Particulate organic carbon dynamics with sediment transport in the upper Yangtze River

Understanding the behaviour of particulate organic carbon (POC) with sediment transport allows for a more accurate estimation of global carbon cycling and the conditions of aquatic ecosystems. River damming alters POC dynamics profoundly by the retention of sediments on which organic carbon (OC) is adsorbed. In this study, we developed a mechanism-based approach to investigate organic carbon (OC) adsorption on river sediment, integrating sediment particle properties (particle size, particle density, surface site density, and particle morphology) and environmental factors (dissolved OC concentration, pH, and suspended sediment concentration). We used this approach to assess the POC concentration in the dammed upper Yangtze River and to compare it with observed POC values in literature; model results and observations correlated very well (R² = 0.89; NSE = 0.83; p < 0.001). OC adsorption on sediment was found to correlate positively with dissolved organic carbon concentration and negatively with pH and suspended sediment concentration. We found that hydroelectric cascade development contributed to a decrease in suspended sediment concentration, with a significant increase in POC concentration both at reservoir sites, and upstream and downstream. The average suspended sediment concentration near the watershed outlet decreased from 2.08 kg m^-3 (individual reservoir period) to 1.57 kg m^-3 (early stage of reservoir construction/operation) and then to 0.01 kg m^-3 (cascade reservoir period). In contrast, the average POC concentration in the dammed river increased from less than, or around 1%, to 1% and finally to 3% during each of these three periods, respectively. Our results highlight the pronounced impacts of cascade reservoirs on river sediment and POC dynamics. By providing a method for assessing OC adsorption on sediment and the dynamics of POC in aquatic systems, this work advances our understanding of carbon cycling in aquatic systems in times of global change.