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Submarine groundwater discharge (SGD) is a significant source of biogenic elements in estuaries, and relevant studies in karstic estuaries are scarce. Krka River Estuary (KRE), located on the eastern Adriatic Sea (Croatia), is a typical oligotrophic stratified karstic estuary. In this study, based on Ra-226 and Ra-228, the total SGD flux into the KRE surface layer was estimated to be (12.8-16.2) x 10(5) m(3)/day. A conservative estimation of the fresh groundwater flux was (5.0-8.3) x 10(5) m(3)/day, which accounts for 10-17% of the Krka River discharge into the estuary. By establishing water and nutrient budgets in the KRE surface layer, we found that SGD dominated the nutrient sources, although it accounted for a small portion of the total inflow water. Specifically, net SGD-derived dissolved inorganic nitrogen (DIN) and silicates contributed 58-90% and 24-64%, respectively, to the total input fluxes. These results indicate that SGD was a major external nutrient source, in which net SGD-derived high DIN flux and high DIN to dissolved inorganic phosphorus ratio may affect productivity in the KRE ecosystem and nearby Adriatic Sea. Additionally, net SGD-derived dissolved inorganic carbon (DIC) flux in the KRE (1.53 mol m(2) day) was much higher than those in most estuaries worldwide, suggesting that the DIC-enriched karst aquifers are important sources for global carbon cycle. Therefore, the impact of net SGD-derived DIC from karst aquifers on coastal seas will likely become more evident and substantial with further development of global climate change, such as sea level rise.

Plain Language Summary Coastal karst aquifers are very vulnerable and sensitive to climate and environmental changes. In this study, we evaluated the impacts of submarine groundwater discharge in a typical oligotrophic highly stratified karstic estuary. Using multiple parameters and mathematical models, we found that the net SGD-derived nutrient flux could affect the productivity of this coastal ecosystem, notably contributing to forming the conditions that lead to the occurrence of phytoplankton blooms. In addition, nutrient-enriched SGDs from the karst aquifers are likely to be important but easily ignored sources for the global carbon cycle. Taken together, our study revealed the impacts of submarine groundwater from the karst aquifers on the coastal seas, which shall become increasingly evident and substantial with further development of global climate change, such as the rise of sea level.