Community Structure And Water Quality Evaluation Of Macrobenthos In Laoguan River Of Danjiangkou Reservoir

Danjiangkou Reservoir,as the water source of the Middle Route Project of South to North Water Transfer,its water quality is directly related to the drinking water safety of more than 20 urban residents along the line.Laostork River,the main stream,is the closest river to Taocha water intake in the middle line project.Its water quality is directly related to the safe operation of the South-to-North Water Transfer Project.From March 2018 to June 2019,six ecological sampling of macrobenthos were conducted in 35 sample points of Laostork River.According to seasons,river reaches and land use types of river basins,the habitat,physico-chemical properties of water quality and biological indexes of different rivers were evaluated.Water Quality single factor analysis showed that the total nitrogen of Laostork River exceeded the standard,and the total phosphorus and ammonia nitrogen were in the safe range.According to chlorophyll a nutrition index,the Laostork River was in the stage of poor nutrition.The comprehensive water qualities under Xuejiaying Beiyu,the end of Muzhai River,Fengwanding River,Guzhuhe Bridge 1,Guzhuhe Bridge 2 and Guzhuhe Bridge assembly point in Xixia county,Shibanhe Bridge upstream and Zhongguan Laoying Bridge in Xichuan county,police station and dam in Taoxi town,Neixiang County were poor,which could be regarded as the polluted rivers and the key objects.A total of 35483 macrobenthos were collected in Laoguan River,belonging to 4 phyla,9 classes,18 orders,51 families and 53 genera.Among them,18522 aquatic insects belonged to 30 families and 30 genera,761 oligochaetes to 3 families and 3 genera,13151mollusks 7 families and 12 genera,and other groups to 8 families and 8 genera.The average density and biomass were 42.24 ind./m~2 and 19.93 g/m~2,and the dominant groups were Cyclobalanus sp.,Corbicula fluminea and Meretrix hupensis.The richness of the Laoguan River in spring and autumn was higher than that in winter and summer,but the density and biomass were the highest in winter.According to the water quality classification by EPT diversity of aquatic insects,the Laoguan River was slightly polluted in spring,slightly and moderately polluted in summer and autumn,and slightly and severely polluted in winter.In the four seasons,the water quality in winter was the worst,so the river in this season should be protected.From the temporal and spatial changes of macrobenthos in different reaches of the old stork river,the groups in the upper reaches were mainly composed of aquatic insects and molluscs in the middle and lower reaches.Light pollution was in the upstream,heavy pollution in the middle and moderate pollution in the downstream.With the human activity strengthen in farmland,the water quality gradually decreased.According to the water quality classification,the upper reaches of the Laoguan River were in class II,the middle reaches were class III,and the lower reaches were class II.According to the topography,vegetation coverage,pollution status,population density and other characteristics,the Laoguan River Basin was divided into four different land types:type I(forest area?90%),type II(forest area?50%,cultivated land population area?50%),type III(forest area?50%,cultivated land population area?50%)and type IV(cultivated land population area?90%).The macrobenthos community structure showed that the larger was the forest area,the more abundant was the aquatic insects.However,the smallest density and biomass of macrobenthos appeared in the type I.According to the diversity index of aquatic insect EPT,the type I watershed was lightly polluted,the type II and the type III were moderately polluted,and the type IV was heavily polluted.From the land use types,the water qualities with more human towns and farmlands were poor,thus reducing agricultural non-point source pollution and urban point source pollution still needs to be strengthened.