Submarine groundwater-derived inorganic and organic nutrients vs. mariculture discharge and river contributions in a typical mariculture bay

Submarine groundwater discharge (SGD) is recognized as one of the most significant pathways for terrestrial nutrient entry into coastal systems. However, SGD-derived nutrient fluxes, particularly organic nutrients, have not yet been considered in the management of mariculture areas. We quantified both saline (12.5 cm day⁻¹) and fresh (1.8 cm day⁻¹) SGD and related nutrient fluxes using the radium quartet and water mass balance model, respectively, and then reassessed nutrient budgets in a typical mariculture bay. Although fresh SGD accounts for 13 % of the total SGD, it carries 7.4–80.5 % of the total SGD nutrients, suggesting that fresh SGD was an important source of new nutrients with high nitrogen to phosphorus (N/P) ratios (585) into the coastal waters. Based on the rebuilt nutrient budgets, total SGD and mariculture discharge were the two major nutrient sources into the coastal water, providing 2.8–21.2 and 1.9–13.9 times more nutrients than local rivers, respectively. The nutrient speciation results revealed that SGD-derived organic nutrients, which are typically ignored, were dominant (i.e., dissolved inorganic nitrogen and dissolved organic phosphorus through SGD contributed about 22.9 % and 41.5 % of all nutrient resources). Furthermore, China-wide upscaling showed that SGD-derived nutrient fluxes in the mariculture area along the coast of China are comparable those of the rivers in China. Therefore, we suggest that SGD-derived inorganic and organic nutrient fluxes should be incorporated into the ecosystem management of mariculture areas.