Coupled effects of human activities and river-Lake interactions evolution alter sources and fate of sedimentary organic carbon in a typical river-Lake system.

Interconnected river-lake systems record sedimentary organic carbon (OC) dynamics and watershed environmental changes, providing valuable information for global carbon budgets and watershed management. However, owing to the evolving river-lake interactions under global change, monitoring OC is difficult, thereby impeding the understanding of OC transport and fate. This study provided new insights into the dynamical mechanisms of OC in a typical river-lake system consisting of Dongting Lake and its seven inlet/outlet rivers (the three inlets of the Yangtze River and four tributaries) over the last century using stable isotope tracing and quantified the influences of climate change and human activities on OC. Results indicated that exogenous OC dominated the OC in the lake (58.2 %-89.0 %) and was lower in the west than in the east due to the differences in the material inputs and depositional conditions within the lake. Temporally, the distribution patterns of OC sources mainly responded to human activities in the basin rather than to climate change. Before 2005, the Yangtze River contributed the most OC (53.5 %-74.6 %), attributed to the high-intensity land use changes (path coefficient (r): 0.48, p-value < 0.01) and agriculture-industry activities (r: 0.44, p-value < 0.001) in the Yangtze River basin that increased soil erosion. After 2005, a large amount of Yangtze River OC was intercepted by the Three Gorges Dam, altering the OC exchange in the river-lake system and shifting OC dominance to the four tributaries (52.2 %-63.8 %). These findings highlight the active response of OC to the river-lake interaction evolution and anthropogenic control, providing critical information for regulating watershed management behavior under global change.