WATER QUALITY AND PLANKTON FLUCTUATION IN GAOZHOU RESERVOIR

The trophic state and plankton distribution were investigated in Gaozhou Reservoir (South China) in flood and dry seasons of 2000 and compared with the data collected ten years ago. It was oligo-mesotrophic, with higher trophic state in flood season than that in dry season. Zooplankton abundance was much higher in dry season than that in flood season and primarily predominated by copepod (Mesocyclops leuckarti, nauplii and copedid) and cladoceran (Ceriodaphnia cornuta and Bosmina longirostris). Phytoplankton abundance and chlorophyll-a concentration was higher in flood season than that in dry season and predominated numerically by Cyanophyta, Chlorophyta and Bacillariophyta. However, Cyanophyta predominated primarily in dry season. Total phosphorous concentration dropped dramatically from 0.021 mg L -1 in 1988 to 0.006 mg L -1 in 2000, while total nitrogen concentration rose from 0.405 mg L -1 to 0.820 mg L -1. As a result, the trophic level was a little lower than that ten years ago and the species composition and the abundance of plankton changed greatly. Phytoplankton was absolutely predominated by Cyanophyta in 1988 and then Cyanophyta, Chlorophyta and Bacillariophyta in 2000. Microzooplankton (rotifer and protozoa) numerically predominated in 1988 and was replaced by macrozooplankton (copepod and cladoceran) in 2000. The comparison analysis showed that the reservoir water quality was improved after twelve years watershed management. Fig 3, Tab 4, Ref 19