The influences of silver carp and bighead carp in bio-manipulation pen on the community structure of cladocera in Lake Gehu

As a dominant group of zooplankton in fresh water worldwide,Cladocera plays an important role in the material circulation and energy flow of water ecosystem,and it is not only a consumer for small animals and algae,but also food for zooplanktivorous fish.Many current studies have showed that non-classical bio-manipulation,a way to prevent water bloom and improve water environment by controlling fierce fish and stocking planktonivorous filter-feeding fish(such as Hypophthalmichthys molitrix and Aristichys mobilis),can be effective.However,to some extent,phytoplankton and community structure of cladocera may be varied with different stocking patterns,fish density and lake type.Lake Gehu,a total area of 164 km2 crossing Wujin and Yixing,is located in the south of Jiangsu province.In recent years,more attention was paid to the researches about the influence of H.ypophthalmichthys molitrix and Aristichys mobilis on lake ecosystems,of which the past domestic studies were mainly focused on aquatic plants,physical and chemical factors,the control of eutrophication and phytoplankton,however,no reports have been made in the bio-manipulation fish pen of Lake Gehu about the influences of abiotic environmental factors and filter-feeding fish on cladocera,and that is our work to fill in the blanks.Thus,from December 2009 to November 2010,a large bio-manipulation fish pen with the stock of silver carp(H.molitrix)and bighead carp(A.mobilis)inside was built to control the cyanobacterial bloom in Lake Gehu,an investigation and laboratory analysis was carried out to compare the variation of the cladoceran community structure between region A outside of the fish pen and regions B,D,C with different ratios of fish(1∶ 4,1∶ 2,1∶ 1.5),which was combined to discuss the influences of bio-manipulation fish pen on cladocera and the effects on improving eutrophic water.Through the identification,32 species of cladocera,belonging to 7 families,14 genera were collected inside and outside of the fish pen where there were no differences between species,and the annual average density and biomass of region A were both higher than any region inside of the fish pen,having a significant difference(P0.05)with region B,respectively.The main dominant species were Bosmina longirostris,Ceriodaphnia cornuta and Moina sp..In August,the density of Moina sp.in region A had significant differences from region B and D,while 3 regions inside of fish pen had no apparent differences,et al.,the results of which indicated that Moina sp.with a larger volume was easier to be preyed by fish than the other two small species.Moreover,some characteristic parameters of water were investigated to depict the correlation between cladoceran and environmental variables.CCA(Canonical Correspondence Analysis)showed that water temperature,permanganate index(CODMn),nitrate nitrogen(NO3-N)and water transparency(SD)were the main abiotic factors affecting the community structure of cladocera.