Environmental DNA reveals shift in phytoplankton community driven by sewage-derived eutrophication in coastal waters

Coastal phytoplankton communities, highly sensitive to environmental fluctuations, serve as critical indicators of marine ecological health under climate change and anthropogenic pressures. Accordingly, this study employed Environmental DNA (eDNA) metabarcoding technology to evaluate the impact of wastewater discharge from the Lingshui River on phytoplankton diversity and community composition in coastal waters. The results showed that there is no significant seasonal difference in the phytoplankton diversity near the discharge outlet of the Lingshui River. However, phytoplankton community richness was significantly greater in summer than in winter. The distribution of the phytoplankton community exhibited seasonal specificity and spatial heterogeneity influenced by nutrient pollution, with three distinct clusters in summer corresponding to heavy, moderate, and light eutrophication levels in the Lingshui River coastal area. DIP was the primary factor affecting phytoplankton richness, whereas BOD and COD were key factors influencing phytoplankton diversity. Temperature (T) and salinity (S) are critical factors determining the spatiotemporal specificity of phytoplankton communities. Co-occurrence network analysis revealed that positive correlations among phytoplankton species exceeded negative correlations in both summer and winter, suggesting that cooperative interactions may outweigh competitive relationships. Five keystone species were identified in summer, among which Skeletonema marinoi was significantly correlated with the organic pollution index (OPI), indicating its sensitivity to organic pollution. No keystone species were identified in the winter. This study revealed that nutrients and organic pollutants from land-sourced wastewater discharge reshaped the phytoplankton community structure in coastal waters, providing an eDNA framework for coastal eutrophication management.