We employed the Regional Ocean Modeling System (ROMS) and the Fennel ecological module to simulate the dispersion and distribution patterns of pollutants and the marine ecological environment in the Hangzhou Bay and the adjacent sea areas. By establishing a hydrodynamic model, we revealed the spatiotemporal variations of tides and currents, and conducted tests on water exchange in the bay with chemical oxygen demand (COD), dissolved inorganic nitrogen (DIN), and phosphate (PO₄) as water quality indicators. The main results are as follows: (1) The hydrodynamic characteristics of the Hangzhou Bay are influenced by the shape of the bay mouth, with tidal height and current increasing from east to west, reaching maximum velocities exceeding 4 m/s. Residual currents are slow, with velocities generally between 10 and 30 cm/s, featuring local eddies that hinder water exchange. The water exchange period is long, with a semi-exchange time of about 46 days. (2) The COD, DIN, and PO₄ concentrations in the bay decrease gradually from northwest to southeast, with the most severe pollution near the Yangtze River Estuary and the northern shore of the bay. The Yangtze River contributes the most pollutants to the bay, while coastal discharge outlets significantly affect water quality degradation in nearshore areas. (3) The nutrient and plankton concentrations at the Yangtze River Estuary are positively correlated with the nutrient flux from the Yangtze River, reaching the highest levels in summer, followed by spring and autumn, and the lowest in winter. The coastal areas of Zhejiang and Fujian are influenced by seasonal changes in coastal currents, with the highest nutrient concentrations in winter, followed by autumn and spring, and the lowest in summer. (4) Over 1982-2012, an increase in the nutrient input from rivers has led to an increase in nearshore nutrient concentrations. The degree and range of phosphorus limitation in the Yangtze River Estuary and the coastal areas of Zhejiang and Fujian have increased, and the increase in plankton biomass is far lower than the increase in nutrient concentrations. The increase in phytoplankton and zooplankton biomass in the Yangtze River Estuary is mainly due to the increased input of phosphorus from the Yangtze River, while the increase in biomass in the coastal areas of Zhejiang and Fujian is influenced by the simultaneous increase in nitrogen and phosphorus concentrations.

Water quality, ROMS, the Hangzhou Bay, East China Sea