Divergence in anthropogenic activities contributed to the eutrophication heterogeneity between river and lake at basin scale: new insights from trophic state index in Asia's largest basin (Yangtze River)

Eutrophication caused by excessive nutrient inputs from intense anthropogenic activities seriously threatens the ecological health of river and lake systems worldwide. However, previous eutrophication studies have focused mostly on lakes, with little attention paid to rivers, and the anthropogenic driving mechanisms of eutrophication heterogeneity between rivers and lakes within large-scale basins remain unclear. In this study, the seasonal and regional characteristics of eutrophication in rivers and lakes within the Yangtze River Basin were analyzed from 2021 to 2023. The relationship between anthropogenic activities and the eutrophication heterogeneity between rivers and lakes was determined by considering population and land use factors. The results showed that the eutrophication of both rivers and lakes was more severe in the downstream area than upstream and midstream area, and intensified during the wet season compared to the normal and dry seasons. On average, 63.2 % and 20.7 % of the river stations were at eutrophic and meso-eutrophic states, respectively; and an average of 26.3 % and 44.2 % of lake stations were at eutrophic and meso-eutrophic states, respectively. Correlation analysis revealed significant differences in human-driven eutrophication mechanisms between rivers and lakes. The trophic states of river systems acutely reflected short-term anthropogenic pressures, while lake systems predominantly manifest the long-term cumulative effects of nutrients generated by land use within the watershed. This study can provide reliable support for targeted management of eutrophication in different water bodies in large-scale basins.