Tidal water exchanges can shape the phytoplankton community structure and reduce the risk of harmful cyanobacterial blooms in a semi-closed lake

For estuaries, inland lakes play a vital role in the ecological balance under the impact of tides. The effect of tides-induced water exchange on phytoplankton community in a semi-closed lake was studied and compared with that of an adjacent closed lake in the Oujiang River mouth in Zhejiang, East China Sea, from June 29, 2020 to June 14, 2021. Results show that the dominant species, abundance, dominance, and diversity of the phytoplankton species between the two lakes were significantly different. In the closed lake, cyanobacteria were the dominant species during the study period. However, in the semi-closed lake, the diversification of the dominant species was greater, and some species of diatoms and green algae became dominant. The average phytoplankton abundance in the closed lake was 6 times of that in the semi-closed lake. The average dominance of cyanobacteria in the closed lake was 0.96, and those in the semi-closed lake and the Oujiang River were 0.51 and 0.22, respectively. Cyanobacterial blooms occurred throughout the study time in the closed lake but not in the semi-closed one. Furthermore, the species diversity richness of the phytoplankton in the semi-closed lake was higher than that of the closed one, and the phytoplankton community between the closed lake and semi-closed lake could be divided into distinctly different groups based on non-metric multidimensional scaling analysis (NMDS) and analysis of similarities (ANOSIM) analysis. The salinity of the water was significantly greater and the transparency significantly smaller in the semi-closed lake than those in the closed lake. Therefore, water exchange driven by local tidal movement increased salinity and decreased transparency of water, which consequently shaped the community structures of different phytoplankton and reduced the risk of a cyanobacterial bloom outbreak in the semi-closed lake.