Cyanobacterial dominance responds nonlinearly to phosphorus additions under nitrogen enrichment in microcosms from a large drinking water reservoir
摘要
Nutrient-driven shifts in phytoplankton communities are key drivers of harmful algal blooms, yet the conditions promoting cyanobacterial dominance remain unresolved. We conducted a 6-day in situ experiment in August 2024 in the mesotrophic Danjiangkou Reservoir (China’s primary water source for the South-to-North Water Division Project), to examine how dissolved inorganic nitrogen (DIN: 0, 2 mg l−1) and phosphorus (DIP: 0, 0.05, 0.1, 0.2 mg l−1) additions regulate phytoplankton community structure. Phosphorus was the primary limiting nutrient: increasing DIP from 0 to 0.05 mg l−1 increased cell concentration and biovolume of phytoplankton and cyanobacteria. Nitrogen addition alone had little effect under lower DIP addition but stimulated cell growth under higher DIP (≥ 0.1 mg l−1). Species richness and diversity peaked at intermediate nutrient additions, whereas evenness declined with increasing DIP under nitrogen enrichment. Cyanobacterial dominance showed a nonlinear response to stoichiometry, increasing with NH4–N:PO4–P molar ratios up to ~30. Community composition shifted markedly: colonial coccoid cyanobacteria including Aphanocapsa, Chroococcus, and Microcystis dominated at 0.05P, while N-fixing taxa, e.g., Dolichospermum and Aphanizomenon, dominated without DIN supply. These results demonstrated that cyanobacterial dominance is governed by threshold responses to nutrient supply and stoichiometry, providing a mechanistic basis for predicting bloom risks under variable nutrient inputs.