Effects of Water Level Fluctuations on Dissolved Organic Carbon Concentration in Sub-Lakes of Poyang Lake

Dissolved Organic Carbon (DOC) is a core environmental factor that sustains aquatic ecosystem stability and regulates the structure and function of lake ecosystems. Poyang Lake, the largest river-connected freshwater lake in China, is characterized by extreme seasonal water level fluctuations (WLFs). To clarify the coupling mechanisms between WLFs and DOC concentrations and identify key influencing environmental factors, quarterly monitoring was conducted at ten sampling sites in two typical regions (the relatively enclosed Cuoji Lake and the extensively interconnected Nanji Wetland) from 2019 to 2022. Results showed that DOC concentrations in Cuoji Lake ranged from 0.54 to 4.61 mg/L (mean: 2.46 ± 0.07 mg/L) with significant seasonal differences (peak: 3.52 mg/L in dry-season January; trough: 2.58 mg/L in flood-season July), while those in Nanji Wetland varied from 1.52 to 9.61 mg/L (mean: 3.82 ± 1.72 mg/L) without marked temporal variation. WLFs were identified as the dominant driver of DOC dynamics, exhibiting a strong negative correlation: flood-season high water diluted DOC through enhanced hydrodynamic mixing, while dry-season low water accelerated vegetation litter decomposition and inhibited DOC transportation, leading to DOC accumulation. Additionally, DOC concentrations were positively correlated with ammonia nitrogen, phosphate, chemical oxygen demand, chlorophyll a, and water temperature. Nutrients promoted microbial decomposition of organic matter, and phytoplankton (dominated by Bacillariophyta and Chlorophyta; Cyanobacteria accounted for 7.06% in Cuoji Lake and 16.02% in Nanji Wetland) released photosynthates, both contributing to DOC accumulation. This study reveals the coupling relationship between WLFs and DOC concentrations, supplements key parameters for global seasonal lake carbon flux estimation, and provides scientific support for wetland carbon cycle conservation, water quality management, and algal bloom prevention in Poyang Lake and similar aquatic ecosystems worldwide.