EFFECTS OF RAINFALL ON SPRING PHYTOPLANKTON COMMUNITY STRUCTURE IN XIANGXI BAY OF THE THREE-GORGES RESERVOIR, CHINA

The Xiangxi Bay showed a serious deterioration of water quality with spring algal blooms occurring repeatedly after the filling of the Three-Gorges Reservoir. A gradual increase in precipitation during spring was observed after the drought in winter and thus rainfall could markedly affect spring phytoplankton community structure. To explore the effects of rainfall on spring phytoplankton blooms in the Xiangxi Bay, an enclosure experiment was conducted during April and May 2008. In the enclosure with rainwater input, despite the high content of total nitrogen and total phosphorus, rainwater had not a significant effect on physico-chemical parameters, phytoplankton and zooplankton. In the reservoir, low rainfalls (<= 20 mm) did not affect physico-chemical parameters, phytoplankton and zooplankton. Moderate rainfalls (similar to 35 mm) had a small effect on phytoplankton and zooplankton but markedly affected physico-chemical parameters such as pH, conductivity, turbidity and nitrate concentration. Heavy rainfalls (>= 64 mm) had significant impacts on physico-chemical parameters, phytoplanIcton and zooplankton. Heavy rainfalls triggered a lot of soil and sand input thus affecting plankton growth through altering ambient environmental conditions. Heavy rainfalls decreased pH, conductivity, transparency, light availability, chlorophyll a concentration but increased nitrate concentrations and turbidity. In addition, the phytoplankton structure and abundance in the Xiangxi Bay were significantly affected by the physicochemical factors changed by heavy rainfalls.