[Spatiotemporal Variations in Nutrient Loads in River-lake System of Changdang Lake Catchment in 2016-2017]

Eutrophication of shallow lakes in the middle and lower reaches of the Yangtze River has become an increasingly serious problem. In this study, we investigated the temporal and spatial variations in nutrient loads (nitrogen, N and phosphorus, P) in the Changdang Lake Catchment located to the northwest of Lake Taihu through field sampling and laboratory analysis in 2016-2017. The results show the severity of the N and P pollution in the Changdang Lake catchment. The mean river water concentrations of TN, NO₃^--N, NH₄⁺-N, TP, Chla, and permanganate index are (3.70±0.76) mg ·L^-1, (1.81±0.42) mg ·L^-1, (1.03±0.61) mg ·L^-1, (0.38±0.31) mg ·L^-1, (25.74±37.00) μg ·L^-1, and (6.35±0.81) mg ·L^-1, respectively. N pollution in the river is more severe in winter and spring than in summer and autumn whereas P pollution in the river is worse in autumn and winter than in spring and summer. Spatially, the magnitude of river N and P pollution follows the order of northern > northwestern > southern > eastern part of the study area. The rivers are in a state of moderate to severe eutrophication. The mean lake water concentrations of TN, NO₃^--N, NH₄⁺-N, TP, Chla, and permanganate index are (2.25±0.94) mg ·L^-1, (0.98±0.47) mg ·L^-1, (0.19±0.14) mg ·L^-1, (0.11±0.03) mg ·L^-1, (18.71±8.76) μg ·L^-1, and (4.59±1.09) mg ·L^-1, respectively. The water quality in Changdang Lake is categorized as worse than class Ⅲ for TN and TP concentrations, which show decreasing trends from the west to the east to the south of the lake. The lake is in a status of slight to moderate eutrophication. The lake water quality is affected by the combination of sewage discharge and non-point source pollutant losses. The inflow rivers including the Danjinlicao River, Tongji River, and Xuebu River are the dominant pollution sources for Changdang Lake. The Danjinlicao River transports 10-12 times the total N and P loads transported by Tongji and Xuebu rivers. Changes in land use and atmospheric deposition are the driving factors of the deterioration of water quality and eutrophication in the catchment.