Seasonal environmental forcing masks thermal discharge effects on plankton communities in a subtropical coastal bay.

Coastal ecosystems adjacent to nuclear power plants may be subject to thermal disturbance; however, the relative importance of seasonal environmental forcing and localized anthropogenic disturbance in shaping plankton community assembly remains poorly understood. In this study, we integrated seasonal environmental monitoring with concurrent surveys of phytoplankton, zooplankton, and nektonic organisms across 24 stations in Daya Bay, southern China, to quantitatively assess the relative influences of seasonal forcing, spatial structure, and potential multi-trophic interactions on plankton community assembly. Environmental conditions and plankton communities exhibited marked seasonal variations but only weak spatial differentiation, indicating that seasonal forcing exerted a stronger influence on community dynamics than did localized spatial heterogeneity. Phytoplankton assemblages remained consistently diatom-dominated, with relatively stable major taxonomic composition across seasons; in contrast, zooplankton displayed pronounced seasonal turnover in dominant taxa and overall diversity. β-diversity was driven predominantly by species turnover, while analyses using NCM and C-score metrics revealed contributions from both stochastic and deterministic processes, with a notable signal of seasonally structured, non-random species segregation. RDA, VPA, and spatial eigenfunction modeling consistently demonstrated that environmental variables explained a larger fraction of the total community variation than did spatial factors. PLS-PM further uncovered both direct environmental effects and indirect pathways mediated through phytoplankton, zooplankton, and nektonic organism, revealing potential bottom-up regulation pathways and inter-trophic associations across multiple plankton and nekton groups. Notably, observed nekton-plankton relationships should not be interpreted as evidence of direct top-down predation, as bottom-trawl sampling inherently biased our nektonic data toward demersal taxa. Collectively, the dominance of strong seasonal forcing and extensive hydrodynamic mixing likely masks localized thermal effects at the spatial and temporal scales examined, underscoring the necessity for long-term, seasonally resolved, and multi-trophic monitoring programs to robustly distinguish natural variability from anthropogenic thermal disturbance.