecosystem services. Specifically, the transport and exchange of dissolved matter between sediments and surface waters is regulated by subsurface flow, which can strongly affect ecological zonation and productivity. Having a quantitative understanding of this subsurface flow is therefore important. Field techniques based on Darcy’s equation or natural tracers are often used separately to assess flows. Here, radon and heat (both natural groundwater tracers) and Darcy’s equation are used simultaneously to quantify the subsurface flow in a tidal wetland (Kooragang Island, Newcastle, Australia) and the results of the independent methods are compared. A steady-state radon mass balance model indicated an overall net subsurface exfiltration of 10.2 ± 4.2 cm/d while a 1D, vertical fluid heat transport model indicated a net exfiltration of 4.3 ± 2.9 cm/d. Flow estimated from analysis of hydraulic heads indicated an exfiltration rate of 3.2 ± 1.8 cm/d. The difference in flow rates is likely due to the localised measurement of the heat and head methods relative to radon, and therefore, these methods are less likely to capture zones of preferential subsurface flow. The main advantage of radon is that it provides the total subsurface flow regardless of the driving force. While head gradient or heat tracer method have the advantage of temporally quantify infiltration and exfiltration, we highlight that these methods may underestimate subsurface flows in highly dynamic coastal systems, such as tidal wetlands where a large portion of the subsurface flow is recirculated seawater. This could potentially lead to errors in solute flux estimates. This study highlights the importance of employing a multi-tracer approach and has implications towards quantifying the hydrological export of dissolved constituents (e.g., carbon and nitrogen) in coastal wetlands.
... Expressed as CO 2 -eq, CH 4 accounted for a major portion of the GHG warming potential in the lagoon (between 46 and 80%), especially in the river stations where the CH 4 was always more important than CO 2 . This is an unusual case since CO 2 is generally the predominant GHG in aquatic coastal ecosystems (Campeau et al. 2014;Sadat-Noori et al. 2018). However, studies have suggested that CH 4 can be the main source of CO 2 -eq emissions in small streams within the fluvial network and in mangrove ecosystems (Campeau et al. 2014;Sea et al. 2018). ...
Increasing eutrophication of coastal waters generates disturbances in greenhouse gas (GHG) concentrations and emissions to the atmosphere that are still poorly documented, particularly in the tropics. Here, we investigated the concentrations and diffusive fluxes of carbon dioxide (CO2) and methane (CH4) in the urban-dominated Jacarepagua Lagoon Complex (JLC) in Southeastern Brazil. This lagoonal complex receives highly polluted freshwater and shows frequent occurrences of anoxia and hypoxia and dense phytoplankton blooms. Between 2017 and 2018, four spatial surveys were performed (dry and wet conditions), with sampling in the river waters that drain the urban watershed and in the lagoon waters with increasing salinities. Strong oxygen depletion was found in the rivers, associated with extremely high values of partial pressure of CO2 (pCO2; up to 20,417 ppmv) and CH4 concentrations (up to 288,572 nmol L−1). These high GHG concentrations are attributed to organic matter degradation from untreated domestic effluents mediated by aerobic and anaerobic processes, with concomitant production of total alkalinity (TA) and dissolved inorganic carbon (DIC). In the lagoon, GHG concentrations decreased mainly due to dilution with seawater and degassing. In addition, the phytoplankton growth and CH4 oxidation apparently consumed some CO2 and CH4, respectively. TA concentrations showed a marked minimum at salinity of ~20 compared to the two freshwater and marine end members, indicating processes of re-oxidation of inorganic reduced species from the low-salinity region, such as ammonia, iron, and/or sulfides. Diffusive emissions of gases from the entire lagoon ranged from 22 to 48 mmol C m−2 d−1 for CO2 and from 2.2 to 16.5 mmol C m−2 d−1 for CH4. This later value is among the highest documented in coastal waters. In terms of global warming potential (GWP) and CO2 equivalent emissions (CO2-eq), the diffusive emissions of CH4 were higher than those of CO2. These results highlight that highly polluted coastal ecosystems are hotspots of GHG emissions to the atmosphere, which may become increasingly significant in future global carbon budgets.
... 222 Rn has been used in numerous studies for quantifying SGD in coastal and estuarine environments (e.g. Burnett et al., 2006;Burnett and Dulaiova, 2003;Sadat-Noori et al., 2018;Stewart et al., 2015;Wong et al., 2013). EC measurements are a simple but useful way to delineate between recycled sea water and fresh water (Short et al., 2015). ...
Coastal ecosystem health and sustainability is tightly coupled to submarine groundwater discharge (SGD) and associated nutrient, carbon and pollutant fluxes. However, there are few studies that systematically analyse the interaction between the terrestrial aquifer system, catchment morphology and coastal SGD. The objective of this study was to evaluate the role of catchment morphology and how this influences the spatial distribution, timing and volume of the SGD flux to a branched tidal creek system, on the barrier island Spiekeroog, Germany. The subsurface salinity was mapped using electrical resistivity tomography (ERT) and a hydrogeochemical survey of the shallow groundwater. Temporal and spatial analysis of geochemical tracers (²²²Rn, Cl⁻) in the tidal creek water was integrated into a transient ²²²Rn mass balance model for quantification of SGD rates during two field campaigns that encompassed spring and neap tides. The ERT mapping indicated that fresh groundwater dominated under the dune ridges down to about 15 m depth, but became progressively more brackish seawards. This is likely due to frequent tidal and storm flooding of low-lying areas. The highest groundwater fluxes into the creek were indicated by high ²²²Rn activities (average 468 Bq m⁻³) towards the dune ridge. Chloride concentrations (up to 17.3 g L⁻¹) increased seawards showing the progressive salinization of water in the creek. The freshwater component of SGD was highly variable in time but was highest at low tide, while the total SGD flux (saline + fresh) was highest when tides changed from inflow to outflow as the rapid pressure release on the local aquifer caused a large hydraulic gradient towards the creek. Comparing the freshwater component of mean daily SGD to the creek (179 m³ d⁻¹) with estimated groundwater recharge rates in the catchment (665 m³ d⁻¹) shows that the fresh groundwater discharge exceeds fresh recharge during spring tides (∼120 %) but is was lower than recharge during neap tides (∼27 %). In this study we show that tidal creeks and their relation to catchment morphology are relevant for understanding the spatial and temporal exchange of fresh and saline water between the catchment and coastal zone.
... We hypothesize that (1) SGD is a source of wastewater and industrial runoff to coastal surface waters, and (2) CEC distribution will be related to a land-use gradient and water residence times. We first quantify SGD using radium isotopes in coastal embayments downstream of a range of urban and industrialized catchments with known SGD (Sadat-Noori et al., 2018;Correa et al., 2020) and leakage from sewer pipes (Sydney Water, 2010). Then, we evaluate these data with respect to land use, population density, and water residence times. ...
Submarine groundwater discharge (SGD) is rarely considered as a pathway for contaminants of emerging concern (CECs). Here, we investigated SGD as a source of CECs in Sydney Harbour, Australia. CEC detection frequencies based on presence/absence of a specific compound were >90% for caffeine, carbamazepine, and dioxins, and overall ranged from 25 to 100% in five studied embayments. SGD rates estimated from radium isotopes explained >80% of observed CEC inventories for one or more compounds (caffeine, carbamazepine, dioxins, sulfamethoxazole, fluoroquinolones and ibuprofen) in four out of the five embayments. Radium-derived residence times imply mixing is also an important process for driving coastal inventories of these persistent chemicals. Two compounds (ibuprofen and dioxins) were in concentrations deemed a high risk to the ecosystem. Overall, we demonstrate that SGD can act as a vector for CECs negatively impacting coastal water quality.
... Shallow gas is often encountered during marine engineering and coastal subway construction. This shallow gas is likely to cause strong blowouts due to disturbances from the engineering processes, and may displace a significant amount of soil due to the water current through a process similar to pockmark formation (Sadat-Noori et al., 2018). Therefore, the formation of pockmarks can also be considered as soil loss during the inflation process. ...
The decomposition of gas hydrates releases gas and water at volumes much larger than the original volume. The gas escapes violently, which destroys seabed sediments and can even cause geological disasters. To explore the effect of natural gas hydrate decomposition on underwater slopes, laboratory experiments were conducted to observe the features of slope failure. Aeration was used to model the gas that was produced by hydrate decomposition, and the results were as follows. (a) Once air filled into the bottom of the underwater slope, the pore pressure at the slope was altered, which eventually caused the slope to fail. (b) The formation of pockmarks corresponded to the variations in the pore-water pressure. (c) The theoretical analysis results showed that the sedimentation rate of fine particles (d < 0.4 mm) was lower than the water flow velocity, indicating that these particles were carried away with the air–liquid two-phase flow, which eventually caused the pockmarks to develop into craters.
Greenhouse gases (GHG) emissions in coastal areas are influenced by both mariculture and submarine groundwater discharge (SGD). In this study, we first conducted a comprehensive investigation on carbon dioxide (CO2) and methane (CH4) emissions affected by SGD in a typical maricultural bay in north-eastern Hainan Island, China. A radon (²²²Rn) mass balance model revealed considerable high SGD rates (179 ± 92 cm d⁻¹) in the bay, and the fluxes of SGD-derived dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) were 150.36 and 3.65 g C m⁻² d⁻¹, respectively. Time-series measurement results, including those for ²²²Rn, CH4, CO2, and physicochemical parameters, indicated that GHG dynamics in the maricultural bay mainly varied with tidal fluctuations, and isotopic evidence further revealed that acetate fermentation was the main mechanism of methanogenesis in the maricultural waters. The water-air fluxes in the maricultural area were 1.05 ± 0.32 and 9.49 ± 3.96 mmol m⁻² day⁻¹ for CH4 and CO2, respectively, implying that Qinglan Bay was a potential source of GHG released into the atmosphere. At the bay-scale, the CO2 emissions followed a spatial pattern, and the CH4 emissions were mainly affected by mariculture. The high CH4 emissions in the maricultural waters caused by maricultural activities, SGD, high temperature, and special hydrology resulted in the formation of the CH4-dominated total CO2-equivalent emissions model. Our study highlights the importance of considering the link between SGD and GHG emissions in maricultural bays when constraining global GHG fluxes.
Submarine groundwater discharge (SGD) is a predominant component of chemical fluxes in the solute budgets in coastal ecosystems because of its large flux and the corresponding concentrated constituents. A quantitative study regarding the mass balance model of ²²⁶Ra and ²²⁸Ra was performed to assess SGD and associated nutrient inputs in Xiangshan Bay (XSB), a typical Chinese aquaculture bay. Based on these mass balance models, the SGD rates in the XSB Channel were 8.9–13.2 cm d⁻¹. The SGD rates in the three corresponding embayments were 3.5–3.7 cm d⁻¹ in Embayment I, 5.6–5.8 cm d⁻¹ in Embayment II, and 4.5–5.8 cm d⁻¹ in Embayment III. Significant spatial variabilities in SGD rates were observed in the different XSB districts, reflecting that ocean dynamics and anthropogenic activities play significant roles at different scales. SGD-derived nutrient fluxes in XSB were the dominant sources of nutrient loading, and their magnitudes were approximately 1–2 orders larger than those of riverine input. SGD rates in the XSB Channel were approximately three times higher than those in the three embayments, but the SGD-derived nutrient rates in the XSB Channel were comparable with those in the three embayments, even significantly lower than that for NO3-N in the inner regions of the embayment. These results imply that aquaculture activities and urbanization jointly influence SGD. When combined with the results of other studies, our observations demonstrate the significant comprehensive effects of multiple factors on SGD, a critical but overlooked nutrient source.
Wetlands play an important role in the global carbon cycle as they can be sources or sinks for greenhouse gases. Groundwater discharge into wetlands can affect the water chemistry and act as a source of dissolved greenhouse gases, including CO2 and CH4. In this study, surface water quality parameters and CO2 and CH4 concentrations were evaluated in a tidal wetland (Hunter Wetlands National Park, Australia) using time series measurements. Radon (²²²Rn), a natural groundwater tracer, was used to investigate the role of groundwater as a pathway for transporting dissolved CO2 and CH4 into the wetland. In addition, water-to-air CO2 and CH4 fluxes from the wetland were also estimated. The results showed a high concentration of radon in wetland surface water, indicating the occurrence of groundwater discharge. Radon concentration had a strong negative relationship with water depth with a determination coefficient (R²) of 0.7, indicating that tidal pumping was the main driver of groundwater discharge to the wetland. Radon concentration also showed a positive relationship with CO2 and CH4 concentrations (R² = 0.4 and 0.5, respectively), while the time series data revealed that radon, CO2, and CH4 concentrations peaked concurrently during low tides. This implied that groundwater discharge was a source of CO2 and CH4 to the wetland. The wetland had an average water-to-air CO2 flux of 99.1 mmol/(m²·d), twice higher than the global average CO2 flux from wetlands. The average CH4 flux from the wetland was estimated to be 0.3 mmol/(m²·d), which is at the higher end of the global CH4 flux range for wetlands. The results showed that groundwater discharge could be an important yet unaccounted source of CO2 and CH4 to tidal wetlands. This work has implications for tidal wetland carbon budgets and emphasizes the role of groundwater as a subsurface pathway for carbon transport.
Salt marshes can sequester large amounts of carbon in sediments, but the relation between carbon storage and exportation remains poorly understood. Groundwater exchange can flush sediment carbon to surface waters and potentially reduce storage. In this study, we estimated groundwater fluxes and associated carbon fluxes using a radon (222Rn) mass balance and sediment carbon burial rates using lead (210Pb) in a pristine salt marsh (North Inlet, SC, USA). We used δ13C to trace carbon origins. We found that groundwater releases large amounts of carbon to the open ocean. These groundwater fluxes have the potential to export 7.2 ± 5.5 g m−2 of dissolved inorganic carbon (DIC), 0.2 ± 0.2 g m−2 of dissolved organic carbon (DOC) and 0.7 ± 0.5 g m−2 of carbon dioxide (CO2) per day. The fluxes exceed the average surface water CO2 emissions (0.6 ± 0.2 g m−2 day−1) and the average sediment carbon burial rates (0.17 ± 0.09 g m−2 day−1). The δ13C results suggest that groundwater carbon originated from salt marsh soils, while the sediment carbon source is derived from salt marsh vegetation. We propose that the impact of salt marshes in carbon cycling depends not only on their capacity to bury carbon in sediments, but also on their high potential to export carbon to the ocean via groundwater pathways.
Saltmarshes are one of the most productive ecosystems, which contribute significantly to coastal nutrient and carbon budgets. However, limited information is available on soil nutrient and carbon losses via porewater exchange in saltmarshes. Here, porewater exchange and associated fluxes of nutrients and dissolved inorganic carbon (DIC) in the largest saltmarsh wetland (Chongming Dongtan) in the Changjiang River Estuary were quantified. Porewater exchange rate was estimated to be (37±35) cm/d during December 2017 using a radon (222Rn) mass balance model. The porewater exchange delivered 67 mmol/(m2·d), 38 mmol/(m2·d) and 2 690 mmol/(m2·d) of dissolved inorganic nitrogen (DIN), dissolved silicon (DSi) and DIC into the coastal waters, respectively. The dominant species of porewater DIN was NH
+4
(>99% of DIN). However, different with those in other ecosystems, the dissolved inorganic phosphorus (DIP) concentration in saltmarsh porewater was significantly lower than that in surface water, indicating that saltmarshes seem to be a DIP sink in Chongming Dongtan. The porewater-derived DIN, DSi and DIC accounted for 12%, 5% and 18% of the riverine inputs, which are important components of coastal nutrient and carbon budgets. Furthermore, porewater-drived nutrients had obviously high N/P ratios (160–3 995), indicating that the porewater exchange process may change the nutrient characteristics of the Changjiang River Estuary and further alter the coastal ecological environment.
The geography patterns and generation mechanisms of greenhouse gases (GHGs) in groundwater, especially in saline groundwater, are critical but rarely studied. Herein, we investigated the GHGs distribution in an aquifer, located upstream of Baiyangdian Lake, China, with a distinctive salinity gradient. A total of 132 groundwater samples were collected from 44 new-constructed wells along the lateral dimensions, and analyzed for dissolved GHGs concentrations, physiochemical parameters, and isotopes. The results showed that the dissolved CO2, CH4 and N2O concentrations ranged from 9.47 to 79.3 mg/L, 1.05 to 56.9 μg/L, and 0.84 to 7.03 μg/L, respectively. The groundwater was supersaturated with GHGs with respect to atmospheric equilibrium, suggesting groundwater discharge as a potential source of GHGs emission. CO2 significantly decreased while CH4 and N2O distinctively increased with the decline of total dissolved solids (TDS) concentration, illustrating an obvious spatial pattern in the GHGs distribution. The CO2 distributions mainly depended on the bicarbonate radical and TDS, indicating carbonate equilibrium as the main process involving in the CO2 generation. CH4 and N2O was primarily generated through the methanogenesis and denitrification processes, respectively. Nutrients including SO4²⁻ and total organic carbon predominately shaped the CH4 distributions, while nitrate mainly governed the N2O distributions. Our study highlights the important roles of hydrochemistry and nutrients in the GHGs generation and distributions, which provides a significant insight on managing the GHGs emissions from the saline groundwater.
Freshened groundwater exists offshore along many coastlines worldwide. Pumping of these offshore resources has been proposed for more efficient oil production as well as drinking and agriculture. Although pumping can occur tens to hundreds of kilometers offshore, these activities may threaten connected onshore aquifer systems. We conducted numerical simulations of variable-density groundwater flow and salt transport in coastal aquifers with different geologic structure subject to offshore pumping to assess potential impacts. Results show that offshore pumping can diminish both onshore groundwater availability and submarine groundwater discharge and can cause widespread land subsidence. Heterogeneity increases water resource vulnerability relative to equivalent homogeneous and layered systems and exacerbates the magnitude of land subsidence. More continuous geologic structure causes propagation of maximum subsidence farther landward. This work suggests that coastal aquifers may be vulnerable to offshore pumping activities and that these effects should be considered in feasibility assessments for offshore drilling.
Estuaries are an important source of greenhouse gases to the atmosphere, but uncertainties remain in the flux rates and production pathways of greenhouse gases in these dynamic systems. This study performs simultaneous high resolution measurements of the three major greenhouse gases (carbon dioxide, methane, and nitrous oxide) as well as carbon stable isotope ratios of carbon dioxide and methane, above and below the pycnocline along a salt wedge estuary (Yarra River estuary, Australia). We identified distinct zones of elevated greenhouse gas concentrations. At the tip of salt wedge, average CO2 and N2O concentrations were approximately five and three times higher than in the saline mouth of the estuary. In anaerobic bottom waters, the natural tracer radon (222Rn) revealed that porewater exchange was the likely source of the highest methane concentrations (up to 1302 nM). Isotopic analysis of CH4 and CO2 showed a dominance of acetoclastic production in fresh surface waters and hydrogenotrophic production occurring in the saline bottom waters. The atmospheric flux of methane (in CO2 equivalent units) was a major (35-53%) contributor of atmospheric radiative forcing from the estuary, while N2O contributed only <2%. We hypothesize that the release of bottom water gases when stratification episodically breaks down will release large pulses of greenhouse gases to the atmosphere.
Vibrio are a genus of marine bacteria that have substantial environmental and human health importance, and there is evidence that their impact may be increasing as a consequence of changing environmental conditions. We investigated the abundance and composition of the Vibrio community within the Sydney Harbour estuary, one of the most densely populated coastal areas in Australia, and a region currently experiencing rapidly changing environmental conditions. Using quantitative PCR (qPCR) and Vibrio-specific 16S rRNA amplicon sequencing approaches we observed significant spatial and seasonal variation in the abundance and composition of the Vibrio community. Total Vibrio spp. abundance, derived from qPCR analysis, was higher during the late summer than winter and within locations with mid-range salinity (5-26 ppt). In addition we targeted three clinically important pathogens: Vibrio cholerae, V. Vulnificus, and V. parahaemolyticus. While toxigenic strains of V. cholerae were not detected in any samples, non-toxigenic strains were detected in 71% of samples, spanning a salinity range of 0-37 ppt and were observed during both late summer and winter. In contrast, pathogenic V. vulnificus was only detected in 14% of samples, with its occurrence restricted to the late summer and a salinity range of 5-26 ppt. V. parahaemolyticus was not observed at any site or time point. A Vibrio-specific 16S rRNA amplicon sequencing approach revealed clear shifts in Vibrio community composition across sites and between seasons, with several Vibrio operational taxonomic units (OTUs) displaying marked spatial patterns and seasonal trends. Shifts in the composition of the Vibrio community between seasons were primarily driven by changes in temperature, salinity and NO2, while a range of factors including pH, salinity, dissolved oxygen (DO) and NOx (Nitrogen Oxides) explained the observed spatial variation. Our evidence for the presence of a spatiotemporally dynamic Vibrio community within Sydney Harbour is notable given the high levels of human use of this waterway, and the significant increases in seawater temperature predicted for this region.
Information about spatial and temporal variability in the distribution and abundance of shark-populations are required for their conservation, management and to update measures designed to mitigate human-shark interactions. However, because some species of sharks are mobile, migratory and occur in relatively small numbers, estimating their patterns of distribution and abundance can be very difficult. In this study, we used a hierarchical sampling design to examine differences in the composition of species, size- and sex-structures of sharks sampled with bottom-set longlines in three different areas with increasing distance from the entrance of Sydney Harbour, a large urbanised estuary. During two years of sampling, we obtained data for four species of sharks (Port Jackson, Heterodontus portusjacksoni; wobbegong, Orectolobus maculatus; dusky whaler, Carcharhinus obscurus and bull shark, Carcharhinus leucas). Only a few O. maculatus and C. obscurus were caught, all in the area closest to the entrance of the Harbour. O. maculatus were caught in all seasons, except summer, while C. obscurus was only caught in summer. Heterodontus portusjacksoni were the most abundant species, caught in the entrance location mostly between July to November, when water temperature was below 21.5°C. This pattern was consistent across both years. C. leucas, the second most abundant species, were captured in all areas of Sydney Harbour but only in summer and autumn when water temperatures were above 23°C. This study quantified, for this first time, how different species utilise different areas of Sydney Harbour, at different times of the year. This information has implications for the management of human-shark interactions, by enabling creation of education programs to modify human behaviour in times of increased risk of potentially dangerous sharks.
Sydney Harbour is a global hotspot for marine and estuarine diversity. Despite its social, economic and biological value, the available knowledge has not previously been reviewed or synthesised. We systematically reviewed the published literature and consulted experts to establish our current understanding of the Harbour's natural systems, identify knowledge gaps, and compare Sydney Harbour to other major estuaries worldwide. Of the 110 studies in our review, 81 focussed on ecology or biology, six on the chemistry, 10 on geology and 11 on oceanography. Subtidal rocky reef habitats were the most studied, with a focus on habitat forming macroalgae. In total 586 fish species have been recorded from the Harbour, which is high relative to other major estuaries worldwide. There has been a lack of process studies, and an almost complete absence of substantial time series that constrains our capacity to identify trends, environmental thresholds or major drivers of biotic interactions. We also highlight a lack of knowledge on the ecological functioning of Sydney Harbour, including studies on microbial communities. A sound understanding of the complexity, connectivity and dynamics underlying ecosystem functioning will allow further advances in management for the Harbour and for similarly modified estuaries around the world.
Groundwater may be highly enriched in dissolved carbon species, but its role as a source of carbon to coastal waters is still poorly constrained. Exports of deep and shallow groundwater-derived dissolved carbon species from a small subtropical estuary (Korogoro Creek, Australia, latitude −31.0478°, longitude 153.0649°) were quantified using a radium isotope mass balance model (233Ra and 224Ra, natural groundwater tracers) under two hydrological conditions. In addition, air-water exchange of carbon dioxide and methane in the estuary was estimated. The highest carbon inputs to the estuary were from deep fresh groundwater in the wet season. Most of the dissolved carbon delivered by groundwater and exported from the estuary to the coastal ocean was in the form of dissolved inorganic carbon (DIC; 687 mmol m−2 estuary day−1; 20 mmol m−2 catchment day−1, respectively), with a large export of alkalinity (23 mmol m−2 catchment day−1). Average water to air flux of CO2 (869 mmol m−2 day−1) and CH4 (26 mmol m−2 day−1) were 5- and 43-fold higher, respectively, than the average global evasion in estuaries due to the large input of CO2- and CH4-enriched groundwater. The groundwater discharge contribution to carbon exports from the estuary for DIC, dissolved organic carbon (DOC), alkalinity, CO2, and CH4 was 22, 41, 3, 75, and 100 %, respectively. The results show that CO2 and CH4 evasion rates from small subtropical estuaries surrounded by wetlands can be extremely high and that groundwater discharge had a major role in carbon export and evasion from the estuary and therefore should be accounted for in coastal carbon budgets.
For decades, ecosystem scientists have used global warming potentials (GWPs) to compare the radiative forcing of various greenhouse gases to determine if ecosystems have a net warming or cooling effect on climate. On a conceptual basis, the continued use of GWPs by the ecological community may be untenable because the use of GWPs requires the implicit assumption that greenhouse gas emissions occur as a single pulse; this assumption is rarely justified in ecosystem studies. We present two alternate metrics – the sustained-flux global warming potential (SGWP, for gas emissions) and the sustained-flux global cooling potential (SGCP, for gas uptake) – for use when gas fluxes persist over time. The SGWP is generally larger than the GWP (by up to ~40%) for both methane and nitrous oxide emissions, creating situations where the GWP and SGWP metrics could provide opposing interpretations about the climatic role of an ecosystem. Further, there is an asymmetry in methane and nitrous oxide dynamics between persistent emission and uptake situations, producing very different values for the SGWP vs. SGCP and leading to the conclusion that ecosystems that take up these gases are very effective at reducing radiative forcing. Though the new metrics are more realistic than the GWP for ecosystem fluxes, we further argue that even these metrics may be insufficient in the context of trying to understand the lifetime climatic role of an ecosystem. A dynamic modeling approach that has the flexibility to account for temporally variable rates of greenhouse gas exchange, and is not limited by a fixed time frame, may be more informative than the SGWP, SGCP, or GWP. Ultimately, we hope this article will stimulate discussion within the ecosystem science community about the most appropriate way(s) of assessing the role of ecosystems as regulators of global climate.
Tide-induced residual circulation and
dispersion in Sydney Harbour has been studied with a vertically integrated,
two-dimensional, primitive equation model. The tide-induced residual
circulation consists of a series of recirculating gyres that are due to the
interaction of the tidal current with the complex coastal geometry and
bathymetry of the harbour. Inclusion of the S2 tide and
homogeneous wind stress in the input forcing does not significantly change the
residual circulation pattern. Residuals are produced mainly by the
M2 tide and there is no significant nonlinear
interaction between the M2 and the
S2 tide. Tracer simulations show that tidal mixing is
limited in the vicinity of the entrance and the flushing rates of different
segments of the harbour vary significantly. Lack of similarity between the
Eulerian and Lagrangian residual fields demonstrates that the net displacement
of material released during a particular phase of the tide has little
connection with the mean current observed at fixed locations and is extremely
sensitive to the timing and location of release.
The influence of heavy metals (copper, lead and zinc) associated with urban
runoff, on assemblages of macrofauna in intertidal soft sediments was studied
in two estuaries in the Sydney region. The patterns of distribution and
abundance of fauna and assemblages was found to vary significantly at several
spatial scales: within bays in an estuary, between bays within an estuary and
between bays from different estuaries. Significant differences were found in
concentrations of heavy metals in sediments, but there was very little
difference among bays in other environmental variables: grain-size
characteristics and organic matter content of sediments. Bays polluted by
heavy metals had significantly different assemblages to unpolluted bays, were
generally less diverse and were characterized by an order-of-magnitude greater
abundance of capitellids, spionids, nereids and bivalves. Unpolluted bays had
greater abundance of crustaceans and several polychaete families, including
paraonids and nephtyids and were generally more diverse. There was a
significant correlation between patterns of assemblages and concentrations of
heavy metals, but not with other environmental variables.
A new system for continuous, highly resolved oceanic and atmospheric measurements of N2O, CO and CO2 is described. The system is based upon off-axis integrated cavity output spectroscopy (OA-ICOS) and a non-dispersive infrared analyzer (NDIR), both coupled to a Weiss-type equilibrator. Performance of the combined setup was evaluated by testing its precision, accuracy, long-term stability, linearity and response time. Furthermore, the setup was tested during two oceanographic campaigns in the equatorial Atlantic Ocean in order to explore its potential for autonomous deployment onboard voluntary observing ships (VOS). Improved equilibrator response times for N2O (2.5 min) and CO (45 min) were achieved in comparison to response times from similar chamber designs used by previous studies. High stability of the OA-ICOS analyzer was demonstrated by low optimal integration times of 2 and 4 min for N2O and CO respectively, as well as detection limits of
We monitored submarine groundwater discharge (SGD) into the Werribee Estuary, Australia, using both chemical and physical methods. SGD occurred at hotspots where 222Rn persisted through a 12 month survey period. A significant correlation between 222Rn and (r2 = 0.8, p < 0.01), as well as between 222Rn and N2O (r2 = 0.6, p < 0.01) at a 222Rn hotspot, and much higher and N2O concentrations in groundwater relative to surface water suggest that elevated and N2O concentrations in the estuary were derived from SGD. Two sampling campaigns over 24 h revealed that variations of 222Rn,, and N2O were controlled by tide-induced hydraulic-head gradient fluctuations and, possibly to a much lesser extent, by tidal pumping and density-driven convection. A two-box 222Rn mass-balance model was used to calculate the rate of SGD into two different layers across the pycnocline of the estuary. A higher total groundwater discharge rate of 0.12 ± 0.09 m d−1 was observed in the surface layer during ebb tide compared with 0.10 ± 0.08 m d−1 during flood tide. Fluxes of groundwater-derived and N2O were higher than the riverine flux at baseflow by more than 30 fold and 20 fold, respectively. SGD derived fluxes exceeded the mean annual riverine and TN fluxes by a factor of ∼ 5 and ∼ 3 respectively. SGD-derived N2O fluxes were 170 µmol m−2 d−1, which are among the highest N2O fluxes observed in estuaries.
We carried out a literature overview to synthesize current knowledge on
CO2 and CH4 dynamics and fluxes with the atmosphere in estuarine
environments. Estuarine systems are highly dynamic in terms of carbon
cycling and emit CO2 to the atmosphere at rates that are quantitatively
significant for the global C cycle. This emission of CO2 to the
atmosphere is strongly supported by the net heterotrophic nature of
these ecosystems. The robustness of the evaluation of the emission of
CO2 from estuarine ecosystems has increased in last years due to
increasing data availability and improvements in the surface area
estimates by types. At present, the lack of sufficient data is the major
limitation in the quantification of the spatial and temporal variability
of CO2 fluxes in estuarine environments. Regarding future observations,
there is also a need for sustained measurements to unravel inter-annual
variability and long-term trends of CO2 and CH4 in estuarine
environments. Indeed, due to the strong linkage with river catchements,
inter-annual variability of CO2 and CH4 in estuarine environments is
expected to be strong. Data used in the present synthesis were either
obtained by the authors, data mined from publications or communicated by
colleagues. There is a need for publicly available and quality checked
data-bases for CO2 and CH4 in estuarine environments. Not only
cross-system meta-analysis of data (CO2, CH4, O2, …) can be
enlightening as explored in the present work, but also considering the
uncertainties in the evaluation of the gas transfer velocity, there
could be a need for future re-evaluations of air-water CO2 and CH4
fluxes, requiring access to the raw pCO2 and [CH4] data.
The air-sea exchanges of CO2 in the world's 165 estuaries and 87 continental shelves are evaluated. Generally and in all seasons, upper estuaries with salinities of less than two are strong sources of CO2 (39 ± 56 mol C m-2 yr-1, positive flux indicates that the water is losing CO2 to the atmosphere); mid-estuaries with salinities of between 2 and 25 are moderate sources (17.5 ± 34 mol C m-2 yr-1) and lower estuaries with salinities of more than 25 are weak sources (8.4 ± 14 mol C m-2 yr-1). With respect to latitude, estuaries between 23.5 and 50 N have the largest flux per unit area (63 ± 101 mmol C m-2 d-1); these are followed by lower-latitude estuaries (23.5-0 S: 44 ± 29 mmol C m-2 d-1; 0-23.5 N: 39 ± 55 mmol C m-2 d-1), and then regions north of 50 N (36 ± 91 mmol C m-2 d-1). Estuaries south of 50 S have the smallest flux per unit area (9.5 ± 12 mmol C m-2 d-1). Mixing with low- p CO2 shelf waters, water temperature, residence time and the complexity of the biogeochemistry are major factors that govern the p CO2 in estuaries, but wind speed, seldom discussed, is critical to controlling the air-water exchanges of CO2. The total annual release of CO2 from the world's estuaries is now estimated to be 0.10 Pg C yr-1, which is much lower than published values mainly because of the contribution of a considerable amount of heretofore unpublished or new data from Asia and the Arctic. The Asian data, although indicating high p CO2, are low in sea-to-air fluxes because of low wind speeds. Previously determined flux values rely heavily on data from Europe and North America, where p CO2 is lower but wind speeds are much higher, such that the CO2 fluxes are higher than in Asia. Newly emerged CO2 flux data in the Arctic reveal that estuaries there mostly absorb rather than release CO2.
Most continental shelves, and especially those at high latitude, are undersaturated in terms of CO2 and absorb CO2 from the atmosphere in all seasons. Shelves between 0 and 23.5 S are on average a weak source and have a small flux per unit area of CO2 to the atmosphere. Water temperature, the spreading of river plumes, upwelling, and biological production seem to be the main factors in determining p CO2 in the shelves. Wind speed, again, is critical because at high latitudes, the winds tend to be strong. Since the surface water p CO2 values are low, the air-to-sea fluxes are high in regions above 50 N and below 50 S. At low latitudes, the winds tend to be weak, so the sea-to-air CO2 flux is small. Overall, the world's continental shelves absorb 0.4 Pg C yr-1 from the atmosphere.
The spatial distribution of chemical contamination and toxicity of surficial sediments in Sydney Harbour, Australia, was investigated in a 3-tiered, hierarchical approach. An initial chemical investigation throughout the entire estuary (Stage 1) indicated wide ranges and different spatial patterns in sediment chemical concentrations. Sediment quality guidelines (SQGs) were used as a preliminary estimate of possible toxicity in Stage 2 of the investigation. Assessment of chemical mixtures indicated that sediments in a small part (similar to 2 %) of the harbour had the highest probability of being toxic (similar to 75 %), whereas sediment in almost 25 % of the port was estimated to have an intermediate (similar to 50 %) probability of being toxic. The SQG assessment in Stage 2 enabled careful stratification of the harbour into areas with different toxicity risks, reducing cost and time commitments in the final tier of assessment. The spatial survey carried out in Stage 3 involved concurrent chemical and ecotoxicological analyses. In this final stage, the degree of response in tests of amphipod survival in whole sediment samples, as well as in tests of microbial metabolism (Microtox (c)) and sea urchin egg fertilisation and embryo development in pore waters, generally increased with increasing chemical concentrations. However, amphipod response was lower than predicted due to relatively low sensitivity of the indigenous amphipod Corophium colo. Areas initially predicted, using SQGs, to be most at risk were highly toxic in the combined chemistry-toxicity investigation, while sediment from areas with the lowest predicted risk were least toxic, but still toxic in at least one ecotoxicological test. The results demonstrate that the empirical approach used for this study, which was originally developed in North America, produced plausible outcomes elsewhere and that observed toxicity, based on SQGs, matched predictions using different species but similar methodologies.
Epibiotic assemblages have been shown previously to differ between pontoons and rocky reefs. This may occur for a variety of reasons, one of which is that pontoons move, whereas reefs do not. Effects of movement of the substratum are particularly pertinent to studies involving settlement panels because these experimental units are often suspended from pontoons such that they can move up and down, or attached to ropes such that they can rotate. The development of epibiotic assemblages on panels that were fixed, moved up and down, or rotated, was studied to test hypotheses about the effects of movement on sessile organisms. The covers of barnacles, sponges and ascidians increased greatly with increasing movement and/or rotation of the substratum, whereas species of red and brown foliose algae and tubiculous polychaetes generally decreased with increasing movement and/or rotation of the substratum. In general, assemblages on panels that moved up and down were most similar to those on rotating panels. One of the most obvious differences was the 2 to 3-fold greater biomass on rotating panels compared to fixed panels, which was attributed to the greater abundance of barnacles. It was concluded that, although movement of a surface can influence the composition of assemblages, this factor on its own may explain only some of the differences reported between pontoons and reefs. Furthermore, the method of deployment of settlement panels was shown to have great effects on the types of assemblages that develop. Differences in water flow were proposed to explain some of the observed patterns, but this idea needs to be investigated further.
A multidisciplinary approach was taken to assess the potential importance of groundwater seepage to nutrient inputs into Manila Bay, Philippines. Three lines of seepage meters were installed in transects along the coast at Mariveles, Bataan Province, during the period 8-10 January 2005. The overall average seepage flux was 5.1 +/- 5.4 cm d(-1) (n= 73) with a range of 0-26 cm d(-1) and a calculated integrated shoreline flux of 12.4 m(3) m(-1) d(-1). Additional methodologies employed included automatic seepage meters, resistivity measurements, sampling for nutrient analyses in both seepage meters and ambient seawater, and use of natural radon as a groundwater tracer. Seepage meter and tracer results provided consistent results of estimates of submarine groundwater discharge into Manila Bay. Many lines of evidence suggest that seepage fluxes are not steady state but are modulated by the tides. Resistivity profiles show that the saline-freshwater interface moves on a tidal timescale, consistent with the observed drop in salinity of the seepage waters as low tide approaches. Our results show that dissolved inorganic nitrogen (DIN) fluxes via submarine groundwater discharge are comparable in magnitude to DIN fluxes from each of the two major rivers that drain into Manila Bay.
Runoff from the urban environment is a major contributor of non-point source contamination for many estuaries, yet the ultimate fate of this stormwater within the estuary is frequently unknown in detail. The relationship between catchment rainfall and estuarine response within the Sydney Estuary (Australia) was investigated in the present study. A verified hydrodynamic model (Environmental Fluid Dynamics Computer Code) was utilised in concert with measured salinity data and rainfall measurements to determine the relationship between rainfall and discharge to the estuary, with particular attention being paid to a significant high-precipitation event. A simplified rational method for calculating runoff based upon daily rainfall, subcatchment area and runoff coefficients was found to replicate discharge into the estuary associated with the monitored event. Determining fresh-water supply based upon estuary conditions is a novel technique which may assist those researching systems where field-measured runoff data are not available and where minor field-measured information on catchment characteristics are obtainable.The study concluded that since the monitored fresh-water plume broke down within the estuary, contaminants associated with stormwater runoff due to high-precipitation events (daily rainfall > 50 mm) were retained within the system for a longer period than was previously recognised.
Dissolved CH4, N2O, O2, and inorganic nitrogen nutrients (NH4 +, NO3 � and NO2�) were measured over tidal
cycles in pristineWright Myo mangrove creek waters during dry and wet seasons. Dissolved CH4 and N2O showed no seasonality (dry season; 491 ± 133 nmol CH4 l�1, 9.0 ± 2.3 nmol N2O l�1, wet season; 466 ± 94 nmol CH4 l�1, 8.6 ± 1.3 nmol N2O l�1). Creek water dissolved gas and inorganic nitrogen distributions reflect sediment porewater release during hydrostatic pressure drop toward low water. Creek water CH4 emission was suppressed by oxidation during rainfall, consistent with changes to dissolved nitrogen speciation, although N2O emissions were unaffected. Scaling up emissions flux estimates from mangrove creek waters and intertidal sediment gives worldwide mangrove emissions�1.3�1011 mol CH4 yr�1 and 2.7�109 mol N2O yr�1; mangrove ecosystems are thus small contributors to coastal N2O emissions but could dominate coastal CH4
emissions. Comparing our data with mangrove CO2 fluxes, mangrove ecosystems could be small net contributors of atmospheric greenhouse gases.
Coastal wetlands can have exceptionally large carbon (C) stocks and their protection and restoration would constitute an effective mitigation strategy to climate change. Inclusion of coastal ecosystems in mitigation strategies requires quantification of carbon stocks in order to calculate emissions or sequestration through time. In this study, we quantified the ecosystem C stocks of coastal wetlands of the Sian Ka'an Biosphere Reserve (SKBR) in the Yucatan Peninsula, Mexico. We stratified the SKBR into different vegetation types (tall, medium and dwarf mangroves, and marshes), and examined relationships of environmental variables with C stocks. At nine sites within SKBR, we quantified ecosystem C stocks through measurement of above and belowground biomass, downed wood, and soil C. Additionally, we measured nitrogen (N) and phosphorus (P) from the soil and interstitial salinity. Tall mangroves had the highest C stocks (987±338 Mg ha) followed by medium mangroves (623±41 Mg ha), dwarf mangroves (381±52 Mg ha) and marshes (177±73 Mg ha). At all sites, soil C comprised the majority of the ecosystem C stocks (78-99%). Highest C stocks were measured in soils that were relatively low in salinity, high in P and low in N∶P, suggesting that P limits C sequestration and accumulation potential. In this karstic area, coastal wetlands, especially mangroves, are important C stocks. At the landscape scale, the coastal wetlands of Sian Ka'an covering ≈172,176 ha may store 43.2 to 58.0 million Mg of C.
We measured the flux of CO2 across the air-water interface using the floating chamber method in three European estuaries with contrasting physical characteristics (Randers Fjord, Scheldt, and Thames). We computed the gas transfer velocity of CO2 (k) from the CO2 flux and concomitant measurements of the air-water gradient of the partial pressure of CO2 (pCO(2)). There was a significant linear relationship between k and wind speed for each of the three estuaries. The differences of the y-intercept and the slope between the three sites are related to differences in the contribution of tidal currents to water turbulence at the interface and fetch limitation. The contribution to k from turbulence generated by tidal currents is negligible in microtidal estuaries such as Randers Fjord but is substantial, at low to moderate wind speeds, in macrotidal estuaries such as the Scheldt and the Thames. Our results clearly show that in estuaries a simple parameterization of k as a function of wind speed is site specific and strongly suggest that the y-intercept of the linear relationship is mostly influenced by the contribution of tidal currents, whereas the slope is influenced by fetch limitation. This implies that substantial errors in flux computations are incurred if generic relationships of the gas transfer velocity as a function of wind speed are employed in estuarine environments for the purpose of biogas air-water flux budgets and ecosystem metabolic studies.
Sydney’s Harbour is an integral part of the city providing natural, social, and economic benefits to 4.84 million residents. It has significant environmental value including a diverse range of habitats and animals. A range of anthropogenic and environmental pressures threatens these including loss and modification of habitats, oversupply of nutrients and introduction of pollutants such as metals, organics, and microplastics, introduction of non-indigenous species and the impacts of recreational fishing. Many people now recognise not only the environmental value of Sydney Harbour, but also the economic and social benefits a healthy harbour provides. Over 80% of residents recognise the importance of maintaining a pollution-free coastal environment and conserving the Harbour’s abundant and diverse marine life. A recent review gathered information to make some first estimates of economic revenues and values associated with Sydney Harbour. Port and maritime revenues ($430 million/yr), ferries ($175 million/yr), cruise ship expenditure ($1025 million/yr), major foreshore events such as New Year’s Eve and the Sydney Festival ($400 million/yr), and also income from culture, heritage, arts and science (over $33 million/yr) inject considerable funds into the Australian economy. Notably, proximity to the harbour enhances Sydney domestic real estate capital by an estimated $40 billion, equivalent to $3775 million/yr and biological ecosystem services were valued at $175 million/yr. Here we provide i) a synthesis of our current understanding of the natural, social, and economic resources of Sydney Harbour, ii) the threats and pressures these resources face, and finally iii) how a new marine management framework is being used to address these threats to the natural, social and economic wellbeing of Sydney Harbour. This review clearly shows that Sydney Harbour is a valuable and valued environment that deserves continuing scientific, social, and economic research to support management now and in the future.
We combined observations of the natural tracer radon (222Rn) with hydrodynamic models across a broad latitudinal gradient covering several climate zones to estimate porewater exchange rates in mangroves. Porewater exchange ranged from 2.1 to 35.5 cm day-1 from temperate to tropical regions and averaged 16.3 ± 5.1 cm day-1. If upscaled to the global weighted mangrove area, porewater exchange in mangroves would recirculate the entire volume of water overlying the continental shelf in less than 153 years. Although porewater exchange (recirculated seawater) and river discharge represent different pathways for water entering the coastal ocean, the estimated global mangrove porewater exchange is equal to approximately one-third of annual global river discharge to the ocean (3.84 x 1013 m3 yr-1). Because biogeochemical processes in mangroves are largely dependent on porewater exchange, these large exchange rates have major implications for coastal nutrient, carbon, and greenhouse gas cycling in tropical marine systems.
Combining air-water equilibrators with a field deployable cavity enhanced laser absorption spectrometer
(CELAS) can generate precise, high resolution, measurements of dissolved CO2 and CH4 concentrations and
d13C values in aquatic systems. However, equilibration response times for combined measurements of CO2
and CH4 isotopologues have not been assessed. We performed laboratory step experiments on six different
equilibrators to constrain CO2 and CH4 equilibration time constants (tau; high-to-low exponential decay constant).
Three equilibrator types were then used in field-based step experiments to determine tau for the individual
isotopologues 12CO2, 13CO2, 12CH4, and 13CH4. In the laboratory experiments, tau ranged from 34–124 s
for CO2 and 117–2041 s for CH4 among the six equilibrators. The ratio between response times of CO2 and
CH4 was substantially lower in the membrane type equilibrators (tau-CH4 ~5 times>tau-CO2) than in the showerhead and marble types (tau-CH4 ~15 times>tau-CO2). Individual isotopologue time constants under a water flow rate of ~5.5 L min-1 ranged from 33.7–43.1 s for 12CO2 and 13CO2, and 177–347 s for 12CH4, and
13CH4. The tau of CO2 isotopologues were within 1 s while tau of CH4 isotopologues were the same. Further
investigations into water flow rate revealed an exponential decrease in equilibration time from 1.5 L min-1
to 9 L min-1 in a marble-type equilibrator. The response time was always longer from high-to-low than low to-
high concentrations. By taking into consideration the equilibration response time, measurements of CO2,
CH4, d13C-CO2, and d13C-CH4 can be resolved in near real-time using appropriate water-air equilibration
devices.
Acrylic fibers impregnated with MnO2 (Mn-fiber) have become a valuable tool for concentrating dissolved radium for oceanographic applications. With four naturally-occurring radium isotopes (223Ra, 224Ra, 226Ra, and 228Ra) of vastly different half-lives (3.6 days to 1600 years), radium can be a powerful tool for tracing terrestrial water discharges into the ocean and studying coastal mixing processes. Several techniques have been outlined in the literature describing the measurement of 226Ra on Mn-fiber via its gaseous daughter, 222Rn. We present a proven, air-tight cartridge design that allows one to use these measurement techniques. We then review the procedures for three radon-based nondestructive measurement techniques for 226Ra on Mn-fiber (via RAD7, RaDeCC, and Rn emanation line systems) and perform an intercomparison among them, using the standard technique of γ-spectrometry as a reference. We find that all methods statistically agree in terms of measured activity. The Rn emanation line and the RaDeCC systems (both based on Lucas cell counting) provide the lowest measurement uncertainties and minimum detectable activities (MDAs) for a given counting time. The RAD7 technique, on the other hand, offers the advantage of being an automated system, thus requiring minimal user interaction. The standard γ-spectrometry technique, while more time-consuming and sample destructive, has the advantage of providing a simultaneous measurement for 228Ra.
Sydney estuary has a long history of environmental degradation and is one of the most modified water ways in Australia due to a highly urbanised catchment (~77 %) and a high population (4.6 million). The objectives of the present study were to map historical land use change from European settlement (1788) to 2010 to determine catchment evolutionary pathways and to estimate catchment loading (total suspended solids, Cu, Pb and Zn) to the estuary over this period. Land use distribution in Sydney catchment, determined for seven time horizons over this period, indicated that a substantial increase in residential land use through subdivision of large estates and an increase in road area resulted in a marked increase in metal loading to Sydney estuary between 1892 and 1936. The decline in industrial activity from a maximum in 1978 (3.9 %) to 1.8 % in 2010 and the introduction of unleaded fuel during this time was accompanied by reduction in metal loading to the estuary. Land use time horizon maps enabled the creation of novel, ternary diagrams to represent temporal evolution in catchment land use. The 15 sub-catchments of Sydney estuary were combined into three major catchment categories, i.e., urban, dense urban and commercial. Present-day annual discharge of stormwater from the Sydney catchment was calculated to be 466,000 ML and annual loadings of total suspended sediment (TSS), Cu, Pb and Zn in tonnes were 49,239, 27, 37 and 57, respectively. Stormwater has superseded industry as the main source of anthropogenic metals to this estuary in recent times.
Research performed in many locations over the past decade has shown that radon is an effective tracer for quantifying submarine groundwater discharge (SGD). The technique works because both fresh and saline groundwaters acquire radon from the subterranean environment and display activities that are typically orders of magnitude greater than those found in coastal seawaters. However, some uncertainties and unanswered problems remain. We focus here on three components of the mass balance, each of which has some unresolved issues: (1) End-member radon - what to do if groundwater Rn measurements are highly variable? (2) Atmospheric evasion -do the standard gas exchange equations work under high-energy coastal mixing scenarios? And (3) "mixing" losses - are there other significant radon losses (e.g. recharge of coastal waters into the aquifer) besides those attributed to mixing with lower-activity waters offshore? We address these issues using data sets collected from several different types of coastal environment.
We quantified groundwater discharge and associated nutrient fluxes to Monterey Bay, California, during the wet and dry seasons using excess (224)Ra as a tracer. Bioassay incubation experiments were conducted to document the response of bloom-forming phytoplankton to submarine groundwater discharge (SGD) input. Our data indicate that the high nutrient content (nitrate and silica) in groundwater can stimulate the growth of bloom-forming phytoplankton. The elevated concentrations of nitrate in groundwater around Monterey Bay are consistent with agriculture, landfill, and rural housing, which are the primary land-uses in the area surrounding the study site. These findings indicate that SGD acts as a continual source of nutrients that can feed bloom-forming phytoplankton at our study site, constituting a nonpoint source of anthropogenic nutrients to Monterey Bay.
Link to text: https://websites.pmc.ucsc.edu/~apaytan/publications/2015_Articles/Lecher%202015%20MB.pdf
Automated cavity ring down spectroscopy was used to make continuous measurements of dissolved methane, nitrous oxide and carbon dioxide in a coral reef lagoon for two weeks (Heron Island, Great Barrier Reef). Radon (222Rn) was used to trace the influence of tidally-driven porewater exchange on greenhouse gas dynamics. Clear tidal variation was observed for CH4, which correlated to 222Rn in lagoon waters. N2O correlated to 222Rn during the day only, which appears to be a response to coupled nitrification-denitrification in oxic sediments, fuelled by nitrate derived from bird guano. The lagoon was a net source of CH4 and N2O to the atmosphere and a sink for atmospheric CO2. The estimated porewater-derived CH4 and N2O fluxes were 3.2- and 24.0-fold greater than the fluxes to the atmosphere. Overall, porewater and/or groundwater exchange were the only important sources of CH4 and a major control of N2O in the coral reef lagoon.
Automated in situ instrumentation captured high-resolution surface water pCO2, CH4 and 222Rn data at the creek mouth, and ~ 500m upstream in a sub-tropical mangrove ecosystem (Southern Moreton Bay, Australia, S27.78°, E153.38°) over a spring-neap-spring tidal cycle (~ 15 days) during November 2013. The partial pressure of CO2 (pCO2) ranged from 385 to 26106 µatm, CH4 from 1.8 to 889 nM, and 222Rn from 280 to 108172 dpm m-3. Average surface water pCO2, CH4 and 222Rn were 4-fold higher at the upstream station. Surface water fluxes of CO2 and CH4 ranged from 9.4 to 629.2 mmol CO2 m-2 d-1 and 13.1 to 632.9 μmol CH4 m-2 d-1 depending upon the gas transfer model used and station location. Creek pCO2, CH4 and 222Rn displayed changes over both semi-diurnal and spring-neap-spring tidal scales. Semi-diurnally, all gases had a significant inverse relationship with water depth. Over the spring-neap-spring cycle, all gases exhibited an inverse relationship with tidal amplitude, with higher values during neap tides than spring tides. Estimated fluxes, porewater observations, and the significant positive relationship between surface water pCO2 and CH4, and 222Rn suggests groundwater exchange (i.e. tidal pumping) drives pCO2 and CH4 within the mangrove creek. We hypothesize that a combination of hourly and weekly groundwater-surface water exchange processes drive surface water pCO2 and CH4 in mangrove creeks. Semi-diurnally, flushing of crab burrows leads to high pCO2 and CH4 concentrations at low tide. During the spring-neap-spring cycle, older groundwater enriched in CO2, CH4 and 222Rn seeps into the creek as tidal amplitude decreases, leading to higher concentrations at neap tides.
Floods frequently produce deoxygenation and acidification in waters of artificially drained coastal acid sulfate soil (CASS) wetlands. These conditions are ideal for carbon dioxide and methane production. We investigated CO2 and CH4 dynamics and quantified carbon loss within an artificially drained CASS wetland during and after a flood. We separated the system into wetland soils (inundated soil during flood and exposed soil during post flood period), drain water and creek water and performed measurements of free CO2 ([CO2*]), CH4, dissolved inorganic and organic carbon (DIC and DOC), stable carbon isotopes, and radon (222Rn: natural tracer for groundwater discharge) to determine aquatic carbon loss pathways. [CO2*] and CH4 values in the creek reached 721 μM and 81 μM respectively two weeks following a flood during a severe deoxygenation phase (dissolved oxygen ~ 0% saturation). CO2 and CH4 emissions from the floodplain to the atmosphere were 17-fold and 170-fold higher during the flooded period compared to the post-flood period, respectively. CO2 emissions accounted for about 90% of total floodplain mass carbon losses during both the flooded and post-flood periods. Assuming a 20- and 100-year global warming potential (GWP) for CH4 of 105 and 27, CH4 emission contributed to 85% and 60% of total floodplain CO2-equivalent emissions respectively. Stable carbon isotopes (δ13C in dissolved CO2 and CH4) and 222Rn indicated that carbon dynamics within the creek were more likely driven by drainage of surface floodwaters from the CASS wetland rather than groundwater seepage. This study demonstrated that >90% of CO2 and CH4 emissions from the wetland system occurred during the flood period and that the inundated wetland was responsible for ~95% of CO2-equivalent emissions over the floodplain.
The impact of groundwater on pCO2 variability was assessed in two coral reef lagoons with distinct drivers of submarine groundwater discharge (SGD). Diel variability of pCO2 in the two ecosystems was explained by a combination of biological drivers and SGD inputs. In Rarotonga, a South Pacific volcanic island, SGD was driven primarily by a steep terrestrial hydraulic gradient, and the water column was influenced by the high pCO2 (5,501 μatm) of the fresh groundwater. In Heron Island, a Great Barrier Reef coral cay, SGD was
dominated by seawater recirculation in permeable sediments (i.e. tidal pumping) and pCO2 was mainly impacted through the stimulation of biological processes. The Rarotonga water column had a relatively higher average pCO2 (549 μatm) than Heron Island (471 μatm), however, pCO2 exhibited a greater diel range in Heron Island (778 μatm) than in Rarotonga (507 μatm). SGD flux rates were quantified using a radon (222Rn) mass balance. The Rarotonga water column received 29.0 ± 8.2 mmol free-CO2 m-2 d-1 from SGD, while the Heron Island water column received 12.1 ± 4.2 mmol free-CO2 m-2 d-1. Both systems were sources of carbon dioxide to the atmosphere (averaging 8.8 ± 3.4 and 2.5 ± 2.1 mmol CO2 m-
2 d-1 in Rarotonga and Heron Island, respectively), with SGD-derived free-CO2 most likely contributing to the outgassing of CO2. Studies measuring the metabolism of coral reefs via changes in carbonate chemistry (e.g. photosynthesis, respiration, calcification, and calcium carbonate (CaCO3) dissolution rates) may need to consider the effects of groundwater seepage on water column carbonate chemistry and greenhouse gas evasion. Local drivers of coral reef carbonate chemistry such as SGD may offer more approachable management solutions to mitigating the effects of ocean acidification (OA) on coral reefs.
Multiple techniques, including thermal infrared aerial remote sensing, geophysical and geological data, geochemical characterization and radium isotopes, were used to evaluate the role of groundwater as a source of dissolved nutrients, carbon, and trace gases to the Okatee River estuary, South Carolina. Thermal infrared aerial remote sensing surveys illustrated the presence of multiple submarine groundwater discharge sites in Okatee headwaters. Significant relationships were observed between groundwater geochemical constituents and 226Ra activity in groundwater with higher 226Ra activity correlated to higher concentrations of organics, dissolved inorganic carbon, nutrients, and trace gases to the Okatee system. A system-level radium mass balance confirmed a substantial submarine groundwater discharge contribution of these constituents to the Okatee River. Diffusive benthic flux measurements and potential denitrification rate assays tracked the fate of constituents in creek bank sediments. Diffusive benthic fluxes were substantially lower than calculated radium-based submarine groundwater discharge inputs, showing that advection of groundwater-derived nutrients dominated fluxes in the system. While a considerable potential for denitrification in tidal creek bank sediments was noted, in situ denitrification rates were nitrate-limited, making intertidal sediments an inefficient nitrogen sink in this system. Groundwater geochemical data indicated significant differences in groundwater chemical composition and radium activity ratios between the eastern and western sides of the river; these likely arose from the distinct hydrological regimes observed in each area. Groundwater from the western side of the Okatee headwaters was characterized by higher concentrations of dissolved organic and inorganic carbon, dissolved organic nitrogen, inorganic nutrients and reduced metabolites and trace gases, i.e. methane and nitrous oxide, than groundwater from the eastern side. Differences in microbial sulfate reduction, organic matter supply, and/or groundwater residence time likely contributed to this pattern. The contrasting features of the east and west sub-marsh zones highlight the need for multiple techniques for characterization of submarine groundwater discharge sources and the impact of biogeochemical processes on the delivery of nutrients and carbon to coastal areas via submarine groundwater discharge.
Widespread sulphidic deposits have accumulated in tropical coastal floodplains throughout the world. Sulphidic soils oxidize when floodplains are drained for urban and agricultural development. As a result, large amounts of sulphuric acid may be released to nearby waterways. Macropores may create excellent conditions for groundwater flow in coastal acid sulphate soils (CASS). An automated radon ((222)Rn) measurement system was used to quantify groundwater inputs into a tidally-dominated estuary that is known to be influenced by acid discharges from CASS (Richmond River Estuary, Australia). A high resolution radon survey along a 120-km long segment of the tidal river identified two areas of preferential groundwater inputs. Intensive time series measurements in one of those areas (the Tuckean Broadwater) demonstrated that groundwater inputs are highly variable over hourly and seasonal time scales and inversely related to surface water pH. Elevated radon concentrations (up to 12 dpm/L) and low pH (as low as 3.3) were observed in surface waters at low tide a few weeks after a large rain event. These results demonstrate that acidic waters are entering the estuary via tidally-modulated groundwater flow pathways. Groundwater discharge rates into drains in the Tuckean Swamp were estimated from a dual-assumption radon mass balance to be 0.09-0.16 and 0.56-0.89 m(3) s(-1) during the dry and wet season, respectively (or 6-10 and 37-59 cm/day if the area is taken into account). While surface runoff increased only 2-fold in the wet season relative to the dry season, groundwater discharge rates increased similar to 6-fold. Since groundwater can be a major driver of surface water quality, radon can be useful in CASS monitoring and management efforts.
We used 222Rn and 224Ra measured in the Upper Gulf of Thailand to estimate 222Rn atmospheric evasion across the water-air interface. The Chao Phraya estuary represents a steady-state source of these tracers and we looked at their horizontal distribution on a transect leading away from the estuary into the Gulf. The isotopes 222Rn and 224Ra have very similar half-lives and are affected in the same manner by mixing processes but only radon will emanate to the atmosphere. We thus are able to estimate the radon air-water exchange rate from the difference in the slopes of the 222Rn and 224Ra horizontal distributions. Estimated gas exchange velocities (k) based on our results were 1.1 cm hr−1 in the dry season (January 2004) and 2.1 cm hr−1 during the wet season (July 2004). These rates agreed reasonably well with some theoretical models developed for lakes, estuaries and coastal systems.
Using recent laboratory and field results we explore the possibility of a cubic relationship between gas exchange and instantaneous (or short-term) wind speed, and its impact on global air-sea fluxes. The theoretical foundation for such a dependency is based on retardation of gas transfer at low to intermediate winds by surfactants, which are ubiquitous in the world's oceans, and bubble-enhanced transfer at higher winds. The proposed cubic relationship shows a weaker dependence of gas transfer at low wind speed and a significantly stronger dependence at high wind speed than previous relationships. A long-term relationship derived from such a dependence, combined with the monthly CO2 climatology of Takahashi [1997], leads to an increase in the global annual oceanic CO2 uptake from 1.4 Gigaton Cyr-1 to 2.2 Gigaton Cyr-1. Although a cubic relationship fits within global bomb-14C oceanic uptake constraints, additional checks are warranted, particularly at high wind speeds where the enhancement is most pronounced.
Measured Bunsen solublllty coefficients reported In the
ilterature are used to derlve functlons that permlt accurate
calculatlon of the concentratlon of methane, carbon
monoxide, and hydrogen In water and sea water at
equilibrium wlth the normal atmosphere. Bunsen
coefflclents are fltted to equations establlshed by Welss
which give Bunsen coefflclents as functions of
temperature and sailnlty. Tables of Bunsen coefficients
coverlng the temperature range -2 to +30 O C and the
saiinlty range 0-40 parts per thousand are calculated for
each gas from the fltted equatlons. The data are also
fltted to an atmospheric equilibrlum solublllty function,
which has a form slmilar to the Bunsen coefficient
equation, but which Includes the atmospheric gas
concentratlon as a variable. Coefficients for thls equation
are glven to allow calculatlon of the concentratlon of
dlssolved methane, carbon monoxide, and hydrogen in
equliibrium wlth moist air at 1 atm total pressure In unlts
of nL/L, nmol/L, nL/kg, and nmol/kg sea water.
The solubility of nitrous oxide in pure water and seawater has been measured microgasometrically over the range 0–40°C. The data have been corrected for nonideality and are fitted to equations in temperature and salinity of the form used previously to fit the solubilities of other gases. The fitted values have a precision of 0.1% and an estimated accuracy of 0.3%. The nonideal behavior of nitrous oxide—air mixtures is discussed, and the solubility of atmospheric nitrous oxide is presented in parametric form. A similar parametric representation for the solubility of atmospheric carbon dioxide is given in the Appendix.
New measurements of the solubility of carbon dioxide in water and seawater confirm the accuracy of the measurements of Murray and Riley, as opposed to those of Li and Tsui. Corrections for non-ideal behavior in the gas phase and for dissociation in distilled water are required to calculate solubility coefficients from these sets of data. Equations for the solubilities of real gases are presented and discussed. Solubility coefficients for carbon dioxide in water and seawater are calculated for the data of Murray and Riley, and are fitted to equations in temperature and salinity of the form used previously to fit the solubilities of other gases.
Using radium (Ra) isotopes and nutrient analy- ses, we found that submarine groundwater discharge (SGD) is an important source of ‘‘new’’ nutrients, particularly nitrogen, to coral reefs around the world. Nitrogen input estimates as- sociated with SGD range from 3 to 800 mmol h1 per meter of shoreline. The use of Ra isotopes allows us to quantify the inorganic nitrogen input from this source of nutrients. Increas- ing coastal population and land use practices may enhance anthropogenic nutrient loading from submarine groundwater contributing to reef degradation.
The relationship between gas transfer velocity and wind speed was evaluated at low wind speeds by quantifying the rate of evasion of the deliberate tracer, SF6, from a small oligotrophic lake. Several possible relationships between gas transfer velocity and low wind speed were evaluated by using 1-min-averaged wind speeds as a measure of the instantaneous wind speed values. Gas transfer velocities in this data set can be estimated virtually equally well by assuming any of three widely used relationships between k600 and winds referenced to 10-m height, U10: (1) a bilinear dependence with a break in the slope at ∼3.7 m s-1, which resulted in the best fit; (2) a power dependence; and (3) a constant transfer velocity for U10 < ∼3.7 m s-1, with a linear dependence on wind speed at higher wind speeds. The lack of a unique relationship between transfer velocity and wind speed at low wind speeds suggests that other processes, such as convective cooling, contribute significantly to gas exchange when the wind speeds are low. All three proposed relationships clearly show a strong dependence on wind for winds >3.7 m s-1 which, coupled with the typical variability in instantaneous wind speeds observed in the field, leads to average transfer velocity estimates that are higher than those predicted for steady wind trends. The transfer velocities predicted by the bilinear steady wind relationship for U10 < ∼3.7 m s-1 are virtually identical to the theoretical predictions for transfer across a smooth surface.
Conventional ground water sampling methods are often expensive and lack the sampling resolution required to categorize geochemical processes in surficial aquifers. Here, we describe the application of a commercially available gas vapor sampling probe to ground water sampling in shallow coastal aquifers. The system involves a small-diameter, shielded-screen well-point sampler that can provide high resolution (∼10 cm) to depths of 10 m or greater depending on the aquifer matrix. Examples of the system’s utility are presented for two geologically contrasting environments (Waquoit Bay, Massachusetts, USA; Ubatuba, Brazil). In Waquoit Bay, the sampler allowed us to resolve a sharp saline transition zone (∼2 m) and the associated changes in nutrients and trace metals that were associated with biogeochemical reactions at this interface. The system also proved useful in Ubatuba, Brazil, where small-scale gradients in sediment composition and permeability controlled the ground water salinity profile.
Direct measurements of submarine groundwaterdischarge (SGD) were taken by three different(continuous heat, heat pulse, and ultrasonic)types of automated seepage meters as well asstandard Lee-type manually operated meters. SGD flux comparisons and the spatial andtemporal variations in groundwater flow wereanalyzed. Seepage rates measured by thedifferent meters agree relatively well witheach other (more than 80% agreement). Comparisons of flux rates as a function ofdistance offshore using exponentialapproximations show that more than fivemeasurement locations (200 m offshore) areneeded for a precise integrated estimation ofSGD offshore within an accuracy of 10%. Thedominant period of seepage variations isestimated to be about 12 hours, which closelymatches the semidiurnal tides in this area. Our analysis also shows that short durationmeasurement periods may cause significantunderestimates or overestimates of the dailyaveraged groundwater flow rates (25%–60% difference when the measurement durationis less than 12 hours). Thus, continuousmeasurements of SGD using automated seepagemeters with high time resolution should enableus to evaluate temporal and spatial variationsof dissolved material transports viagroundwater pathways. Such inputs may affectbiogeochemical phenomena in the coastal zone.
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.