Effects of silver carp (Hypophthalmichthys molitrix) on spring phytoplankton community structure of Three-Gorges Reservoir (China): results from an enclosure experiment

Serious phytoplankton blooms occurred repeatedly every spring in many bays of the Three-Gorges Reservoir. However, little was known about the impact of phytoplanktivorous fish on spring phytoplankton community structure in the Three-Gorges Reservoir. In this study, an enclosure experiment was conducted to assess the impact of silver carp (Hypophthalmichthys molitrix) on the plankton community structure and water quality of the Three-Gorges Reservoir during 2 April to 2 May 2008. The field experiment was performed in six enclosures. Stocking silver carp into enclosures caused a dramatic change in the aquatic ecosystem. For example, pH, transparency, DO and phosphate were reduced, while chlorophyll-a concentration and turbidity increased dramatically. Furthermore, during the enclosure experiment, some zooplankton such as rotifer and copepoda were significantly reduced, and some phytoplankton and protozoa were significantly increased by silver carp. The predation by silver carp might have increased phytoplankton biomass through reducing the densities of some zooplankton which directly fed on phytoplankton. It was concluded that silver carp was not suitable for clearing spring phytoplankton (<20 μm) blooms in the Three-Gorges Reservoir at the present time.