Spatiotemporal patterns of phytoplankton communities in coastal waters near the Taishan Nuclear Power Plant.

Coastal nuclear power plants are expanding rapidly, yet their ecological effects on phytoplankton in open coastal waters remain difficult to distinguish from strong seasonal background variability. We combined pre-operation surveys in 2015-2016 with monthly post-operation monitoring in 2019 around the Taishan Nuclear Power Plant in the northern South China Sea. Reference-based difference-in-differences (DiD) models and continuous-distance analyses provided little evidence for persistent near-field post-operation divergence. Instead, cross-period changes were mainly characterised by broader environmental differences, including lower chemical oxygen demand and higher nitrate after operation. Further analyses of the 2019 monthly data showed that spatial gradients were seasonally bounded and concentrated in late spring to summer, especially from May to August. The cumulative observed phytoplankton taxon pool was smaller after operation, Shannon diversity decreased in spring-early summer and winter, and multivariate analyses indicated clear before-after differences in community composition, including a relative increase in dinoflagellates. These community-level differences indicate broader before-after reorganization rather than a stable year-round distance gradient from the outfall. Our findings support season-aware monitoring that prioritizes periods of strongest detectability and integrates physicochemical variables with phytoplankton community composition. Because the pre- and post-operation surveys were conducted in different years and used partially different station networks, these results should be interpreted as operation-associated ecological patterns rather than definitive causal effects of plant operation.