Potential Linkages Between Submarine Groundwater (Fresh and Saline) Nutrient Inputs and Eutrophication in a Coastal Aquaculture Bay

Submarine groundwater discharge (SGD) plays a crucial role in nutrient budgets of coastal systems, encompassing both submarine fresh groundwater discharge (SFGD) and recirculated saline groundwater discharge (RSGD). Despite its significance, the specific importance of these components in mariculture bays has not been thoroughly assessed. Here, utilizing Ra isotopes and water-salt mass balance model, we show that SFGD flux (1.1 ± 0.4 cm d⁻¹) represented only 17% of the SGD in the Zhenzhu Bay, a typical mariculture bay along the South China Sea. Interestingly, the nutrient contribution from SFGD surpassed that from RSGD, accounting for 82% of the dissolved inorganic nitrogen (DIN) flux within the SGD. Analysis of the monthly satellite Chlorophyll-a (Chl-a) data confirmed that the decline in phytoplankton biomass can be linked to the limited dissolved silicate (DSi) transported by SFGD. Additionally, the elevated nitrogen to phosphorus ratio (241:1) and reduced silicon to nitrogen ratio (0.5:1) in SFGD compared to the Redfield ratio suggested that SFGD characterized by nitrogen excess and silica deficient, which likely played a role in transitioning from biogenic element constraints in coastal water. This shift may impact the proportions and functionality of the phytoplankton community, potentially mitigating water eutrophication. These findings underscore the significant influence of SGD on nutrient dynamics and the ecological environment in the Zhenzhu Bay.