Study On Spatio-temporal Variation Trends Of Water Quality And Its Correlation With Phytoplankton In Lake Wuliangsuhai Based On Water Pollution Index(WPI)

Lake Wuliangsuhai,the largest shallow lake of the Yellow River in China,is an important component of the Hetao region irrigation system.The water quality directly affects the safety of residential use and sustainable development of the local economy.In this study,lake Wuliangsuhai was selected as the research area and total nitrogen(TN),total phosphorus(TP),dissolved oxygen(DO),and chemical oxygen demand(COD)were monitored as the typical water quality parameters.The spatio-temporal variation on water quality of lake Wuliangsuhai was analyzed in a long time sequence from 1998 to 2017,and Cluster analysis and Daniel trend test methods were employed to analyse the water quality spatio-temporal variation trends of dry season and the wet season in 2014 and 2017 based on water pollution index(WPI).During the phytoplankton investigation in 2017,Pearson correlation analysis and Redundancy Analysis(RDA)are used to discuss the relationship withphytoplankton and WPI,in order to provide a theoretical basis for the comprehensive treatment and protection of lake Wuliangsuhai water environment.The main conclusions are as follows:(1)The results show that the water quality,which varys from inferior V to V water quality standard class of lake Wuliangsuhai improved from 1998 to 2017,spatial variations of WPI(TN),WPI(TP),and WPI(DO)followed the order of inlet >center and outlet,while spatial variations of WPI(COD)showed the order of outlet >inlet > center.TN was the critical pollution parameter through the years.The reasons for the variations in water quality in lake Wuliangsuhai were as follows: non-point source pollution and fish farming activities,point source pollution such as industrial wastewater and domestic sewage,point source pollution such as industrial wastewater and domestic sewage and ecological water compensation from the Yellow River.(2)On a yearly scale,January,April,November in 2014 and January,April in 2017 are divided into the dry season,meanwhile,June,July,August in 2014 and June,July,August and October in 2017 are divided into the wet season through temporal cluster analysis.The cluster analysis that is used in spatial variations,groups all 13 sampling sites of the basin into three statistically significant clusters,including the highly polluted region,the moderately polluted region,and the less polluted region both in2014 and 2017.WPI(TN),WPI(TP),WPI(DO),WPI(COD)is aggravated from the dry to the wet season in 2014.The surface water quality reached the V and inferior V,and TN,TP,COD are the most significant parameters.WPI(COD)was higher thanWPI(TN),WPI(TP)and WPI(DO)in the dry season,while WPI(TN),WPI(TP)and WPI(DO)were higher than WPI(COD)in the wet season in 2017.The the water quality in 2017 has improved compared with 2014.The surface water quality reached the V,and COD is the main pollution parameter.(3)In lake Wuliangsuhai,it identified that 91 species belong to 7 phyla during the3 seasons phytoplankton investigation in 2017.Chlorophyta,Bacillariophyta and Cyanophyta were the main phylums.For temporal variation of phytoplankton abundance and biomass,it was the highest in the August,followed by October and April.Redundancy Analysis(RDA)results revealed that phytoplankton species were closely related with WPI(TP),WPI(DO)in April,WPI(TN),WPI(DO)in August and WPI(TN),WPI(TP)in October.Pearson correlation analysis indicated that phytoplankton abundance and biomass were positive correlated with WPI(DO)significantly in April.