Using 222Rn to estimate submarine groundwater discharge (SGD) and the associated nutrient fluxes into Xiangshan Bay, East China Sea

Submarine groundwater discharge and ocean acidification: Implications from China's coastal waters

Ocean acidification (OA) is a global environmental concern, and submarine groundwater discharge (SGD) is a potentially process that enhances OA. This review summarizes the relationship between two types of constituents carried by SGD into China's seawater and OA. 1) Current research predominantly concentrates on constituent fluxes from SGD, neglecting its ecological impacts on carbon and nutrients budgets, as well as the mechanisms between carbon and nutrients. 2) Uncertainties persist in SGD research methods and acidification characterization. 3) There's a need to enhance quantitative research methods of SGD-OA, particularly in areas with intricate biogeochemical processes. Effective identification methods are crucial to quantify SGD's contribution to OA. Investigating core scientific questions, including SGD's impact on OA rates and scales, is paramount. While the primary focus is on SGD-OA research in China, insights gained from novel perspectives could have broader value for coastal management globally.

Seasonal characteristics of groundwater discharge controlled by precipitation and its environmental effects in a coal mining subsidence lake, eastern China

Many regions have formed subsidence lakes due to underground mining in the world. However, seasonal variations of lacustrine groundwater discharge (LGD) rate and solute fluxes in the coal mining subsidence were rarely reported. In this study, we conducted four seasonal samplings in a coal mining subsidence, during which samples for stable water (δ18O) and radioactive (222Rn) isotopes were collected to quantify the seasonal dynamics of LGD rates. The LGD rates estimated from the 222Rn mass balance model were 10.2 ± 8.7, 5.5 ± 3.2, 11.5 ± 7.8, and 7.8 ± 4.5 mm d-1 in summer, autumn, winter and spring, respectively. According to the 18O mass balance model, the corresponding LGD rates were 15.1, 7.3, 15.6, and 11.3 mm d-1 in summer, autumn, winter and spring, respectively. We found a significant correlation between precipitation and LGD rates, suggesting precipitation was recognized as the main control factor for seasonal variations of LGD rates. Based on this correlation, the extrapolated LGD rates over a year ranged from 3.1 to 12.7 mm d-1 with an average of 8.8 mm d-1. Moreover, the fluxes of dissolved silicon (DSi), iron (Fe), and manganese (Mn) from LGD in autumn were (1.6 ± 0.9) × 105, (1.9 ± 1.1) × 104, and (1.1 ± 0.6) × 104 mol a-1, respectively. Correspondingly, in winter they were (3.5 ± 2.4) × 105, (4.1 ± 2.8) × 103, and (2.8 ± 1.9) × 103 mol a-1, respectively. This study demonstrated significantly seasonal variations of LGD, with precipitation being the main control factor of LGD in the coal mining subsidence lake. The fluxes of dissolved substance (DSi, Fe, Mn) from LGD need to be emphasized because they may have important impacts on the ecological stability in coal mining subsidence lakes.

Coastal eutrophication driven by long-distance transport of large river nutrient loads, the case of Xiangshan Bay, China

With accelerating anthropogenic activities, the overloading of land-derived nutrients and the resultant eutrophication are threatening coastal aquatic habitats worldwide. In semi-enclosed coastal bays, eutrophication is always considered a local problem that can be mitigated by nutrient reduction at a regional scale. However, as the main nutrient drains major global river discharges can have far-reaching effects over hundreds of kilometers alongshore, which are usually not precisely recognized in local coastal zone management. Here, we first quantified the contributions from both local and remote nutrient sources in Xiangshan Bay (XSB), a eutrophic semi-enclosed bay in China 200 km south of the mouth of the Changjiang River (CJR, the world's third largest river), employing a salinity-based conservative mixing model. We found that the nutrients in Xiangshan Bay were mainly supplied by intruded coastal water fed by CJR discharge, contributing 63 % of dissolved inorganic nitrogen (DIN), 65 % of dissolved silicon (DSi), and 49 % of dissolved inorganic phosphorus (DIP) during the summer of 2017, and 75 % of DIN, 75 % of DSi and 60 % of DIP during the winter of 2019. Additionally, long-term interannual trends in the nutrient concentrations of XSB were generally synchronous with those of the downstream portion of the CJR, indicating that CJR discharge seems to be a strong driver of the eutrophication observed in XSB. In contrast, the impact of local nutrient inputs, such as riverine sewage drainage, aquaculture, biogenic activities, and elemental recycling, was much lower and was regionally limited to the inner bay. Interestingly, the DIP contributions of the local and remote sources were similar, indicating the greater relevance of the internal process. Overall, to mitigate eutrophication in large river-adjacent coastal bays, the inter-regional united practices for nutrient source regulation and ecosystem restoration should be permanently applied along the entire river basin-estuary-coastal continuum.

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.

Anthropogenic As pollution mediated by submarine groundwater discharge in a marine ranch

Arsenic (As) pollution, is a global problem, threatening human health and ecological security, especially in the bay environment with dense population and human activities. Among potential pathways of As into the bay, submarine groundwater discharge (SGD) has not received adequate attention due to its invisibility. We determined As and 222Rn activity concentrations in different water mass. Spatial variation of dissolved As concentration in the groundwater was large and attributed to the adjacent local industries. By combining 222Rn mass balance modeling with As concentrations measured, the SGD-derived As fluxes was conservatively estimated to be 1310 kg As d-1 and 5880 kg As d-1 in the dry and wet seasons, respectively. The migration of arsenic may be enhanced by rainfall and dissolved carbon. The amount of SGD derived As input to the bay was greater than the total combined As input from river discharge, atmosphere, sewage drainage, and diffusion from sediment.

Patterns and variability in the microplastic contamination along the southwest coast of India with emphasis on submarine groundwater discharge sites

Beach sediments of the southwest coast of India were analysed to estimate the microplastic contamination with emphasis on the submarine groundwater discharge (SGD) zones. Both SGD and non-SGD sites were assessed for abundance, morphotype and polymer type of microplastics. Microplastic load was 230.429 ± 62.87 particles per 100 g. Fibre, mainly blue, was the abundant morphotype, followed by fragment, foam and film. The polymer types were POLYETHYLENE (PE) (30.77 %), POLYPROPYLENE (PP) (26.92 %), POLYAMIDE (PA) (19.23 %), POLYSTYRENE (PS) (11.54 %), ETHYLENE VINYL ACETATE (EVA) (7.692 %) and POLYVINYL CHLORIDE (PVC) (3.846 %). The SGD zones exhibited higher microplastic contamination with statistically significant variations from non SGD sites. The study accounts the levels of microplastic contamination along the southwest coast of India, a major fishery zone. The higher abundance of microplastic in the SGD zones indicates the significance of subterranean groundwater through flow as a pathway of anthropogenic contaminants towards marine ecosystems.

Riverine and submarine groundwater nutrients fuel high primary production in a tropical bay

River discharge has long been recognized as a major source of nutrients supporting high primary production (PP) in Bandon Bay, while submarine groundwater discharge (SGD) and atmospheric deposition have largely been overlooked. In this study, we evaluated contributions of nutrients via river, SGD and atmospheric deposition, and their roles on PP in the bay. Contribution of nutrients from the three sources during different time of the year was estimated. Nutrients supply from Tapi-Phumduang River accounted for two-fold the amount from SGD while very little supply was from atmospheric deposition. Significant seasonal difference in silicate and dissolved inorganic nitrogen were observed in river water. Dissolved phosphorous in river water was mainly (80 % to 90 %) of DOP in both seasons. For the bay water, DIP in wet season was two-fold higher than in dry season while dissolved organic phosphorus (DOP) was only one half of those measured in dry season. In SGD, dissolved nitrogen was mostly inorganic (with 99 % as NH₄⁺), while dissolved phosphorous was predominantly (DOP). In general, Tapi River is the most important source of nitrogen (NO₃^-, NO₂^-, and DON), contributing >70 % of all considered sources, especially in wet season, while SGD is a major source for DSi, NH₄⁺ and phosphorus, contributing 50 % to 90 % of all considered sources. To this end, Tapi River and SGD deliver a large quantity of nutrients and support high PP in the bay (337 to 553 mg-C m^-2 day^-1).

Submarine groundwater discharge: An Asian overview

Submarine groundwater discharge (SGD) is the combination of fresh and saline groundwater flux to marine system through continental boundaries regardless of its chemical composition and factors influencing the flow. We have discussed the SGD studies in the Asian context; SGD has been studied in various parts of Asia, including China, Japan, South Korea, and Southeast Asia. In China, SGD has been studied in several coastal regions, including the Yellow Sea, the East China Sea, and the South China Sea. In Japan, SGD has been studied in the Pacific coast, where it has been found to be an important source of fresh water to the coastal ocean. In South Korea, SGD has been studied in the Yellow Sea, where it has been found to be an important source of fresh water to the coastal ocean. In Southeast Asia, SGD has been studied in several countries, including Thailand, Vietnam, and Indonesia. Recently the SGD studies acquired much development India, the research on SGD in India is limited, and more studies are needed to understand the SGD process, its impact on the coastal environment, and the management strategies, Groundwater extraction for irrigation, industry, and domestic use is increasing in India, which can affect the SGD process in coastal aquifers. Overall, the studies suggest that SGD is an important process in Asian coastal regions, playing a role in the supply of fresh water and the transport of pollutants and nutrients.

The effects of subterranean estuary dynamics on nutrient resource ratio availability to microphytobenthos in a coastal lagoon

Causal links between subterranean estuary (STE) dynamics, their climatological drivers, and the ecology of coastal ecosystems have remained elusive. Yet, establishing these connections is essential for fully integrated management of coastal ecosystems. We test, in a semi-arid climate, whether the composition of submarine groundwater discharge (SGD) to a lagoon can be regulated by the annual oscillation of the local STE, itself driven by groundwater recharge variability. We study STE outflow samples gathered monthly for a year in the Ria Formosa lagoon, examining the temporal dynamics of salinity, E_H, pH (Total scale), dissolved oxygen and nutrient (PO₄^3-, NO₂^-, NO₃^-, NH₄⁺, and Si(OH)₄) concentrations under the local hydrological regime. The objectives were threefold: (1) to determine the annual variability of nutrient content and N:P:Si stoichiometry in SGD into the lagoon; (2) to identify the main drivers of variability in SGD composition and stoichiometry and their interactive effects; (3) to discuss links to, and implications for, ecosystem function that could help define expectations of cause-effect relationships and be useful for environmental management of the lagoon and similar systems elsewhere. We find that the terrestrial groundwater recharge cycle drives the expansion and contraction of the subterranean estuary on annual timescales, causing the pH of SGD to fluctuate in opposition to continental groundwater level. The annual dynamics of the STE and the resulting pH oscillation determine the annual variability of nutrient composition ratio in SGD and shape benthic primary production dynamics. When saltwater intrusion occurs, the pH within SGD increases, enhancing nitrification and desorption of exchangeable phosphorus, while silicate fluxes increase with seawater retreat. The result is that nutrient resource ratio availability for coastal primary production depends on the fresh groundwater level. This implies that ecosystem function in such systems is more tightly related to the dynamics of linked groundwater reservoirs than previously thought.

Application of radon (222Rn) as an environmental tracer in hydrogeological and geological investigations: An overview

Radon (²²²Rn) is a colourless, odourless, inert, and radioactive noble gas (t_1/2 = 3.8 days) that emanates from rocks and soils as a result of the alpha decay of its parent, radium (²²⁶Ra) in the decay series of uranium-238, is the focus of this study. Radon is produced in the crystal lattice of the minerals and emanates out through alpha recoil. It dissolves in water, and is also found in soil and air. Its distribution in water is more pertinent for scientific investigations. It can be measured by various methods. Certain properties of radon enable it to serve as an ideal tracer, viz., short-half life, inertness, high abundance in groundwater than surface water, preferential partitioning, sensitivity to sudden changes in subsurface conditions, non-invasiveness etc. This paper reviews the state-of-the-art techniques on the measurement of dissolved radon in water and its potential applications as a tracer and precursor in several hydrogeological and geological applications like understanding the surface water - groundwater interactions, hydrograph separation of streams, estimation of Submarine Groundwater Discharge (SGD), study of hydrodynamics and water balance of lakes, earthquake predictions, locating geological structures (faults/lineaments), geochemical explorations, NAPL contamination studies etc. Among the various applications presented, radon based approach is found to be more reliable in water resources domain than seismic precursory studies. The interpretations based on radon study in the above applications will pave the way for the improved understanding of the hydrological processes, and thus, help the planners and water managers for the sustainable development and management of water resources.

Tidally driven submarine groundwater discharge to a marine aquaculture embayment: Insights from radium and dissolved silicon

For understanding the significance of tidal pumping for driving submarine groundwater discharge (SGD), two time series observations were conducted over tidal cycles in typical mariculture areas of Sanmen Bay, China. Based on ²²⁴Ra and dissolved silicon (DSi), the tide-driven SGD fluxes showed significant negative correlations with tidal height, and were 16.4 ± 5.6 cm d^-1 and 21.7 ± 13.9 cm d^-1 at two coastal sites of the Sanmen Bay, highlighting the potential of DSi in SGD calculation when coastal waters are on short time scales. Furthermore, nutrient fluxes through tidally influenced SGD were estimated to be 11.5-26.5 mmol m^-2 d^-1 for dissolved inorganic nitrogen, 0.06-0.34 mmol m^-2 d^-1 for dissolved inorganic phosphorus and 11.5-32.1 mmol m^-2 d^-1 for DSi, which were higher than those from surface loadings, revealing that tide-driven SGD with large amounts of nutrient and high N:P ratios may significantly contribute to the development of marine aquaculture.

Radon and Salinity Mass Balance Constraints on Groundwater Recharge on a Semi-Arid Island (Catalina, California)

Quantifying the freshwater component of submarine groundwater discharge (SGD) is critical in the analysis of terrestrial influences on marine ecosystems and in assessing the water budget and groundwater recharge of coastal aquifers. In semi-arid to arid settings, this quantification is difficult because low SGD rates translate into low concentrations of groundwater solutes in coastal waters. In this study, fresh SGD (FSGD) was quantified for Toyon Bay on Catalina Island, California, for wet and dry seasons using a combination of radon and salinity mass balance models, and the results were compared to watershed-specific groundwater recharge rates obtained from soil water balance (SWB) modeling. Calculated FSGD rates vary only slightly with season and are remarkably similar to the recharge estimates from the SWB model. While sensitivity analyses revealed FSGD estimates to be significantly influenced by uncertainties in geochemical variability of the groundwater end-member and fluctuations of water depth, the results of this study support the SWB-model-based recharge rates. The findings of this study highlight the utility of the radon-and-salinity-mass-balance-based FSGD estimates as groundwater recharge calibration targets, which may aid in establishing more refined sustainable groundwater yields.

Coastal eutrophication in China: Trend, sources, and ecological effects

Eutrophication in coastal waters caused by excess nutrient inputs has occurred widely on a global scale. Due to the rapid economic development over the last four decades, most of the Chinese coastal waters have experienced a eutrophic process. Major observed trends of coastal eutrophication include two periods, a slow development from the 1970s to 1990s and a fast development after 2000, with major contributions of increased nitrogen (N) and phosphorus (P) from river inputs, atmospheric deposition, and submarine groundwater discharge (SGD). Nutrient composition and stoichiometry have been significantly changed, including increased ammonium, bioavailable organic N and P, and asymmetric ratios between N, P and silicate (Si). Most of these changes were related to the rapid increases in population density, fertilizer application, sewage discharge, aquaculture and fossil fuel combustion, and have resulted in distinctly increased harmful algal blooms. Coastal eutrophication combined with the effects of climate change is projected to continually grow in coming decades. Targeted research is therefore needed on nitrogen reduction and control, potential adaptation strategies and the consequences for ecosystems and economic sustainability.

Submarine groundwater discharge and associated nutrient flux from southwest coast of India

The nutrient discharge associated with submarine groundwater discharge (SGD) into the Arabian Sea has been investigated for the first time using the Radon isotope (²²²Rn) mass balance model at three sites along the southwest coast of India. The SGD flux varied in the range of 49.48×10⁴ - 335.84×10⁴ m³/day, with high (low) discharge during the low (high) tide. SGD delivers a considerable amount of the nutrient into the Arabian Sea with 2.10×10⁴ to 11.66×10⁴ mol/day dissolved inorganic nitrogen (DIN), 1.23×10² to 56.31×10² mol/day dissolved inorganic phosphate (DIP), and 7.28×10⁴ to 24.44×10⁴ mol/day dissolved silicate (DSi). This significant nutrient input to the coastal waters through SGD is mainly attributed to the land-use practices like agricultural activities, improper waste disposal, and thickly populated coastal settlement zones. The increase in the nutrient discharge may lead to phytoplankton bloom in the nearshore environment and can accelerate seasonal coastal hypoxia over the western Indian shelf. This evidence of considerably high nutrient flux through SGD advocates the importance of understanding SGD associated flux along the southwest coast of India to maintain a sustainable ecological balance.

High leaf mass per area Oryza genotypes invest more leaf mass to cell wall and show a low mesophyll conductance

The intraspecific variations of leaf structure and anatomy in rice leaves and their impacts on gas diffusion are still unknown. Researches about the tradeoff between structural compositions and intracellular chemical components within rice leaves are still lacking. The objectives of the present study were to investigate the varietal differences in leaf structure and leaf chemical compositions, and the tradeoff between leaf structural tissues and intracellular chemical components in rice leaves. Leaf structure, leaf anatomy, leaf chemical composition concentrations and gas exchange parameters were measured on eight Oryza sativa L. genotypes to investigate the intraspecific variations in leaf structure and leaf anatomy and their impacts on gas exchange parameters, and to study the tradeoff between leaf structural compositions (cell wall compounds) and intracellular chemical components (non-structural carbohydrates, nitrogen, chlorophyll). Leaf thickness increased with leaf mass per area (LMA), while leaf density did not correlate with LMA. Mesophyll cell surface area exposed to intercellular airspace (IAS) per leaf area, the surface area of chloroplasts exposed to IAS and cell wall thickness increased with LMA. Cell wall compounds accounted for 71.5 % of leaf dry mass, while mass-based nitrogen and chlorophyll concentrations decreased with LMA. Mesophyll conductance was negatively correlated with LMA and cell wall thickness. High LMA rice genotypes invest more leaf mass to cell wall and possess a low mesophyll conductance.

Possible effect of submarine groundwater discharge on the pollution of coastal water: Occurrence, source, and risks of endocrine disrupting chemicals in coastal groundwater and adjacent seawater influenced by reclaimed water irrigation

This study investigated occurrence, source, and risks of endocrine disrupting chemicals (EDCs) in coastal groundwater and adjacent seawater influenced by reclaimed water irrigation in a typical coastal region of China. All target EDCs were detected in coastal groundwater and reclaimed water while only estrone, bisphenol A (BPA), and nonylphenol were detected in seawater. Concentrations of BPA that was the predominant EDC in coastal groundwater ranged from 35.9 to 52.9 ng/L and estradiol was easy to accumulate in groundwater under reclaimed water irrigation. Concentrations of all target EDCs in seawater ranged from 18.9 to 30.9 ng/L, much lower than those in groundwater. Ecological risks posed by EDCs in groundwater and seawater were very high. Estrone contributed to 51.3%-62.9% of total acute risk quotients for seawater while detected 17-α-ethynylestradiol contributed to 41.1%-56.2% of total acute risk quotients for groundwater. Estradiol equivalent concentrations of target EDCs in groundwater/seawater were in the range of (3.5-7.6)/(1.4-2.3) ng/L while non-cancer risks posed by EDCs in groundwater/seawater were acceptable. Dual-isotope analysis illustrated that reclaimed water was the main source of EDCs in coastal groundwater. About 82% of EDCs was discharged into the Laizhou Bay through the submarine groundwater discharge based on the flux analysis. The pollution of the coastal groundwater through reclaimed water irrigation subsequently led to EDCs pollution of the adjacent seawater through the submarine groundwater discharge. Therefore, effective control of EDCs in reclaimed water for irrigation will be beneficial to control EDCs in groundwater and seawater of the coastal regions.

Improved Approach for the Investigation of Submarine Groundwater Discharge by Means of Radon Mapping and Radon Mass Balancing

The paper presents an improved approach for investigating submarine groundwater discharge (SGD) based on radon mapping and radon mass balancing in the coastal sea. While the use of radon as an environmental tracer in SGD studies is well-established, we identified based on our longstanding experience six methodical shortcomings of the conventional approach and suggest corresponding developments. The shortcomings include: (1 and 2) inadequate consideration of both detection equipment response delay and influence of tidal stage; (3 and 4) incorrect quantification of radon losses, due to offshore mixing and degassing resulting in a potentially incorrect radon mass balance; (5) inaccurate determination of the terrestrial groundwater endmember, due to inhomogeneous radon distribution in the coastal aquifer; and (6) difficulties in distinguishing between discharged fresh groundwater and recirculated seawater. The improved approach is practically demonstrated in a step by step manner in a large-scale field study, which was carried out in False Bay (South Africa) and which consisted of two parts, namely (i) qualitative SGD localization along the entire False Bay coastline based on coastal radon distribution patterns and (ii) quantitative SGD investigation within a defined coastal area of interest (AOI) based on a radon mass balance (RMB). The plausibility of the AOI related results was evaluated by a hydrogeological model, used for qualitative SGD localization, and a hydrological model, applied for estimating groundwater recharge within the AOI catchment.

Groundwater nutrient inputs into an urbanized tropical estuary system in Indonesia

Groundwater discharge is known to transport nutrients into estuaries at several locations around the world. However, few studies report groundwater-associated nutrient fluxes from tropical developing regions such as Southeast Asia, even though this area shows the strongest human modifications in the coastal zone worldwide. We investigated groundwater nutrient flux into two streams and estuaries (Awur and Sekumbu Bay) in the urban area of Jepara, Indonesia, and its relation with the land usage surrounding the estuaries. We found that average concentrations of NO₃, NH₄, and PO₄ in Jepara's aquifer reached 145μM, 68μM, and 14μM, respectively, and our results indicate that these were mainly originated from untreated sewage, agriculture, and manure input. Approximately 2200tonNyear^-1 and 380tonPyear^-1 were removed in the soil and aquifer before the nutrients were discharged into the river. The total groundwater discharge into the river and estuary was estimated to 461×10³m³d^-1, or up to 42% of the river discharge. Discharge of groundwater-associated NO₃ (72×10³mold^-1), NH₄ (34×10³mold^-1), PO₄ (5×10³mold^-1), and additional surface runoff may contribute to eutrophication and a decrease of nearshore surface water quality. Nutrient concentrations in groundwater, river, and coastal seawater in the Jepara region are similar to those found in major urban areas in Southeast Asia, e.g. Manila and Bangkok, even though Jepara has smaller size and population. Thus, our results indicate that medium populated cities with highly modified regional land use can contribute a significant amount of nutrient discharge in the coastal area and should be included in global assessments of nutrient budget calculation.

Chemical inputs from a karstic submarine groundwater discharge (SGD) into an oligotrophic Mediterranean coastal area

The impacts of nutrient and other chemical inputs released by a submarine groundwater discharge (SGD) on the marine environment of an oligotrophic Mediterranean coastal area (Messiniakos Gulf, SE Ionian Sea) are investigated through a multidisciplinary approach. Nutrients and organic pollutants associated with the SGD are presented to study the chemical characteristics of the SGD and to investigate its effect on the marine ecosystem in comparison to freshwater discharges of the water bodies of Messinia Prefecture. Nutrient and organic pollutant fluxes were calculated from (214)Bi-based SGD estimates. An average of 22×10(3) mol of silicate per month and 8×10(3) mol of nitrate per month were released via the SGD. Nutrient concentrations at the mouth of the SGD were three times higher than in Messiniakos Gulf, and NO3(-) was the primary Dissolved Inorganic Nitrogen form discharged by SGD. Organic pollutant concentrations associated with agricultural activities were low at the SGD. The implementation of a Eutrophication Index (E.I.) showed that the water column at the SGD site corresponds to Moderate/Bad ecological quality, whereas the status switches rapidly to Good at a small distance from the SGD. Coastal areas influenced by river or sewage discharge correspond to a Moderate/Good ecological status. The BENTIX index used for the classification of the ecological quality status of the benthic macroinvertebrate communities showed that the SGD has a minor influence compared to the other freshwater discharges in Messiniakos Gulf. Though the SGD has a considerable outflow, morphology and hydrodynamics of the area favor the rapid dispersion of the upwelling water and degrades the SGD's effect even on a regional scale.