Linkage between nitrogen loss, river transport, lake accumulation and water quality properties in plain river network basin

Various forms of nitrogen (N) discharged by high-intensity human activities in the Yangtze River Delta are transported into the lake along the river channel, accelerating the lake's N cycle and increasing the eutrophication ecological risk. Taihu Lake is a typical eutrophic shallow lake, suffering from cyanobacteria blooms for decades due to excessive exogenous nutrient load. In this study, the coupling relationship between basin N loss and lake response was established by combining N flow and exogenous nutrient load. The results showed striking spatiotemporal differences and the large tributaries input the majority of N. Three evolution stages of the lake ecosystem were classified, i.e., Stage A (1980–1997) with slow increasing N load; Stage B (1998–2006) with high-level N load despite some controlling methods; Stage C (2007 to present) with the strengthening of N management in lake basin after the Water Crisis, the N load has gradually decreased, while the water flow is increasing by the year. Environmental N export in the basin was 581.46 kg/ha N in 2021, and a total of 32.06 Gg N was finally drawn into the lake. Over the recent two decades, the noticeable expansion of built-up land from 8.21 % to 21.04 % associated with its environmental impacts i.e., urban heat island effect, hard pavement, and ecological fragility deserves attention. Accordingly, the rapid climate change of the basin became the key factor driving the tributaries' hydrologic conditions (r_∂ = 0.945). The developed social economy dominated the sewage discharge (r_∂ = 0.857). The N inputs and losses to the environment in the basin can be further exacerbated without control. Meanwhile, the lake would respond to the exogenous input. In addition to the self-cleaning part of the lake, the N accumulation rate of the surface sediment ranged from 3.29 to 10.77 g N/(m²·yr) of Taihu Lake. To meet the pollutant control target, around 66.28 Gg anthropogenic N needs to be reduced in the upper stream area yearly. Clarifying the N flow and its environmental burden can mitigate its damage to the ecosystem and take on the refined management on the watershed scale.