Study on the changes of phytoplankton communities composition and their responses to environmental factors in the Changjiang River Estuary-Hangzhou Bay in the summer of 2004－2023

Phytoplankton play an essential role in marine ecosystems, with their community composition changes being influenced by the combined effects of anthropogenic activities, climate change, and hydrodynamic processes. The Changjiang River Estuary-Hangzhou Bay region has faced increasing environmental pressures, such as nutrient enrichment, harmful algal blooms, and hypoxia, driven by intensive human activities. However, long-term studies on phytoplankton community composition change and its environmental drivers remain insufficient. By utilizing summer monitoring data (2004－2023) from national marine ecological environment monitoring work, this study systematically analyzed the characteristics of phytoplankton communities across four segments (Changjiang River Estuary mouth, Outer Changjiang River Estuary mouth, Hangzhou Bay and Zhoushan coastal waters) and identified the key environmental influencing factors. The results showed that: the dinoflagellate/diatom ratios increased in Changjiang River Estuary mouth and Outer Changjiang River Estuary mouth, but it decreased in Hangzhou Bay and Zhoushan coastal waters. This phenomenon was associated with variations in the concentrations of dissolved inorganic nitrogen (DIN) and soluble reactive phosphate (SRP), as well as the ratios of DIN/SRP and NH4-N/DIN. Skeletonema costatum persisted as the dominant species across all segments but exhibited declining dominance. Concurrently, Pseudo-nitzschia pungens among the dominant phytoplankton species demonstrated increasing frequency in Outer Changjiang River Estuary mouth and Zhoushan coastal waters, whereas the frequency of Coscinodiscus spp. gradually diminished in Hangzhou Bay and Zhoushan coastal waters. Spatially, progressive seaward increases were observed in dinoflagellate/diatom ratios, diversity index, and richness index along the estuarine gradient, potentially associated with offshore gradients of DIN, SRP, and suspended solids concentrations. Redundancy analysis and generalized additive models revealed that nutrients (particularly DIN and SRP) and sea surface salinity were the key environmental influencing factors of phytoplankton community composition in Changjiang River Estuary mouth and Zhoushan coastal waters. This study provides basic data and technical support for understanding the changing patterns of coastal marine ecosystems under the influence of human activities and global climate change.