Temporal variability of dissolved inorganic nitrogen and key environmental drivers in a dam-induced subtropical urban lake

• Temporal shifts and water quality drivers of DIN in lake were studied. • Seasonal DIN variability was modeled using Random Forest. • The impact of drivers on DIN was quantified by Partial Least Squares Path Modeling. • Physical indicators, nutrients, and Chl-a were the main factors affecting DIN. The dissolved inorganic nitrogen (DIN) is a critical indicator of the organism growth and ecological balance within aquatic systems. Various water quality determinants influence the temporal variations in DIN concentration, making the identification of key drivers essential for a deeper understanding of nutrient cycling and the formulation of targeted management strategies. However, the impact of hydrological alterations on the key drivers of DIN and its seasonal variability remains unclear. This study investigates seasonal variations in DIN in Hanfeng Lake, a subtropical urban lake located in the Three Gorges Reservoir (TGR) area, over the period from 2021 to 2023. The results indicated that DIN concentration was seasonally significantly different, showing higher values in winter and spring than that in summer and autumn. Random Forest modelling indicated that the temporal variations in DIN concentration were closely related to the notable seasonal fluctuations in key water quality indicators such as the temperature (T), Secchi depth (SD), and concentrations of dissolved phosphorus (DP), dissolved silica (DSi), chlorophyll-a (Chl-a), which were predominantly attributable to hydrological alterations associated with reservoir management and external pollutant inputs from agricultural fertilization. Partial Least Squares Path Modeling confirmed that physical indicators, like T and SD, were the primary drivers of variations in DIN concentration, while DP, DSi, and Chl-a also had significant effects. Based on these insights, the study recommends to minimize the agricultural fertilizer application in spring and summer for decreasing external inputs, and optimize reservoir operation strategies in autumn and winter for controlling hydrological alterations.