Advances in the study of submarine groundwater discharge (SGD) in China

Submarine groundwater discharge (SGD) is a critical land-ocean process in coastal areas and an essential component of the global hydrological cycle. Thus, hydrologists and oceanographers are paying more attention to SGD. SGD transports large amounts of materials into the ocean, including nutrients, carbon, and metals, and thus plays a vital role in the cycling of marine materials and affects the ecological environments of nearshore areas. This study examined the research status of SGD and its environmental effects in China, including a systematic analysis of radium activities and nutrient contents in groundwater, SGD rates, and the contribution of SGD to nutrients in various coastal ecosystems (bays, estuaries, continental shelves, and lagoons). The results showed the median concentrations of groundwater nutrients (dissolved inorganic nitrogen DIN, dissolved inorganic phosphorus DIP, and dissolved inorganic silicon DSi), SGD rates, and SGD-derived DIN and DIP in estuarine areas far exceeded those in the other three ecosystems studied. This result could be attributed to the complex hydrodynamic conditions of estuary areas as well as the considerable influence of human activities. Conversely, the lowest SGD rates and three inorganic nutrient species via SGD were observed in large scale continental shelves. SGD-derived nutrient fluxes were comparable to riverine inputs along the entire coast of China, which significantly affected the nutrient budget, nutrient composition, and primary productivity of coastal waters. Finally, this review suggests further research of SGD in three aspects: (1) accurate assessment, (2) its environmental effects, and (3) its regulatory strategies.