Responses Of Water Quality In The Western Inflowing Rivers Of Lake Taihu To The Land Use/Land Cover In The Upstream Watersheds

Water quality in drinking water sources plays a vital role in maintain the security of water supply to the citizens in the surrounding residential areas.In recent decades,the development of social economy,coupled with urbanization and intensified human activities have resulted in an increased trophic level,and a declined water quality coupled with the outbreak of algal blooms in inland waters in China.Lake Taihu watershed locates in the highly urbanized Yangtze River delta with dense river networks and population,and nuisance algal blooms occurred in the summer during the past three decades.Lake Taihu locates in the central watershed and its water quality impacts significantly to the social development of the whole watershed.External nutrient inputs from the catchment contributed directly to the eutrophication process of the lake.The predominant inflowing rivers in the catchment locate in the western region.It is therefore important to trace the trophic level variabity of the inflowing rivers and the lake itself and how water quality in the western inflowing rivers may respond to the land cover in the corresponding upstream catchments.Water samples for water quality measurements were collected from the western inflowing rivers and the corresponding lake regions.Land use data in the upstream watersheds were extracted using Arc GIS software.The variability of water quality in the western inflowing rivers and the corresponding lake regions and how water quality were influenced by upstream land cover were analyzed using Pearson and redundancy analyses.We concluded that:（1）There are temporal and spatial heterogeneity of water quality in the inflowing rivers and the corresponding western lake regions.Temporally,the mean concentrations of total nitrogen（TN）and ammonium（NH₄⁺-N）in the non-flood season were higher than that in the flood season,while the mean concentration of total phosphorus（TP）and chemical oxygen deman COD_Mnwere higher in the flood season than in the non-flood season.Spatially,the water quality in the northern lake is worse than that in the southern half of the lake.（2）Both single-factor index evaluation and the average water quality pollution index methods were used to evaluate the water quality,but both methods indicatde that the inflowing rivers and lake regions were seriously polluted.Nitrogen and phosphorus were the major factors underlying the trophic status in the rivers and the western lake regions.（3）The concentration of TN,TP,and NH₄⁺-N in the western lake regions increased significantly with increasing TN,TP,and NH₄⁺-N in the western inflowing rivers（P<0.01）.No significant relationship was found between COD_Mnconcentration in the lake and in the upstream western inflowing rivers（P=0.65）.（4）Significant correlations were found between the water quality of the river into the lake and land use at different scales.At the buffer zone scale,the concentration of water quality parameters,including TN,TP,and NH₄⁺-N of rivers entering the lake were significantly and positively correlated with the proportion of urban and forest land use.At the watershed scale,water quality parameters,including TN,TP,NH₄⁺-N,and COD_Mnare positively correlated with the proportion of waterbody and forest land use.Compared with the buffer zone scale,the whole upstream watershed scale land use can better used to trace the variability of water quality in the inflowing rivers,with an explanation rate of 76.8%.Among them,the waterbody land use explained38.8%of the variability of water quality in the inflowing tributaries.