Effects Of Sediment Dredging On Planktonic Ecosystem Of Shuangqiao River

The effects of sediment dredging events on the aquatic ecosystem and ecological feedback of plankton on sediment dredging of Shuangqiao River in Chaohu Basin were researched in the present study,a one-year investigation on the aquatic ecosystem was operated in the river in 2017-2018.The water quality,the plankton community structures,the biodiversity index,functional groups of the phytoplankton community were compared and analyzed in pre-dredging and post-dredging period.And we finally evaluated the effectiveness of sediment dredging of Shuangqiao River.The results are summaried below:The undredging reach of Shuangqiao River had no significant changed on the concentration of nitrogen(N)and phosphorus(P)throughout the year.At first,p H and dissolved oxygen(DO)values of the dredging reach decreased,and then increased,the turbidity(NTU)showed a fluctuating rising trend.Sediment dredging greatly reduced the concentration of nitrogen and phosphorus in the Shuangqiao River,and the concentration of nitrogen and phosphorus decreased by 48.09%and 19.09%,respectively.The reduction of ammonia nitrogen(NH4~+-N)was the largest,reaching 61.26%.The water quality of Shuangqiao River has improved after dredging,but the river still retained eutrophic state.A total of 17 crustacean species were identitied in the dredging reach of ShuangqiaoRiver,including cladocerans by 8 taxa(4 genera)and copepods by 9 taxa(4 genera).Moina micrura,Diaphanosoma brachyurum,Sinocalanus dorrii and Mesocyclops leuckarti were dominant species in the pre-dredging period.Mesocyclops leuckarti,Limnoithona sinensis and Sinocalanus dorrii were the dominant species in the post-dredging reach.The copepods had obvious numerical advantages,but the cladocerans had no dominant species after sediment dredging.The dominant species changed from Moina micrura of the cladocerus in pre-dredging to Mesocyclops leuckarti of copepods.There were significantly different in density and biomass of crustaceans between pre-dredging and post-dredging period.After dredging,the average density of crustaceans decreased by58.85%,and the biomass decreased by 86.40%.The biggest change in density and biomass during sediment dredging at each sampling site was sampling 2,which decreased by 82.26%and 92.09%respectively.The percentage decrease of zooplankton biomass was greater than the density,which demonstrated the miniaturized tendency of crustacean.Meanwhile,a total of 71 phytoplankton species(8 phyla)were identified in the dredging reach of Shuangqiao River.There was little effect on the species composition of phytoplankton in sediment dredging,and the phytoplankton community was Cyanophyta–Chlorophyta–Diatom both in undredging and dredging reach.The relative density of Cyanophyta accounted for more than 50%of all sampling sites,but after dredging,the biomass of Cyanophyta decreased,which of Diatom rose continuously.As the dredging project progressed,the concentration of water nutrients decreased,and the average density and biomass of phytoplankton decreased by 91.48%and 77.71%after dredging,respectively.After dredging,the diversity index of crustaceans in Shuangqiao River had increased,while the diversity index of phytoplankton fluctuated drastically.The Simpson index,Shannon-Weiner index and Margalef index of integrated zooplankton and phytoplankton indicated that Shuangqiao River was still in Mesotrophic-Meso-eutrophic state.In total,according to FG classification method,23 phytoplankton functional groups were identified:M,H1,H2,MP,S1,LO,X2,X1,J,F,N,G,X3,C,D,Y,W1,LM,E,T,SN,K,and P.The number of phytoplankton functional groups increased from 18 to 22after dredging.The phytoplankton functional groups were dominated by the M functional group(mainly comprised of Microcystis spp.)and the MP and P functional groups(mainly composed of Diatoms).At the same time,group M had an absolute advantage at all sampling sites during the entire period of sediment dredging.The main functional group of each sampling sites in the pre-dredging period was group M+MP+C,and it was changed to group M+MP+LO+F in the initial post-dredging period,and group M+MP+P after half a year of dredging.The dominant phytoplankton functional group converted from group M,MP,P,W1,Y to group C,F,J,M,MP,P,Y.Pearson analysis of zooplankton,redundancy analysis(RDA)and Pearson analysis of phytoplankton functional groups showed that the nitrogen content of Shuangqiao River had a significant effect on planktonic ecosystem.Among them,p H,temperature,ammonium nitrogen,nitrate nitrogen,total dissolved nitrogen,total nitrogen and nitrogen:phosphorus ratio were positive to the biomass of crustacean zooplankton.Total nitrogen,ammonia nitrogen,nitrate nitrogen,nitrogen-phosphorus ratio,and temperature were the main factors affecting the phytoplankton functional groups.Combined with the research results,it was shown that the sediment dredging could significantly reduce the internal nutrients of river(nitrogen and phosphorus)in a short time.To a certain extent,the operation of sediment dredging improved the structure of plankton community,and the aquatic ecosystems of river would be healthy.Therefore,after dredging,Shuangqiao River was still in a state of eutrophication.For the urban inland river,such as Shuangqiao River,while carrying out ecological restoration,the input of external pollution would be strictly controlled.