Changes of phytoplankton and water environment in a highly urbanized subtropical lake during the past ten years

Phytoplankton and water quality changes in highly urbanized lakes affect the surrounding water safety. However, due to the complexity and variability of natural changes and human disturbances, it is difficult for multi-year research with yearly sampling frequency to cover accurate changes of phytoplankton and water environment or provide constructive suggestions for managers. Based on monthly monitoring data spanning 2011–2020 in a highly urbanized subtropical lake (Hongze Lake, China), Mann-Kendall test, ANOVA analysis and variation partitioning analysis were used to assess the changes of phytoplankton and water environment, and detect dynamic responses of phytoplankton to environmental changes. Rising water temperature during winter and spring, the decrease in nitrate, and the increase in water flow and turbidity were the main environmental characteristics from 2011 to 2020. The average and maximum abundance of Chlorophyta, Bacillariophyta, and Cryptophyta significantly declined, while changes in Cyanobacteria were characterized by an increase of N2-fixing filamentous cyanobacteria and a decrease of non-filamentous cyanobacteria. The rising water temperature during spring may promote the early growth of N2-fixing filamentous cyanobacteria. The decrease in nitrate mainly resulted in the decrease of Chlorophyta and non-filamentous cyanobacteria, and the increase of N2-fixing filamentous cyanobacteria during summer and autumn. The increase of turbidity and water flow inhibited the growth of Chlorophyta, Bacillariophyta, Cryptophyta, and non-filamentous cyanobacteria, but created favourable conditions for the growth of N2-fixing filamentous cyanobacteria. In summer and autumn, managers should focus on the proliferation of N2-fixing filamentous cyanobacteria when precipitation increase, nitrogen nutrients decrease, and non-filamentous cyanobacteria risk under opposite conditions. These findings greatly improved our understanding of the dynamic response of phytoplankton communities to natural changes and anthropogenic disturbances in the urbanized subtropical lakes, and can be used to develop lake management strategies.