Trend analysis and cause diagnosis of eutrophication in Lake Huanggai in the middle reaches of Yangtze River

In order to evaluate the state and temporal variations of eutrophication in Lake Huanggai and diagnose the main causes of changes, the comprehensive trophic stale index was used to evaluate the eutrophication level and variance tendency of water body, and the pollution index method was used to evaluate the pollution degree of surface sediments. The correlations between the concentration of nitrogen and phosphorus in the sediments and in the water were analyzed. Based on the correlations of nitrogen and phosphorus between sediments and water, and the water quality of the river entering into the lake, the main causes of eutrophication in Lake Huanggai were preliminarily inferred. All of the analyses above were based on water quality data of environmental monitoring stations from 2015 to 2021 and four times of intensive monitoring data of the whole lake in autumn 2021. The results showed that from 2015 to 2021, the TLI(%) scores of Lake Huanggai were 44.14, 45.91, 42.39, 49.79, 49.01, 49.62 and 52.77, respectively, which showed a trend of increase year by year. The state of eutrophication changed from moderate to mild, and the eutrophication degree was higher in summer and autumn than in winter and spring. The contribution rates of Til (SD), Til (TN) and TLI (TP) were 28%, 18% and 16%, respectively. The increase of nutrient concentration and decrease of transparency were the main driving factors of eutrophication in Lake Huanggai. The average concentrations of TP and TN in the sediments of Lake Huanggai were 791 mg/kg and 2691 mg/kg, respectively, indicating heavy pollution and high release risk. The TN correlation between surface sediments and surface water was weak, while the TP correlation was strong. However, it was greatly affected by other factors, such as wind speed and water depth, etc. When the wind speed over ihe lake was high, P in surface sediments in shallow water area was easy to release into the overlying water. The main driving factors of water eutrophication in Lake Huanggai in recent years were the increase of nutrient concentration and decrease of transparency caused by the inflow of rivers and the re-suspension of sediments under the action of winds and waves. It is suggested that measures such as estuarine wetland restoration and aquatic vegetation restoration should be taken to slow down the eutrophication trend of Lake Huanggai. © 2023 Science Press. All rights reserved.