Groundwater-driven nutrient inputs to coastal lagoons: The relevance of lagoon water recirculation as a conveyor of dissolved nutrients

The connection between Submarine Groundwater Discharge and seawater quality: The threat of treated wastewater injected into coastal aquifers

Submarine Groundwater Discharge (SGD) delivers nutrients to the coastal sea triggering phytoplankton blooms, eutrophication, and can also serve as a pathway for contaminants. Wastewater treatment plants (WWTP) including injection wells in coastal areas influence coastal aquifers and might impact the composition and magnitude of SGD fluxes. In tourist areas, wastewater treatment may be less efficient and larger in volume during high seasons, potentially impacting nutrient fluxes from SGD and exacerbating environmental impacts. This study analyzes the nutrient transfer from treated wastewater injection in karstic aquifers to the coastal sea via SGD, considering the impacts of tourism seasonality. This study is conducted in Cala Deià, a small cove in the Balearic Islands, a Mediterranean tourist destination. The findings suggest that the seasonality of tourism, leading to variations in the volume of wastewater treated in the WWTP, influences the dynamics of the coastal aquifer. This leads to increased SGD water and nutrient fluxes to the sea in summer, i.e. the peak tourist season. The measured DIN, DIP, and DSi inventories in the cove are much larger in August than in April (3, 10, and 1.5 times higher, respectively) due to higher input of nutrients in summer due to SGD impacted by the WWTP. These elevated nutrient flows can support algal blooms in the cove, compromising water quality for local swimmers and tourists. Indeed, in August, shoreline stations exhibited eutrophic Chl-a concentrations, with peaks reaching approximately 4 mg Chl-a L-1. These elevated levels suggest the presence of an algal bloom during the survey. The anthropogenic origin of SGD-driven nutrients is traced in seawater and seagrass meadows, as evidenced by high ∂15N signatures indicative of polluted areas. Thus, the high pressure exerted on coastal areas by tourism activities increased the magnitude of SGD nutrient fluxes, thereby threatening coastal ecosystems and the services they provide.

Variability of the trophic state in a coastal reef system associated with submarine groundwater discharge in the Mexican Caribbean

Submarine groundwater discharges (SGD) have been associated with important sources of nutrients between the land and oceans that can generate eutrophication conditions. This study aims to analyze the behavior of nitrogen and phosphorus using the mixing curve method, to examine the variation of the trophic state using the Karydis Index, and to evaluate the δ15N in benthic organisms to trace the origin of nitrogen in neap tide (November) and spring tide (January) in the Manatí Cenote, and Nohoch-Teek reef lagoon in the Mexican Caribbean. Nitrogen and phosphate enrichment was in the Manatí Cenote during neap and spring tides. This enrichment was particularly noticeable in the reef lagoon during low tides in the areas influenced by SGD. In the Cenote, differences in the nitrate trophic state were observed, indicating an eu-mesotrophic condition during neap tide and a mesotrophic condition during spring tide. However, no significant differences were observed for ammonium (oligo-mesotrophic), nitrites, or phosphate compounds (oligotrophic). The trophic state reef lagoon exhibited a similar pattern but with different spatial variations. In both systems, phosphorus was a limiting nutrient, while δ15N suggested anthropogenic nitrogen uptake by several benthic organisms.

Comparing nearshore and embayment scale assessments of submarine groundwater discharge: Significance of offshore groundwater discharge as a nutrient pathway

Submarine groundwater discharge (SGD) can influence biogeochemical cycles in coastal seas by delivering nutrients from the seafloor. Comparison between the nearshore and embayment scale assessments of SGD against river water discharge would be crucial for understanding biogeochemical impacts on the coastal seas because the discharge pattern (non-point or point pathway) is different. Here, we quantified SGD contribution to rivers in nutrient budgets at two scales within a coastal embayment (Obama Bay, Japan) by mass balance models of radon and radium isotopes. We then compared the SGD contribution between the two scales by the meta-analysis for regional data sets conducted in nearshore and embayment scales. The estimated SGD rates in the nearshore and embayment scales in the bay were 7.8 cm d-1 and 20.0 cm d-1, indicating that offshore SGD was more significant than nearshore. The ratios of nutrient fluxes derived from SGD to rivers (SGD:River) in the nearshore scale were 1.7 for dissolved inorganic nitrogen (DIN), 3.0 for phosphorus (DIP), and 0.5 for silica (DSi), while those in the embayment scale increased to 10.4 for DIN, 18.5 for DIP, and 3.9 for DSi. This result indicates that SGD-derived nutrients become more important at larger spatial scales. Meta-analysis revealed that the difference in the contribution of SGD to rivers was affected by the seafloor size and there was no significant difference in SGD rates between nearshore and embayment scale studies. However, our regional study shows the site-specific pattern that SGD rates in the embayment scale were higher than those in the nearshore scale. Overall, we clarified that SGD can be a crucial nutrient pathway for coastal embayments regardless of the spatial scales and contribute to coastal nutrient biogeochemistry in more offshore areas.

Effect of submarine groundwater discharge on nutrient distribution and eutrophication in Liaodong Bay, China

Driven by the anthropogenic activities associated with coastal settlements, eutrophication has become a global issue. Submarine groundwater discharge (SGD) is a significant continuous pathway for transporting nutrients from land to coastal waters, but its influence on eutrophication in Liaodong Bay (LDB) has received limited attention. In this study, radium isotopes and nutrient data from coastal waters were analyzed to evaluate the SGD flux and its implications for potential eutrophication in LDB. We found that the mean concentrations of dissolved inorganic nitrogen (DIN), phosphorous (DIP), and silicate (DSi) in groundwater were higher than those of seawater and river water. Based on 223Ra and 228Ra mass balance models, the SGD fluxes were estimated to be (0.53-2.03) × 109 m3/d, of which the fresh SGD accounted for 4 %-15 %. SGD is a vital invisible source of nutrients, contributing more than 79 % of the total inputs of DIN, DIP, and DSi into LDB. With high DIN/DIP ratios (average=85.8) and large nutrient inputs, SGD may significantly drive the phosphorus limitation and eutrophication in LDB. This study shows that SGD-derived nutrient fluxes should be considered in the assessment of water eutrophication for the formulation of future environmental management protocols in coastal systems.

Long-term trends of salinity in coastal wetlands: Effects of climate, extreme weather events, and sea water level

Coastal freshwater ecosystems play major roles as reservoirs of biodiversity and provide many ecosystem services and protection from extreme weather events. While they are of particular importance worldwide, they are affected by a large variety of anthropogenic threats, among which salinization has been less studied, particularly regarding large temporal and spatial data sets based on real case scenarios, while salinity can impact biodiversity and ecosystem functioning. In this study, we investigated the variations of salinity across long-term (1996-2020) and seasonal (monthly records) temporal scales and spatial (varying distance to the coastline) scales in water bodies of two typical temperate coastal wetlands situated on the Atlantic coast of France. We complemented our analyses with models of sea water levels computed at both sites across 2000-2020. Our detailed data set allowed for highlighting that salinity in ponds varied seasonally (higher during summer, due to decreased precipitation and higher temperature), but also spatially (higher closer to the seashore, which pattern increased through time). Over the long term, decreased precipitation but not increased temperature induced increasing salinity. We also highlighted contrasted long-term patterns of salinity changes on these two coastal wetlands, with one site were salinity decreased over time linked to the responses to marine flood, allowing to document the temporal dynamics of salinity following a massive intrusion of sea water. Complementarily, at both sites, water levels at high tides increased through time, a pattern which can induce additional salinization. To our knowledge, our study is the first to investigate long-term changes in salinity in coastal wetlands through natural processes (e.g. seaspray, seasonal variations) and ongoing climate perturbations (e.g. marine surges linked to extreme weather events, increased temperature and decreased precipitations).

Socio-hydrogeological survey and assessment of organic pollutants to highlight and trace back pollution fluxes threatening a coastal groundwater-dependent ecosystem

Despite being a vector of pollution towards connected ecosystems, groundwater is often underestimated or not taken into account in management frameworks. To fill this gap, we propose to add socio-economic data to hydrogeological investigations to identify past and present pollution sources linked to human activities at watershed scale in order to forecast threats towards groundwater-dependent ecosystems (GDEs). The aim of this paper is to demonstrate, by a cross-disciplinary approach, the added value of socio-hydrogeological investigations to tackle anthropogenic pollution fluxes towards a GDE and to contribute to more sustainable management of groundwater resources. A survey combining chemical compounds analysis, data compilation, land use analysis and field investigations with a questionnaire was carried out on the Biguglia lagoon plain (France). Results show a pollution with a two-fold source, both agricultural and domestic, in all water bodies of the plain. The pesticide analysis reveals the presence of 10 molecules, including domestic compounds, with concentrations exceeding European groundwater quality standards for individual pesticides, as well as pesticides already banned for twenty years. On the basis of both the field survey and the questionnaire, agricultural pollution has been identified as very local highlighting the storage capacity of the aquifer, whereas domestic pollution is diffuse over the plain and attributed to sewage network effluents and septic tanks. Domestic compounds present shorter residence time within the aquifer highlighting continuous inputs, related to consumption habits of the population. Under the Water Framework Directive (WFD), member states are required to preserve the good ecological status, quality and quantity of water bodies. However, for GDEs it is difficult to achieve the 'good status' required without considering the groundwater's pollutant storage capacity and pollution legacy. To help resolve this issue, socio-hydrogeology has proved to be an efficient tool as well as for implementing effective protection measures for Mediterranean GDEs.

Nitrogen in surface aquifer - Coastal lagoons systems: Analyzing the origin of eutrophication processes

Coastal lagoons can act as sinks and sources of a wide range of substances, including nutrients, and pollutants. In these ecosystems, primary production is limited more by nitrogen than by phosphorus. For this reason, they are significantly vulnerable to nitrate pollution. In this study, a joined analysis of surface and ground water was conducted to determine the origin, occurrence and processes affecting nitrogen fate in a Mediterranean coastal aquifer-lagoon system. This included the analysis of water levels, as well as hydrochemical and isotopes data evolution for a 4-year period, which revealed two important insights of nitrogen cycling within the system. Firstly, we detected different origins for nitrate pollution (a mixture of sewage, manure and chemical fertilizers), as well as their nearly complete attenuation in the alluvial aquifer due to heterotrophic and autotrophic processes, favoured by the presence of organic matter and Fe-minerals in its sediments. Secondly, due to its rapid assimilation, inorganic nitrogen peaks in the lagoons were mainly detected after storm events. While nitrate peaks may be attributed to surface water runoff, ammonium peaks may be related to organic nutrient cycling. In contrast, we did not detect continuous and low N inputs, associated to groundwater flow. These results depict the need of a more integrated management strategy of these aquifer-lagoon systems.

High spatial heterogeneity and low connectivity of bacterial communities along a Mediterranean subterranean estuary

Subterranean estuaries are biogeochemically active coastal sites resulting from the underground mixing of fresh aquifer groundwater and seawater. In these systems, microbial activity can largely transform the chemical elements that may reach the sea through submarine groundwater discharge (SGD), but little is known about the microorganisms thriving in these land-sea transition zones. We present the first spatially-resolved characterization of the bacterial assemblages along a coastal aquifer in the NW Mediterranean, considering the entire subsurface salinity gradient. Combining bulk heterotrophic activity measurements, flow cytometry, microscopy and 16S rRNA gene sequencing we find large variations in prokaryotic abundances, cell size, activity and diversity at both the horizontal and vertical scales that reflect the pronounced physicochemical gradients. The parts of the transect most influenced by freshwater were characterized by smaller cells and lower prokaryotic abundances and heterotrophic production, but some activity hotspots were found at deep low-oxygen saline groundwater sites enriched in nitrite and ammonium. Diverse, heterogeneous and highly endemic communities dominated by Proteobacteria, Patescibacteria, Desulfobacterota and Bacteroidota were observed throughout the aquifer, pointing to clearly differentiated prokaryotic niches across these transition zones and little microbial connectivity between groundwater and Mediterranean seawater habitats. Finally, experimental manipulations unveiled large increases in community heterotrophic activity driven by fast growth of some rare and site-specific groundwater Proteobacteria. Our results indicate that prokaryotic communities within subterranean estuaries are highly heterogeneous in terms of biomass, activity and diversity, suggesting that their role in transforming nutrients will also vary spatially within these terrestrial-marine transition zones.

Groundwater discharge locally shapes the rocky shore macroinvertebrate community in South-Southwest Portugal

Groundwater discharge is an essential process in the functioning of coastal aquatic ecosystems due to its significant role in nutrient cycling, geochemical mass balances and primary productivity. However, the occurrence patterns, importance, and effects of this discharge on rocky shores communities remain largely unknown. We assessed the importance of groundwater discharge into the highly ecologically important intertidal ecosystems. We compared the benthic macroinvertebrate composition and abundance between discharge and no-discharge sites, replicated for five shores in South and Southwest Portugal. This robust replicated feature across shores and regions is a particularly novel contribution to the field. Groundwater discharge significantly affected the biological communities' abundance across all shores, but not biodiversity patterns. The algae Enteromorpha sp., snail Melaraphe neritoides and lichen Verrucaria maura can potentially be used as bioindication tools for shifts in groundwater discharge quantity and qualitative patterns. Our study validates the importance of this commonly overlooked local disturbance factor in regulating intertidal communities.

Groundwater discharge as a driver of methane emissions from Arctic lakes

Lateral CH₄ inputs to Arctic lakes through groundwater discharge could be substantial and constitute an important pathway that links CH₄ production in thawing permafrost to atmospheric emissions via lakes. Yet, groundwater CH₄ inputs and associated drivers are hitherto poorly constrained because their dynamics and spatial variability are largely unknown. Here, we unravel the important role and drivers of groundwater discharge for CH₄ emissions from Arctic lakes. Spatial patterns across lakes suggest groundwater inflows are primarily related to lake depth and wetland cover. Groundwater CH₄ inputs to lakes are higher in summer than in autumn and are influenced by hydrological (groundwater recharge) and biological drivers (CH₄ production). This information on the spatial and temporal patterns on groundwater discharge at high northern latitudes is critical for predicting lake CH₄ emissions in the warming Arctic, as rising temperatures, increasing precipitation, and permafrost thawing may further exacerbate groundwater CH₄ inputs to lakes.

CH₄ inputs to Arctic lakes via groundwater discharge are an important pathway that links CH₄ production in thawing permafrost to emission via lakes. Here the authors unravel the role and drivers of groundwater inflows for CH₄ emissions from Arctic lakes.

Thermal-Based Remote Sensing Solution for Identifying Coastal Zones with Potential Groundwater Discharge

Submarine Groundwater Discharge (SGD) is an essential process of the hydrological cycle by hydraulically connecting the land and sea. However, the occurrence, importance and effects of SGD remain largely underexplored. Here, we developed and validated a straightforward tool for mapping potential SGD areas in coastal ecosystems of Portugal. Our approach was based on the premise that relatively cooler groundwater discharging to warmer coastal waters manifests in the thermal band of satellite imagery acquired during the summer months. We then used Landsat 8 thermal infrared imagery (TIR) to derive sea surface temperature and standardized temperature anomalies maps. The results confirmed the capacity of TIR remote sensing for identifying SGD areas. The thermal analysis enabled us to acquire a useful visual-spatial correlation between the location of thermal anomalies and potentiometric surfaces of coastal aquifers. This way, over 20 potential SGD areas were identified. Our study makes an important contribute to our current SGD research status by developing a cost-efficient tool which can be used as a first level approach for large areas. Further investigation is needed to quantify the SGD and its potential effect in the receiving ecosystems, especially those located within environmentally protected areas.

Submarine karstic springs as a source of nutrients and bioactive trace metals for the oligotrophic Northwest Mediterranean Sea

Groundwater springs in karstified carbonate aquifers are known to transport carbon, nutrients and trace elements to the coastal ocean. The biogeochemical significance of submarine karstic springs and their impact on coastal primary production are often difficult to quantify. We investigated several karstic springs, including the first-order Port-Miou spring, in an urbanized watershed that is also severely impacted by sewage effluent (Calanques of Marseille-Cassis, France). Karstic springs were elevated in major nutrients and bioactive trace metals over Mediterranean seawater, with relatively low concentration ranges. Groundwater NO₃^- was likely derived from atmosphere-aquifer interactions, while DOC:DON ratios reveal that NO₂^- and NH₄⁺ was autochthonously produced during mixing between karst groundwater and seawater. Submarine groundwater discharge (SGD) during March 2018 (wet season, baseflow conditions) was 6.7 ± 2.0 m³ s^-1 for the entire investigated coastline, determined from simultaneous ²²⁴Ra and ²²⁶Ra mass balances. The contribution of groundwater PO₄^3-, the major limiting nutrient of the Mediterranean Sea, sustained only 1% of primary production adjacent to sewage outfall, but between 7 and 100% of the local primary production in areas that were not impacted by sewage. Groundwater and seawater Fe:DIN and Fe:DIP ratios suggest that Fe was not a limiting micro-nutrient during the period of study, where bioactive trace metal fluxes were dominated by sewage and atmospheric deposition, although excess Fe from groundwater may locally enhance N fixation. Groundwater solute fluxes may easily vary by a factor of two or more over time because karst aquifers are sensitive to precipitation, as is the case of the regional carbonate karstified aquifer of Port-Miou, highlighting the critical importance of properly characterizing nutrient and trace metal inputs in these coastal environments.

Submarine groundwater discharge and its implication for nutrient budgets in the western Bohai Bay, China

Submarine groundwater discharge (SGD) supplies substantial quantities of nutrients from land to oceans. However, SGD and associated nutrient fluxes have long been ignored in Bohai Bay, which is subjected to the serious environmental problem. Here, we investigated the concentrations of radon (²²²Rn) and nutrients in groundwater and surface water in the western Bohai Bay during May 2017. The flushing time in the bay was estimated to be 38.8-58.3 days based on tidal prism model. The SGD flux was estimated to be 7.3 ± 4.8 cm d^-1 based on ²²²Rn mass balance model. The SGD associated nutrient fluxes were estimated to be (6.3 ± 4.1) × 10⁷ mol d^-1 for dissolved inorganic nitrogen (DIN), (1.2 ± 0.8) × 10⁶ mol d^-1 for dissolved inorganic phosphorus (DIP) and (7.5 ± 4.9) × 10⁷ mol d^-1 for dissolved inorganic silicon (DSi). By establishing nutrient budgets, we found that SGD was a major source, contributing 80.8% of all source for DIN, 90.7% of all source for DIP and 78.4% of all source for DSi into the western Bohai Bay. This study shows that SGD associated nutrient fluxes may have significant impact on nutrient budgets in the western Bohai Bay.

Remobilization of dissolved metals from a coastal mine tailing deposit driven by groundwater discharge and porewater exchange

Mining impacts on coastal environments have been extensively studied around the world. However, the role of Submarine Groundwater Discharge (SGD) and Porewater Exchange (PEX) as pathways for pollutants from mining waste deposits into seawater has been largely overlooked. Portmán Bay is located in the Cartagena-La Unión Pb-Zn sulphur mining district in Murcia, SE of Spain. The disposal of about 60 million tons of metal-rich mine tailings from 1957 to 1990 led to the infill of most of the bay. Although the effects of metals on indicator organisms have been shown previously, there is a major lack of knowledge on the release of dissolved metals from the emerged tailing deposit into the sea, more than 25 years after the closure of the mining activities. Samples for Ra isotopes (²²³Ra, ²²⁴Ra, ²²⁶Ra and ²²⁸Ra) and dissolved metals (Ag, Cd, Co, Pb, Zn) were analyzed in porewaters and seawater in order to separately estimate SGD and PEX driven dissolved metal fluxes. Our results show a continuous release of dissolved metals into the sea driven by both PEX and SGD. Most of dissolved metals are remobilized and released into the water column by PEX, which is a ubiquitous mechanism acting along the shoreline. Although SGD only represents 13% of the water flow, it drives large fluxes of dissolved Fe into the sea, mainly restricted to the west side of the bay. Large inputs of dissolved Fe²⁺ from the anoxic tailings deposit trigger a massive precipitation of iron hydroxides that enables the removal of most dissolved metals from the water column. This study highlights the role of PEX and SGD as significant mechanisms for the land to ocean transfer of dissolved metals from coastal mine tailings deposits.

Application of Stable Isotopes of Water to Study Coupled Submarine Groundwater Discharge and Nutrient Delivery

Submarine groundwater discharge (SGD)—including terrestrial freshwater, density-driven flow at the saltwater–freshwater interface, and benthic exchange—can deliver nutrients to coastal areas, generating a negative effect in the quality of marine water bodies. It is recognized that water stable isotopes (18O and 2H) can be helpful tracers to identify different flow paths and origins of water. Here, we show that they can be also applied when assessing sources of nutrients to coastal areas. A field site near a lagoon (Ringkøbing Fjord, Denmark) has been monitored at a metric scale to test if stable isotopes of water can be used to achieve a better understanding of the hydrochemical processes taking place in coastal aquifers, where there is a transition from freshwater to saltwater. Results show that 18O and 2H differentiate the coastal aquifer into three zones: Freshwater, shallow, and deep saline zones, which corresponded well with zones having distinct concentrations of inorganic phosphorous. The explanation is associated with three mechanisms: (1) Differences in sediment composition, (2) chemical reactions triggered by mixing of different type of fluxes, and (3) biochemical and diffusive processes in the lagoon bed. The different behaviors of nutrients in Ringkøbing Fjord need to be considered in water quality management. PO4 underneath the lagoon exceeds the groundwater concentration inland, thus demonstrating an intra-lagoon origin, while NO3, higher inland due to anthropogenic activity, is denitrified in the study area before reaching the lagoon.

Characteristics of groundwater discharge to river and related heavy metal transportation in a mountain mining area of Dabaoshan, Southern China

Groundwater discharge to river and the related heavy metal transportation were estimated for Dabaoshan, a mountain mining area where extensive mining activities had been conducted over 40 years. In the lower reach of the mining area, shallow aquifers were contaminated by varies heavy metals due to the discharge of acid mine drainage. Polluted aquifers act as long-term pollution sources to the surrounding gaining rivers, even after the mining activities were stopped. The natural tracer ²²²Rn was measured for river water of the Hengshi River and groundwater adjacent to the river channel in both wet and dry seasons. The total groundwater discharge rate was estimated to be 17.4-26.7 × 10³ m³ day^-1 in wet season and 1.9-2.1 × 10³ m³ day^-1 in dry season; and the river recharge was 5.6 ± 1.0 × 10³ m³ day^-1 in wet season and 2.1 ± 1.0 × 10³ m³ day^-1 in dry season. Compared with other mining and natural/artificial factor influenced areas, groundwater discharge rate in Dabaoshan was much lower, but the magnitudes of groundwater-borne Cu, Zn, Mn and Co fluxes were comparable or even much higher. This suggested that groundwater-derived heavy metal fluxes were significant pollution sources to river in the mountain mining area. Meanwhile, the results also suggested that concentrations of Cd, Pb, Cu, Ni, Mn, Fe, Zn and Tl in groundwater increased where the recharge of river water to groundwater occurred, suggesting the recharge of river water can affect heavy metal concentrations of the beneath aquifers, even in a gaining river.

Groundwater discharge to coastal streams - A significant pathway for nitrogen inputs to a hypertrophic Mediterranean coastal lagoon

Near-shore and direct groundwater inputs are frequently omitted from nutrient budgets of coastal lagoons. This study investigated groundwater-driven dissolved inorganic nitrogen (DIN) inputs from an alluvial aquifer to the hypertrophic Or lagoon, with a focus on the Salaison River. Piezometric contours revealed that the Salaison hydrogeological catchment is 42% bigger than the surface watershed and hydraulic gradients suggest significant groundwater discharge all along the stream. Hydrograph separation of the water flow at a gauging station located 3 km upstream from the Or lagoon combined with DIN historical data enabled to estimate that groundwater-driven DIN inputs account for 81-87% of the annual total DIN inputs to the stream upstream from the gauging station. A radon mass balance was performed for the hydrological cycle 2017-2018 to estimate groundwater inflow into the downstream part of the stream. Results showed that (1) DIN fluxes increased by a factor 1.1 to 2.3 between the gauging station and the Salaison outlet, (2) the increase in DIN was due to two groundwater-fed canals and to groundwater discharge along the stream, the latter represented 63-78% of the water flow. This study thus highlights the significance of groundwater driven DIN inputs into the Salaison River, which account for 90% of the annual DIN inputs. This is particularly true in the downstream part of the river, which, on averages, supplies 48% of total DIN inputs to the river. These downstream DIN inputs into the Or lagoon were previously not taken into account in the management of this and other Mediterranean lagoons. The inputs will probably affect restoration processes for many years due to their residence time in the aquifer. This study throws light on a rarely documented source of 'very-nearshore' groundwater discharge to coastal streams in water and nutrient budgets of coastal zone ecosystems.

A Novel Approach To Quantify Air-Water Gas Exchange in Shallow Surface Waters Using High-Resolution Time Series of Dissolved Atmospheric Gases

Gas exchange across the air-water interface is a key process determining the release of greenhouse gases from surface waters and a fundamental component of gas dynamics in aquatic systems. To experimentally quantify the gas transfer velocity in a wide range of aquatic settings, a novel method based on recently developed techniques for the in situ, near-continuous measurement of dissolved (noble) gases with a field portable mass spectrometer is presented. Variations in observed dissolved gas concentrations are damped and lagged with respect to equilibrium concentrations, being the result of (a) temperature (and thus solubility) variations, (b) water depth, and (c) the specific gas transfer velocity ( k_i). The method fits a model to the measured gas concentrations to derive the gas transfer velocity from the amplitude and the phase lag between observed and equilibrium concentrations. With the current experimental setup, the method is sensitive to gas transfer velocities of 0.05-9 m/day (for N₂), at a water depth of 1 m, and a given daily water temperature variation of 10 °C. Experiments were performed (a) in a controlled experiment to prove the concept and to confirm the capability to determine low transfer velocities and (b) in a field study in a shallow coastal lagoon covering a range of transfer velocities, demonstrating the field applicability of the method.