| In recent years, eutrophication has become a major factore deteriorating the water quality of lakes.Eutrophication was gradually deepened since 1980s.Ecosystem structure and function have been severely degraded, and lake quality has been deteriorated. Control of phytoplankton is one of the keys to restore lake. Phytoplankton is not only relevant to nutrients, but also zooplankton (especially Daphnia).The current researches on biological control only consider the effects of filter-feeding fish on zooplankton. However, in shallow lakes, in addition to effects of filter-feeding fish predation on zooplankton, sediment resuspension also has effects on zooplankton. Taihu is a typical shallow lake, and sediment resuspension caused by wind and waves is one of the important characteristics.Sediment resuspension can be directly or indirectly impact on the ecosystem, such as light, nutrient release and absorption, thereby affecting phytoplankton. Sediment resuspension also has direct or indirect effects on zooplankton. Understanding the effects of sediment resuspension processes on zooplankton will help us to reveal the mechanism of changes of zooplankton community structure, so as to provide the basis for manipulating zooplankton and lake restoration.An experimental study of sediment resuspension was carried from May to June 2008. We set up three treatments:(strong resuspension, SR, daily 11 hour's duration of resuspension);(weak resuspension, WR, daily 5 hour's duration of resuspension); (control treatment, C, without resuspension) with constant exogenous nutrient loading. Each treatment has three replicates.The results show that resuspension treatments have significant impacts on zooplankton community structures.Daphnia was dominant in the control group, while rotifers and small cladocera were dominant in the resuspension groups.When the sediment resuspension occurs, the trend of the succession is large zooplankton (such as Daphnia and copepods) to the small zooplankton (such as rotifers and Bosmina).To explore impacts of different levels of sediment resuspension on zooplankton communities, an experiment with further increased sediment resuspension was carried from April to May 2009.We set up three treatments:(strong resuspension, SR, daily 24 hour's duration of resuspension);(weak resuspension, WR, daily 5 hour's duration of resuspension);(control treatment,C, without resuspension).Each treatment had three replicates. The experimental results show that the numbers of Bosmina coregoni and rotifer in WR were higher than SR, indicating that extremely high sediment resuspension can inhibit the growth and reproduction of small sized zooplankton.Third experimental study was carried out from 11 to 12 September 2009 to access the effect of sediment resuspension on the feeding of Daphnia carinata. We set up four treatments:no sediment resuspension without zooplankton (C), no sediment resuspension with zooplankton (Z); sediment resuspension without zooplankton (R) and sediment resuspension with zooplankton (RZ).Each treatment had four replicates.The results show that sediment resuspension decreased the feeding and growth rate of Daphnia carinata. The sediment got into the Daphnia carinata filters and reduced the feeding efficency on phytoplankton. Sediment also affects the food digestibity and assimilation in Daphnia carinata.Therefore, the effects on Daphnia carinata feeding is likely the main reason for reduced the numbers in sediment resuspension group.Our results suggest that sediment resuspension is one of the major determinants on the changes of zooplankton community structures.In shallow lakes, sediment resuspension is frequent and sometimes intensive. Even if the number of filter-feeding fish is reduced, large Cladocera may be still difficult to be restored due to sediment resuspension. Therefore, when food web manipulation is used to restore a lake, in addition to considering the control of fish, one must also take into account the effect of sediment resuspension.
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