Characterization of Zooplankton Community Structure and Environmental Factors in the Qingyi/Guxi River Network

Qingyi River and Guxi River,located in the border area of Jiangsu Province and Anhui Province,connect the Yangtze River and Shijiu Lake.These rivers,along with the Qingshan River,Yunliang River and other small connector streams,form a complex river network in the lower reaches of the Yangtze River.The hydrology in the network is complex,influenced by the Yangtze River and regulation of impounded waters,and this is reflected in the zooplankton community structure.Rotifer and crustacean populations were investigated in 2012 during the normal(October) and dry(December) seasons at six sample sites in the river network.The species composition of zooplankton and variations in the standing stock were determined and the inter-specific relationship of the zooplankton populations and the relationship of the zooplankton community to environmental factors were discussed.The results provide basic data to support continued study of the zooplankton community and manage zooplankton succession in the river network.Samples for qualitative analysis of rotifers were collected with a No.25 plankton net and fixed with 5% formalin.For quantitative analysis,2 L water composite samples were collected by mixing water from the surface,middle and bottom layers at each sample site and fixing with Lugol's solution.Crustacean samples for qualitative analysis were collected with a No.13 plankton net.Crustacean samples for quantitative analysis were collected by filtering 20 L of water through a No.25 plankton net and fixing with 5% formalin.Zooplankton in all samples were classified and identified in the laboratory.The water quality parameters,NH+4-N,NO-2-N,TN,TP,CODMn,Chl.a,were determined according to Water and Wastewater Monitoring Analysis Methods.In total,31 species of Rotifera(dominant species:Keratella cochlearis and Synchaeta sp.),14 species of Cladocera(dominant species:Bosmina sp.) and 20 species of Copepoda(dominant species:Microcyclops varicans,Limnoithona sinensis and Thermocyclops sp.) were identified.Rotifera density was the highest(average value,350.69 ind / L),followed by Copepoda(3.44 ind / L) and Cladocera(0.15 ind / L).Seasonal variation of species richness and population density of rotifers was different than that for crustaceans.For rotifers,species richness and population density were higher in normal season than in dry season,while species richness and population density for crustaceans displayed the opposite patterns.Canonical correspondence analysis(CCA) indicates that the zooplankton community structure and distribution are influenced by nitrogen,phosphorus,CODMnand Chl.a.During dry season,there is a significant positive correlation between the population densities of Rotifera,Cladocera and Copepoda(P 0.01) and the density of all three populations with Chl.a(P 0.05).In conclusion,zooplankton community structure and the succession of dominant species in the river network follows specific patterns determined by the hydrological condition after flood(high flow) season and the inter-specific relationship of rotifers and crustaceans.During dry season,the restricted food supply is a primary factor in the competition among zooplankton populations.