Hydrological variability disrupts nutrient–chlorophyll relationships in Dongting Lake, China

The general relationship between nutrient concentrations and chlorophyll a (Chl a) in lakes has been established across broad spatial scales. However, effective guidance for managing eutrophication in river-connected lakes is difficult to implement because of the complexity of hydrological systems. In this study, the dynamic relationship between nutrients and Chl a, as well as their responses and mechanisms to driving factors under varying hydrological conditions, were investigated using an 18-year dataset from a large river-connected lake (Dongting Lake). Total nitrogen (TN) and total phosphorus (TP) concentrations were higher during the dry season than during the flood season. In contrast, Chl a concentration decreased significantly from 3.17 μg/L in the flood season to 2.61 μg/L in the dry season, exhibiting an asynchronous response of Chl a to nutrient concentrations. The trophic state of Dongting Lake was borderline between mesotrophic and eutrophic, with a trophic state index (TSI) ranging from 49.96 to 51.08 across hydrological seasons. Chl a concentration increased significantly with increasing TN and decreasing TP. Dongting Lake was classified as either oligo-mesotrophic or eutrophic for TN concentrations of ≤ 1.29 mg/L and > 1.48 mg/L, respectively, during the dry season. The trophic state of Chl a was influenced by the combined limitation of N and P, with the strength of N limitation for oligo-mesotrophic conditions (Chl a) increasing to 13.89 % during the dry season. Hydrometeorological factors, such as water level and water temperature, significantly influenced the trophic state of Chl a during different hydrological seasons. Our findings indicate that the trophic state of Dongting Lake is highly responsive to hydrological variations, highlighting the need for careful consideration of the nutrient–Chl a relationship and hydrological conditions in policymaking, management planning, and ecological engineering efforts, which are crucial for improving water quality in river-connected lakes.