Responses of coastal phytoplankton communities to herbicide exposure: enhanced resistance coupled with reduced resilience

Coastal ecosystems face increasing anthropogenic disturbances, making the survival strategies of phytoplankton communities under stress a critical issue in marine ecology. The community rescue theory suggests that exposure history can enhance phytoplankton's ability to withstand lethal stress, though the mechanisms remain unclear. This study utilized two-phase mesocosm experiments to simulate exposure history and lethal pressures. By combining tolerance and heritability tests, the mechanisms by which exposure history enhanced phytoplankton tolerance were investigated. The results demonstrated that: (1) Exposure history enhanced the community tolerance threshold to atrazine through ecological (the relative abundance of dinoflagellates increased by 13.6–66.4 %) and plastic processes (the EC50 of sensitive populations increased by 12.3–114.9 %). And this enhancement was positively correlated with exposure intensity but accompanied by suppression of community biomass. (2) Rescue was more likely to occur in large-scale communities, suggesting that high biomass was a prerequisite for populations/communities to survive the period of biomass collapse. Our findings aligned with the observation in in situ marine environments: long-term exposure to herbicides enhanced community tolerance (EC50 from 97.19 ± 6.8 nmol L⁻¹ to 115.5 ± 7.8 nmol L⁻¹) and delayed the collapse of communities under lethal pressure. However, this acquired tolerance was not heritable, and rescue still led to the loss of nearly half of rare taxa, potentially hindering the community's ability to withstand other environmental stressors. Our findings elucidate how phytoplankton communities achieve rescue through structural reorganization, providing crucial theoretical underpinnings for disturbance assessment in coastal ecosystems.