The Water-Sediment Regulation Scheme Drives Phytoplankton Dynamics in the Yellow River Estuary

The Water-Sediment Regulation Scheme (WSRS) of the Yellow River is a large-scale anthropogenic intervention designed to alleviate downstream channel siltation, yet its long-term ecological effects on estuarine phytoplankton communities remain contentious. Based on a 12-year (2011–2022) continuous monitoring dataset from the Yellow River Estuary (YRE), including two consecutive non-WSRS years (2016–2017) as a baseline, this study systematically analyzed the impacts of the WSRS on phytoplankton abundance, community structure, and dominant species succession, as well as the underlying regulatory mechanisms. Our results demonstrate that the WSRS significantly reduced salinity (p < 0.001) and delivered pulsed loads of dissolved inorganic nitrogen (DIN) and silicate (DSi), accounting for 11.64–40.63% of annual fluxes, while intensifying phosphorus-limited stoichiometric imbalance (DIN/DIP mean: 361.67 ± 124.57). Phytoplankton abundance exhibited extreme interannual fluctuations during WSRS years, with an annual mean abundance (957.08 × 104 cells/m3) nearly six times higher than that of non-WSRS years (165.88 × 104 cells/m3), and displayed a distinct nearshore inhibition and offshore stimulation spatial pattern, with the transition zone corresponding to the salinity front zone. At the community level, the WSRS did not alter the absolute dominance of diatoms but drove a directional succession from larger centric diatoms toward small, needle-shaped, low-salinity-tolerant species (Synedra sp., Nitzschia delicatissima); this shift is co-regulated by bottom-up environmental filtering (low salinity and high nutrients) and top-down size-selective grazing. This study reveals the net enhancing effect of the WSRS as a pulsed anthropogenic disturbance on estuarine phytoplankton communities and its dual regulatory mechanisms, providing long-term empirical evidence and a theoretical basis for ecological impact assessment and the adaptive management of large-scale river regulation projects.