Impacts of Coastal Nutrient Increases on the Marine Ecosystem in the East China Sea During 1982-2012: A Coupled Hydrodynamic-Ecological Modeling Study

In the past decades (1982-2012), the rapid increase in nitrogen and phosphorus inputs has led to a two-fold increase in nitrogen and phosphorus concentrations in the East China Sea. To investigate how the increasing nutrients in the East China Sea have been affecting the marine environment and growth of plankton, this paper establishes a modeling framework using the coupled ocean hydrodynamics-ecological model. Observational sea surface temperature, salinity, nutrients, and chlorophyll-a are compared with the predictions to validate the model performance. The study reveals that the concentrations of dissolved inorganic nitrogen (DIN) and phosphate (PO43--P) are highest at the Yangtze River Estuary, slightly lower in the Zhejiang-Fujian nearshore, and much lower in the open sea, exhibiting significant seasonal variations closely related to the biomass of plankton. Over the past decades, DIN and PO43--P concentrations at sea surface have increased by 294% and 253%, respectively, in the Yangtze River Estuary, and by over 291% and 76% in the Zhejiang-Fujian nearshore, both of which are characterized by high nitrogen-phosphorus ratios. As a result, the biomass of plankton at sea surface has increased by 26% averaged over the two regions. Sensitivity experiments indicate that the biomass enhancement in the Yangtze River Estuary is primarily attributed to the increase in phosphorus, whereas that over the Zhejiang-Fujian nearshore is explained by the combined effects of both nitrogen and phosphorus increases. This study contributes to the development of ecological conservation strategies for rivers and coastal seawaters in China in the future.