The important role of submarine groundwater discharge (SGD) to derive nutrient fluxes into River dominated Ocean Margins – The East China Sea

Submarine groundwater discharge (SGD), which has been recognized as an important pathway for the transport of terrestrial chemical components (i.e., nutrients, trace elements and other contaminants) to the ocean, plays an important role on the biogeochemical cycling in marine environment, especially coastal/marginal seas. In the present work, the initial results on the amount of SGD into the continental shelf of the East China Sea (ECS), one of the marginal seas with large riverine input from the Changjiang River (Yangtze River) are evaluated. According to the locations and features of water masses in the ECS, radium (Ra) isotopes mass balance model was built and the conservative and non-conservative (excess) components of Ra were evaluated. Using the inventories of excess ²²⁸Ra and ²²⁶Ra, the residence time of water in the ECS was estimated as 1.30 ± 0.27 years. Then the SGD flux was estimated to be (5.42 ± 0.14) × 10¹¹ m³ yr⁻¹, which were 47 ± 1% of the total river discharge into the ECS along the coast. Nutrient fluxes driven by SGD were estimated to be (7.32 ± 0.19) × 10¹⁰, (1.79 ± 0.05) × 10⁹ and (1.59 ± 0.04) × 10¹¹ mol yr⁻¹ for dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP) and dissolved inorganic silicate (DSi), respectively, which were about 0.7, 2.2 and 1.4 times, respectively, of the riverine inputs. Furthermore, SGD-driven nutrient had obviously high DIN/DIP ratios, which could lead to a number of large-scale environmental problems to the ECS, such as the frequent harmful algal blooms and hypoxia especially in the estuary and coastal area.