Single and multiple stressors alter phytoplankton biomass and diversity with contrasting responses of Bacillariophyta and other phytoplankton

Shallow lakes provide vital ecological services yet face escalating threats from climate warming, eutrophication, and imidacloprid pollution. Understanding how these stressors individually and interactively impact phytoplankton—the cornerstone of aquatic food webs—is critical for ecosystem health. Through an 8-month, multi-seasonal mesocosm experiment, we assessed the individual and combined effects of these stressors on phytoplankton biomass and community structure and diversity. We found that nutrients and imidacloprid, individually and combined, increased total phytoplankton biomass while favoring Cyanobacteria (dominant: Microcystis aeruginosa ) and Chlorophyta. Crucially, despite imidacloprid-induced suppression of their primary copepod grazers, Bacillariophyta (diatom) biomass declined disproportionately due to their lack of direct abiotic benefits or compensatory mechanisms, coupled with intensified competition from cyanobacteria that gained multifaceted advantages. Bacillariophyta exhibited unique vulnerability, with severe species losses under imidacloprid, contrasting sharply with cyanobacterial resilience. Warming, imidacloprid, and their combination reduced total species richness, while all single stressors decreased Shannon diversity and equitability. While individual stressors significantly impacted phytoplankton communities, their combined effects did not consistently exacerbate these impacts and frequently mitigated—or even reversed—the direction of negative effects induced by individual stressors, particularly those from imidacloprid exposure. This study underscores that environmental risk assessments must avoid overestimating individual stressors in lake ecosystems subject to multiple pressures. Concurrently, it highlights the necessity of prioritizing monitoring differential phytoplankton groups (particularly vulnerable Bacillariophyta versus resilient Cyanobacteria). Such approaches are critical to developing targeted management strategies, accurately projecting ecological trajectories of phytoplankton communities, and ultimately safeguarding the sustainable health of shallow lake ecosystems. • Nutrient and imidacloprid boosted total phytoplankton, Cyanophyta, and Chlorophyta biomass. • Warming and imidacloprid, alone or combined, reduced phytoplankton richness. • Warming and imidacloprid reduced Bacillariophyta biomass and richness. • Single stressors (warming, nutrient, imidacloprid) reduced the Shannon index and equitability. • Stressor combinations mitigated the extreme negative effects of individual stressors, especially imidacloprid.