Study on the Distribution Characteristics and Influencing Factors of Phytoplankton Communities in Poyang Lake, China, Under Summer High-Temperature and Drought Conditions

Extreme heat and drought in Poyang Lake may contribute to pollutant accumulation and phytoplankton blooms. In order to understand these dynamics, phytoplankton assemblages and physicochemical parameters were compared between a high-temperature drought year (2022) and a reference year (2023). Population statistics, physicochemical parameter measurements, correlation analysis, and redundancy analysis (RDA) were used in the study to investigate phytoplankton communities and environmental factor impacts under high-temperature droughts. The results indicate the following: (1) There were 97 species of phytoplankton belonging to 7 divisions, primarily Chlorophyta, Bacillariophyta, and Cyanobacteria. The average phytoplankton density was 14.31 × 106 cells/L, and the biomass was 7.72 mg/L. Cyanobacteria accounted for 83.90% of the total phytoplankton abundance; Microcystis flos-aquae was the dominant species throughout the lake. (2) Habitat conditions vary significantly from region to region (p < 0.05). Transparency (SD), total nitrogen (TN), total phosphorus (TP), and nitrate nitrogen (NO3-N) are key environmental factors affecting the structure of phytoplankton communities in the lake. Notably, the low SD likely resulted from both high turbidity and phytoplankton abundance, particularly cyanobacterial blooms. (3) The comparison between 2022 and 2023 suggests that the high-temperature and drought conditions in 2022 may have contributed to a substantial decline in lake water level and to the accumulation of nitrogen and phosphorus. These changes could, in turn, have favored phytoplankton aggregation and growth. However, because the comparison covers only two years and lacks continuous water temperature observations, these associations should be considered preliminary.