The Roles of Nutrient Loading and River Discharge on the Phytoplankton Dynamics in the Semi-Enclosed Sea

Understanding spatiotemporal variations in phytoplankton biomass is crucial to the health of marine ecosystems. Therefore, the Finite Volume Community Ocean Model (FVCOM)-based Ecological Model (Integrated Compartment Model) was implemented to assess nutrient and phytoplankton dynamics in the Bohai Sea from 2010 to 2019. From 2010 to 2013, dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) levels in the Bohai Sea were higher (∼0.400 and 0.030 mg/L, respectively) in nearshore areas compared to other years. During the summer and fall, the spatial distribution of the DIN/DIP ratio in the Bohai Sea exhibited a double-core structure. Higher values (>100) were primarily concentrated in the central region of the Liaodong Bay and south of the Central Basin near the Yellow River Estuary. Statistical analyses and numerical experiments revealed that increasing DIP loading from rivers had a greater effect on phytoplankton biomass in the Laizhou and Bohai Bays than increasing DIN loading. However, the phytoplankton biomass in the Liaodong Bay was strongly influenced by both increasing DIN and DIP loading from rivers. Notably, the increase in phytoplankton biomass resulting from increasing DIN loading exceeded that from increasing DIP loading by 17% in the Liaodong Bay. The reduction in river discharge weakened circulation at the river mouths, thereby partially retaining surface phytoplankton. This was more predominant in the nearshore areas of the Yellow River owing to the higher river discharge in August 2019. This study provides valuable insights for the management and conservation of marine ecosystems.