Evaluating nutrient distribution and eutrophication pattern in a shallow impounded lake: Exploring the influence of floods

Water quality management in shallow impounded lakes is challenging due to nutrient's enrichment and algal blooms. Lake Hongze is a reservoir for the South-to-North Water Diversion Project's Eastern Route and an essential water source for Jiangsu Province, China, and its water quality closely relates to the local aquatic ecosystem and affects the water supply security of the surrounding areas. The spatial and seasonal patterns of total nitrogen, total phosphorus (TP), and chlorophyll-a (Chl-a) in the lake were investigated and the effects of floods on these patterns were assessed. Hydrological data and trophic state parameters were analyzed using 7 years of monitoring data from 16 water sampling sites throughout the lake. The statistical analysis revealed the seasonal variation characteristics affected by floods and the differences in material transport continuity between inflow and outflow boundaries. Eutrophication assessment using the trophic level index and Chl-a concentrations also indicated eutrophication was concentrated at the southeast side of the lake. Spatial interpolation of Chl-a using the ordinary kriging method clarified that existence and movement of the localized eutrophication area in Lake Hongze. The mass balance calculations of TP indicated that a substantial amount of phosphorus entered the lake during the flood season, however, most severe algal blooms occurring after the flood season. The onset of algal blooms exhibits a significant time lag in response to phosphorus input, primarily due to the influence of hydrodynamic processes within the lake during the flood season.