he long-term changes of zooplankton, including in species composition, abundance and regression analysis between biomass and water environment have been studied at three sampling stations (fromⅠ to Ⅲ) of Lake Xihu (West Lake), Hangzhou, a shallow eutrophic lake treated by water pumping and dredging. StationⅠis located in the Xiaonanhu (drawing region), the bay of Lake Xihu, Station Ⅱ and Ⅲ are in the central lake and north-end of the lake (discharge region), respectively. Quantitative sampling of each group of zooplankton were taken monthly from each station in 1990, 1995 and 2003. The main purposes of this paper are to describe long-term changes in zooplankton communities of Lake Xihu, and to discuss the possible mechanisms of the change.
During the survey of 2003, 69 species of zooplankton were identified, among them 26 species were Protozoa, 27 Rotifera, 8 Cladocera and 8 Copepoda. From 1990 to 2003, the annual average density of zooplankton at Station Ⅰfrom 1270 ind./L increased to 1583 ind./L, from 3229 ind./L to 11022 ind./L at Station Ⅱ and from 3161 ind./L to 7390 ind./L at Station Ⅲ, the annual average biomass at Station Ⅰfrom 0.186 mg/L increased to 0.705 mg/L, from 0.665 mg/L to 1.399 mg/L at Station Ⅱand from 0.740 mg/L to 1.195 mg/L at Station Ⅲ. 99% of abundance and 78.0% of biomass of zooplankton were protozoans and rotifers.
During 1990—2003, some of the dominant species of zooplankton, such as Tintinnoinea, Polyarthra Trigla and Bosmina longirostris increased their percentage in abundance remarkably, whilst the proportion of Diaphanosoma leuchtenbergianum was decreased at three stations, the proportion of Trichocerca pusilla was increased during 1990—1995 after water pumping treatment and declined in 2003 after dredging, Coleps hirtus, Keratella cochlearis and Diaphanosoma brachyurum, which were dominated in 1980s, have been disappeared from three stations in recent years.
The abundance of Protozoa and Rotifera at Stations Ⅱ and Ⅲ were higher than that at Station Ⅰ for their higher trophic level. Diaphanosoma leuchtenbergianum, Bosmina longirostris, Bosminopsis deitersi and Sinocalanus dorrii were more common at Station Ⅰ for its lower trophic level, whereas Moina micrura and Thermocyclops dybowskii were more popular at Stations Ⅱ and Ⅲ. After dredging treatment, Protozoa biomass only at Station Ⅰwas positively correlated with Chlorophyll-a concentration and CODMn. There was a significant positive relationship between Rotifera biomass and Chlorophyll-a concentration at three stations, biomass of Cladocera at each station was positively correlated with Chlorophyll-a concentration and CODMn, and biomass of Copepoda at Stations Ⅱ and Ⅲ was positively correlated with Chlorophyll-a concentration and CODMn.
During 1990—2003, the biomass of Rotifera had positive linear relationships with pH value and chlorophylla-a concentration, and a negative relationship with transparency in Lake Xihu. The most significant ecological factor affected on rotifers biomass was chlorophylla-a concentration after pumping water from Qiantang River in 1995, and the ecological factor was the pH value of Lake water after dredging in 2003.
The differences in water current and trophic level are responsible for the heterogeneous distribution of each part of zooplankton in Lake Xihu. The abundance of Protozoa and Rotifera has been increased rapidly, following the eutrophic process of the lake water.