Abstract: The spatiotemporal dynamics and dominant drivers of phytoplankton communities in the Bohai Rim estuaries remain poorly understood and require urgent elucidation. Combining microscopic identification of phytoplankton and analysis of aquatic environmental factors, statistical methods including Mantel test, co-occurrence network analysis and partial least squares path modeling (PLS-PM) were adopted to explore the spatiotemporal distribution characteristics of phytoplankton community structure and identify its key influencing factors across five typical estuaries around the Bohai Rim. Results showed that water salinity ranged from 0.10‰ to 30.20‰, water temperature varied between 0.00 and 34.40 ℃. The mass ratio of total nitrogen to total phosphorus was 15-1350, indicating an overall characteristic of high nitrogen and low phosphorus. A total of eight phyla of phytoplankton were identified, among which Cyanophyta, Chlorophyta and Bacillariophyta were the dominant phyla. The dominant species included Pseudanabaena sp., Microcystis sp., and Oscillatoria sp. Phytoplankton community structure exhibited distinct spatiotemporal differentiation. Spatially, phytoplankton showed an 83.3% dissimilarity in dominant species among different salinity zones. Temporally, community structure showed obvious seasonal variation. Cyanophyta was absolutely dominant in summer, whereas Bacillariophyta prevailed in winter. Mantel tests revealed that salinity was significantly correlated with species-level community composition spatially, while water temperature had significant temporal correlations with both phylum and species-level community composition. In contrast, water nutrients showed weak correlations with phytoplankton community structure. Phytoplankton co-occurrence network analysis further confirmed that salinity was generally negatively correlated with the relative abundance of dominant species, and that the strength of interspecific correlations among phytoplankton decreased significantly in non-freshwater environments. PLS-PM analysis demonstrated that water temperature and salinity indirectly regulated chlorophyll-a concentration and phytoplankton community diversity by modulating the abundance of key phytoplankton species. Overall, phytoplankton community assembly in estuaries around the Bohai Sea was jointly driven by spatial filtering effects of salinity and seasonal variations in water temperature, whereas direct regulation by nutrients was relatively weak in this region.

Key words: phytoplankton, community assembly, temperature, salinity, keystone species