Interannual shifts in July cyanobacterial communities toward bloom-forming taxa amid nutrient decline and warming in Lake Changhu.

Warming and declining nutrient concentrations are reshaping cyanobacterial communities in shallow eutrophic lakes, but how these concurrent changes affect cyanobacterial community composition and dominance remains unclear. Using a consistent July time series from Lake Changhu, a shallow eutrophic lake in the middle Yangtze River basin, we examined long-term changes in phytoplankton composition, cyanobacterial functional groups, and their associations with environmental and meteorological variables from 2012 to 2025. Lake Changhu remained moderately eutrophic, although total nitrogen (TN) and total phosphorus (TP) concentrations in 2025 were 42.1% and 45.3% lower than in 2012, respectively, and soluble reactive phosphorus (SRP) also declined significantly. Despite these nutrient declines, cyanobacteria became increasingly dominant in later survey years, with Shannon-Wiener diversity and Pielou's evenness declining after 2018. Dominant cyanobacteria shifted toward bloom-forming genera, including Microcystis, Dolichospermum, Aphanizomenon, Planktothrix, and Planktothricoides. The proportion of bloom-forming cyanobacteria increased from 2.2% in 2012 to 98.3% in 2025, whereas filamentous and diazotrophic cyanobacteria showed no significant monotonic trends. Phytoplankton resource use efficiency increased, and among functional ratios, only the bloom-forming/non-bloom-forming ratio was negatively related to dissolved inorganic nitrogen (DIN) and SRP, with a stronger relationship for SRP. Several bloom-forming genera were associated with temperature-related gradients, and a significant SRP × warming interaction indicated that the association between warming and bloom-forming dominance varied with SRP conditions. Overall, Lake Changhu experienced a long-term shift toward bloom-forming cyanobacteria amid declining nutrient concentrations and warming. Integrating cyanobacterial traits with nutrient and temperature monitoring may improve assessment of bloom-forming potential.