Threshold responses of plankton community structure to nutrient gradients and environmental drivers in the Yangtze...

Increasing nitrogen and phosphorus inputs from the Yangtze River have intensified estuarine eutrophication, posing critical threats to coastal aquatic ecosystems. This study investigates decadal (2011-2022) spatiotemporal dynamics of phytoplankton and zooplankton communities in the Yangtze Estuary, unraveling their responses to environmental perturbations. Phytoplankton communities underwent a significant compositional shift toward improved diversity (higher species richness) despite reduced cell density, transitioning from diatom-dinoflagellate dominance to more diverse assemblages. In contrast, zooplankton communities deteriorated, exhibiting marked declines in species richness and biomass, with copepods, larvae, and coelenterates emerging as dominant taxa. Nutrient analyses revealed interannual decreases in dissolved inorganic nitrogen (DIN) and phosphate (PO₄-P) concentrations, with a slight increase in silicate (SiO₄-Si), and annual average N:P ratios consistently exceeding the Redfield ratio (16:1), indicating persistent phosphorus limitation. Spatially, outer estuarine regions maintained the highest biodiversity and biomass, forming distinct ecological gradients in species richness and evenness compared to inner estuarine zones. Canonical correspondence analysis and Mantel tests revealed strong associations between planktonic assemblages and key environmental drivers: DIN, PO₄-P, SiO₄-Si, and salinity. TITAN (Threshold Indicator Taxa Analysis) further identified critical ecological thresholds with ranges of 0.289-1.360 mg/L for DIN, 0.002-0.023 mg/L for PO₄-P, and 0.685-1.631 mg/L for SiO₃-Si. These findings bridge the gap between plankton community dynamics and nutrient thresholds in large river estuaries, providing mechanistic insights for evidence-based eutrophication management and sustainable coastal ecosystem governance.