Cultivated land-wetland complex increases humified DOM inputs and elevates eutrophication potential in Lake Lugu

Dissolved organic matter (DOM) serves as a critical biogeochemical nexus linking terrestrial and aquatic carbon cycles, profoundly influencing ecosystem structure and functionality. Although land uses, seasonal variations, and trophic status collectively influence the spectral characteristics of lake DOM, the mechanistic drivers underlying its spatiotemporal dynamics remain poorly understood, particularly in ecologically fragile plateau lake regions. In this study, we systematically characterized physicochemical parameters, trophic state, and DOM spectral properties in the nearshore waters of Lake Lugu across different land use types, using seasonal water quality monitoring coupled with absorption and excitation-emission matrix (EEM) fluorescence spectroscopy. Our results indicated that the water in the Caohai (CH) lake region adjacent to the cultivated land-wetland complex exhibited significantly higher Trophic State Index (TSI) values than those in other lake regions. CH serves as a significant source of nutrients input to Lake Lugu, particularly during the rainy season. Additionally, DOM components analysis revealed that autochthonous components, C3 (humic-like substances derived from macrophyte decomposition) and C4 (tryptophan-like fluorescence produced by phytoplankton), constitute the primary components of DOM in Lake Lugu. Across lake areas with elevated trophic status, the fluorescent components of DOM demonstrates a higher proportion of allochthonous components alongside enhanced aromaticity and humification, indicating that eutrophication process remodels its spectral characteristics and fluorescence components of DOM. Furthermore, under the combined influence of rainfall and prevailing winds, the CH cultivated land-wetland complex facilitated the transport of substantial amounts of humic-like substances into Lake Lugu. Plateau lakes serve as sensitive indicators for global change, and this study provides a theoretical foundation for eutrophication prevention and carbon sink management for plateau lakes.