Effects of water level fluctuations on phytoplankton in a Changjiang River floodplain lake (Poyang Lake): Implications for dam operations

As one of the few remaining lakes that is freely connected with the Changjiang (Yangtze) River, Poyang Lake exhibits highly variable water level fluctuations. In this study, we examined the phytoplankton biomass and community including two hydrological and climatic variables (high water flow velocity, low water temperature in low water season and low water flow velocity, high water temperature in high water season). Diatoms dominated (40-87% of total) the community in periods of low water levels. In periods of high water levels, the community was dominated by cyanobacteria (45-93% of total). Various functional groups were also analyzed to assess the effects of hydrological conditions on phytoplankton community structure. The well-developed populations during the low water periods were representatives of functional groups P (Aulacoseira granulata, 28.7% of total) and Y (Cryptomonas spp., 12.4% of total). In the high water periods, A. granulata and Microcystis spp. (M and LM) increased to 39.8% and 12.7% of the total phytoplankton biomass, respectively. We then used parabolic curves to identify the optimal lake water levels for each phytoplankton group. The total phytoplankton, diatom, cyanobacteria, and green algae responded positively to low water level and negatively to high water level. The water level of 14-15 m is crucial for phytoplankton biomass patterns and succession through the alternations of washing-out and dilution effects and biological competition. These results support the hypothesis that water level control projects (e.g., dams) may affect phytoplankton biomass and assemblage structure patterns in Poyang Lake by altering its seasonal hydrology.