River inflow and seawater intrusion shape distinct phytoplankton communities in jinghai bay, a coastal bay of the Yellow Sea.

Phytoplankton are key primary producers that shape the structure and function of estuarine ecosystems. However, the distinct effects of riverine inputs and seawater intrusion on phytoplankton communities remain unclear. This study investigated the seasonal dynamics of phytoplankton in Jinghai Bay during spring and summer 2023, focusing on the contrasting influences of river inflow and seawater intrusion. Our data revealed two distinct zones: a river-influenced zone (RIZ) with low salinity and high nutrients, and a seawater-intrusion zone (SIZ) with high salinity and low nutrients. In spring, the SIZ exhibited higher phytoplankton abundance and evenness, while the RIZ showed greater species richness. During summer, the RIZ had higher abundance and simplified dominant species, whereas the SIZ maintained higher diversity and evenness. Environmental drivers also differed between the two zones: phytoplankton dynamics in the RIZ were mainly shaped by DIN, DIP, and temperature, while those in the SIZ were governed by salinity gradients. Redundancy analysis further revealed that salinity positively correlated with phytoplankton diversity in the SIZ but negatively with dominant species in the RIZ. These findings suggest that river inflow promotes phytoplankton growth by altering community composition, while seawater intrusion imposes salinity-driven selection. Overall, our results highlight the contrasting mechanisms through which river inflow and seawater intrusion regulate phytoplankton traits in estuarine ecosystems.