Long-term trends and rising levels of refractory dissolved organic matter in a suburban plateau lake: Impacts of hydrological changes

Dissolved organic matter (DOM) characteristics and concentrations in lakes are strongly associated with terrestrial input, phytoplankton dynamics, and physicochemical environment. Hydrological conditions can affect multiple aspects of the lake environment, thereby interfering with DOM cycling. This study investigates the long-term trends and drivers of DOM accumulation in Lake Erhai, a subtropical plateau lake in southwestern China, focusing on the role of hydrological processes in driving its accumulation and persistence. By analyzing data from 1992 to 2023—including bulk chemical analysis, 3D-EEM fluorescence spectroscopy, degradation experiments and bayesian structural equation modeling (BSEM), it is concluded that a 174 % increase in water residence time (WRT), from 2.8 years to 7.8 years, driven by reduced inflow and outflow volumes, has promoted the accumulation of refractory DOM (RDOM), raising chemical oxygen demand (COD_Mn) and presenting substantial challenges challenges to water quality management. Degradation experiments revealed limited biodegradability of DOM (15 % over 28 days) and minimal photodegradation (13.5 % over 72 h), with more than 80 % remaining in a refractory state. Spectroscopic analyses revealed compositional shifts in DOM with prolonged WRT, characterized by decreased humic-like substances and increased protein-like compounds, indicating a progressive transition from allochthonous to autochthonous DOM dominance. BSEM analysis identified a significant temporal shift in DOM drivers: during the initial phase (1992–2010), human activity pressure (HAP) and riverine input quality (RIQ) collectively explained 70 % of the variance, with natural drivers contributing less than 20 %; whereas in the subsequent phase (2010–2023), anthropogenic influences diminished as hydrological and climatic factors became predominant, with hydrological regime (HR) and climatic factors (CF) jointly accounting for 87 % of RDOM variance, reflecting a transition from anthropogenic to climate-hydrological driven accumulation patterns. This research underscores the critical role of hydrological residence time in determining DOM composition, sources, and persistence in plateau lakes following partial decoupling of external pollution sources. The findings highlight the dual influence of climate and hydrology on lakes experiencing significant pressures from reduced water resources and increasing water demand, challenging conventional management strategies focused exclusively on external nutrient control. The case of Lake Erhai demonstrates the necessity for integrated management approaches that address both external and internal DOM dynamics to support sustainable water quality and ecosystem integrity.