In the context of global climate change, frequent summer heavy rainfall events act as significant disturbances to the ecosystem functions of shallow lakes. This study examined the response of phytoplankton community structure and dynamics to heavy rainfall in Lake Changhu, a shallow eutrophic lake, through monthly monitoring during the summer months (June–August) of 2020–2022. The results revealed that heavy rainfall induced substantial water level fluctuations and shifts in key environmental parameters. Marked interannual variations were observed in the phytoplankton community, with the highest species richness in summer 2021 and lowest in 2022. While Chlorophyta dominated in species composition, Cyanobacteria overwhelmingly dominated in abundance, with key taxa including Dolichospermum flos-aquae L., Pseudanabaena limnetica L., Oscillatoria princeps V., Microcystis wesenbergii K., and Merismopedia minima B. Both phytoplankton abundance and biomass peaked in summer 2021. Community diversity indices were consistently lower in June compared to July–August, indicating higher environmental stress and a more simplified community structure during the initial rainfall period. A comprehensive water quality evaluation suggested that Lake Changhu was in a lightly to moderately polluted state. Correlation and redundancy analyses (RDA) identified rainfall, water temperature, and nutrient concentrations as the primary environmental drivers shaping phytoplankton community succession. These findings systematically elucidate the mechanistic responses of phytoplankton to heavy rainfall disturbances, offering a scientific foundation for ecological resilience assessment and adaptive management of shallow lakes under climate change.

Aquatic eukaryotic communities play important roles in biogeochemical nutrient cycling and ecosystem functioning and are also sensitive to changes in aquatic environmental conditions. Qianmudang Reservoir is a typical shallow drinking-water reservoir supplying approximately 500,000 residents in Haiyan County, Zhejiang Province, China. However, the seasonal dynamics of its aquatic eukaryotic communities remain poorly understood. In the present study, we used 18S rRNA gene amplicon sequencing to characterize aquatic eukaryotic communities across four seasonal surveys in 2025. A total of 4903 eukaryotic amplicon sequence variants (ASVs) were obtained. ASV richness and Shannon diversity were highest in summer. Cryptomonas (Cryptophyta) showed the highest overall relative sequence abundance at the genus level, TSAR accounted for the greatest relative sequence abundance at the supergroup level. Community composition showed pronounced seasonal differentiation, and indicator value analysis identified 17 season-associated genera. Only 33 ASVs were detected across all four seasons, and β-diversity partitioning revealed that seasonal community dissimilarity was dominated by ASV turnover rather than nestedness-resultant differences. Partial db-RDA identified associations between community composition and total phosphorus, dissolved oxygen and water temperature, where total phosphorus had the strongest marginal effect. Variation partitioning further showed that environmental variables and sampling period jointly explained a large proportion of community variation. These findings revealed pronounced seasonal restructuring of aquatic eukaryotic communities at both taxonomic and ASV levels and highlight the importance of ASV turnover and occupancy patterns for understanding temporal biodiversity dynamics in shallow drinking-water reservoirs.