The Nutrient Budget of a Highly Eutrophic Coastal System With Inefficient Nutrient Retention: The Radial Sand Ridges, Southwestern Yellow Sea

Abstract Estuarine‐coastal systems serve as filters of terrestrial nutrients in the land‐ocean continua. A proportion of land‐sourced nutrients reach the benthos where they are removed by remineralization and denitrification or buried and lost. The filter efficiency ( FE ), quantifying the proportion of nutrients either maintained in the coastal system or lost via burial and denitrification, differs according to physical, chemical, and ecological conditions of estuarine‐coastal zones and should be assessed on a system‐specific basis. Here, field data and a hydrodynamic‐biogeochemical model are used to investigate the nutrient budget and FE of a highly eutrophic Chinese coastal sea, the radial sand ridges (RSRs). Results indicate that rivers are the primary nutrient source, exceeding atmospheric deposition and sediment remineralization. Due to high turbidity and light‐limited primary production, most terrestrial nutrients are transported to the adjacent seas rather than being locally assimilated. Hence, FE is extremely low in the RSRs, averaging 5.1% and 1.7% for nitrogen and phosphorus, respectively. In the well‐flushed summer, physical export exceeds the entire combined terrestrial sources, lowering nutrient concentrations in the RSRs. Numerical experiments reveal that the annual nutrient export is primarily sensitive to winds and the cross‐shore concentration gradient. Our study underlines the significance of atmospheric, terrestrial, and hydrodynamic influences on coastal eutrophication and recommends that efforts to improve coastal FE are as important as reducing nutrient loading. Applicability of the modeling approach to other coastal zones for filter function assessment, and local and broader implications of this regional study are discussed.