Water Residence Time and Temperature Drive the Dynamics of Dissolved Organic Matter in Alpine Lakes in the Tibetan Plateau

Lakes are hotspots of dissolved organic matter (DOM) degradation and play an active role in carbon cycles. Alpine lakes are under the most immediate impacts of climate change and act as sentinels of alpine ecosystem's response to global warming, yet the primary environmental drivers of DOM variability in alpine lakes remain unidentified. Here, we sampled and analyzed DOM from 25 alpine lakes (>4,000 m above mean sea level) across the Tibetan Plateau. We found that the water residence time (WRT) of lakes and mean annual temperature (MAT) accounted for 30%-59% of the variance in the abundance of chromophoric DOM (CDOM) and fluorescent DOM (FDOM). Both CDOM and FDOM decreased with increasing WRT and MAT. Molecular-level analysis of selected DOM samples showed that a higher WRT corresponded to DOM with lower molecular intensities, lower proportions of aromatics, lower oxidized compounds, and higher proportions of reduced and nitrogen-containing compounds. These results indicate that in-lake DOM processing dominated the regional variability of CDOM and FDOM. Under the scenario of climate warming and lake enlargement due to increasing precipitation and glacier melting, increased amount of DOM could be processed and removed from Tibetan lakes. Our findings are the first that identified WRT as a significant predictor of the amount of various DOM pools in Tibetan lakes and this observation may be broadly applicable to alpine lakes since they share similar DOM source-sink characteristics. The simple empirical models (DOM vs. WRT + MAT) established here can be integrated into more sophisticated numerical models in predicting changes in the carbon cycle of alpine lakes.