Salinity and Turbidity Gradients Driven Spatial Heterogeneity of Phytoplankton Community in the Eutrophic Macrotidal Qiantang River‐Estuary‐Coastal Sea Continuum

Abstract The river‐estuary‐coastal sea continuum is usually characterized by drastic tidal fluctuations and significant spatial gradients in salinity, turbidity, and nutrients. However, the response of the phytoplankton community in the continuum to such physicochemical variations remains poorly understood. Here, three cruises in dry, dry‐to‐wet transition, and wet seasons during 2022–2023 were conducted to explore the spatial variation in phytoplankton community and its main drivers in the eutrophic, macrotidal Qiantang River‐estuary‐coastal sea continuum. Our results revealed notable spatial heterogeneity in phytoplankton community composition across four subregions (i.e., tidal freshwater zone, upper estuary, middle estuary, and lower estuary). Generalized additive models showed that the spatial variation in phytoplankton abundance was largely explained by physical properties (i.e., salinity and turbidity). Redundancy analysis further confirmed that salinity and turbidity explained more variation in community composition than nutrients. The significant distance‐decay relationship indicated that dispersal limitation profoundly influences the spatial distribution pattern of the phytoplankton community. Deterministic processes dominated community assembly in the continuum, and the relative importance of environmental filtering and stochastic processes in structuring the phytoplankton community varied across seasons. Variation partitioning analysis confirmed that the biogeographical pattern of phytoplankton was largely conditioned by the spatially structured environmental variations. These findings highlight the contributions of environmental filtering and neutral processes in shaping the spatial distribution of phytoplankton and enhance our understanding of how pronounced environmental gradients, such as salinity fluctuations and the turbidity maximum zone, regulate the spatial variation of the phytoplankton community in the macrotidal river‐estuary‐coastal sea continuum.