Plankton community divergence under weak physicochemical gradients across a port-estuary-aquaculture coastal mosaic

Coastal port-bay systems are increasingly influenced by shipping activities, riverine inputs, and aquaculture expansion, posing growing risks to ecosystem stability and harmful algal blooms (HABs). Yet how contrasting local disturbance regimes shape plankton community trajectories under weak spatial gradients in bulk water chemistry remains poorly resolved. In particular, it is unclear whether adjacent nearshore sectors experiencing different dominant pressures can develop divergent ecological pathways despite similar physicochemical backgrounds. Using repeated surveys at 24 fixed stations (12 for plankton sampling) in 2021 and 2024 across three pressure-defined sectors off Yangpu Port (estuary-harbor complex, port-dominated waters, and aquaculture-influenced zone), we show that although spatial differentiation in routine hydrochemical parameters was generally weak, phytoplankton and zooplankton communities showed distinct region-specific restructuring. Zooplankton communities exhibited more pronounced interannual changes in the port-affected sectors, whereas their response was weaker in the aquaculture-influenced region. The aquaculture sector showed a pronounced shift toward diazotroph-influenced assemblages, while port-affected waters exhibited stronger structural reorganization. These findings suggest that community restructuring and HAB-related intensification can occur without clear spatial deterioration in bulk water quality, with the relative abundance of harmful algal taxa increasing from 2.455.82% in 2021 to 12.82-22.30% in 2024. Taken together, these results imply that reliance on bulk physicochemical metrics alone could potentially overlook early ecological changes, highlighting the value of integrating community-based indicators into monitoring and adaptive management frameworks for rapidly developing coastal zones.