Net Submarine Groundwater-Derived Dissolved Inorganic Nutrients and Carbon Input to the Oligotrophic Stratified Karstic Estuary of the Krka River (Adriatic Sea, Croatia)

Submarine groundwater discharge (SGD) is a significant source of biogenic elements in estuaries, and relevant studies in karstic estuaries are scarce. Krka River Estuary (KRE), located on the eastern Adriatic Sea (Croatia), is a typical oligotrophic stratified karstic estuary. In this study, based on ²²⁶Ra and ²²⁸Ra, the total SGD flux into the KRE surface layer was estimated to be (12.8–16.2) × 10⁵ m³/day. A conservative estimation of the fresh groundwater flux was (5.0–8.3) × 10⁵ m³/day, which accounts for 10–17% of the Krka River discharge into the estuary. By establishing water and nutrient budgets in the KRE surface layer, we found that SGD dominated the nutrient sources, although it accounted for a small portion of the total inflow water. Specifically, net SGD-derived dissolved inorganic nitrogen (DIN) and silicates contributed 58–90% and 24–64%, respectively, to the total input fluxes. These results indicate that SGD was a major external nutrient source, in which net SGD-derived high DIN flux and high DIN to dissolved inorganic phosphorus ratio may affect productivity in the KRE ecosystem and nearby Adriatic Sea. Additionally, net SGD-derived dissolved inorganic carbon (DIC) flux in the KRE (1.53 mol · m² · day) was much higher than those in most estuaries worldwide, suggesting that the DIC-enriched karst aquifers are important sources for global carbon cycle. Therefore, the impact of net SGD-derived DIC from karst aquifers on coastal seas will likely become more evident and substantial with further development of global climate change, such as sea level rise.