Spatiotemporal water quality variations and their relationship with hydrological conditions in Dongting Lake after the operation of the Three Gorges Dam, China

Abstract

Identifying the spatiotemporal characteristics of water quality and related driving factors is essential for lake water quality management. To further understand the unique relationship between hydrological conditions and water quality in river-connected lakes, we analysed the spatiotemporal variations of water quality and hydrological factors in Dongting Lake from 1991 to 2018 and identified the effects of the operation of the Three Gorges Dam (TGD) on lake water quality. The total nitrogen (TN) and total phosphorus (TP) concentrations showed general increasing trends in the pre-TGD period, in contrast to a significant increasing TN trend and a decreasing TP trend in the post-TGD period, the latter of which was closely linked to a prominent decrease of water and suspended sediment discharge after the initiation of TGD operation. TN concentrations showed significant spatial variations during the study period, with the highest concentration in East Dongting Lake and the lowest concentration in West Dongting Lake. Nutrient inputs were the dominant pollution source determining the water quality of Dongting Lake; water discharge and water level were also important factors controlling the spatiotemporal variability of water quality. The contribution of the four tributaries to lake water quality was much greater than that of the three channels, accounting for 89.94–92.25% of N inputs and 74.10–75.85% of P inputs. Moreover, the contribution of the three channels to lake water quality decreased after TGD operation, mainly because of reduced nutrient input, water discharge, and suspended sediment discharge. This study indicates that controlling pollutant effluents in tributaries and monitoring hydrological conditions within the lake are the key factors necessary to improve the water quality in Dongting Lake and may provide a practical reference for water quality management in river-connected lakes worldwide.