Crucial Role of Tributary Bays in the Carbon Burial of Three Gorges Reservoir

Tributary bays, which are formed in river tributaries for reservoir construction with a relatively lentic environment, have been estimated to hold unique biogeochemical processes and to be involved in ecological issues (e.g., algal bloom). Nevertheless, the role of tributary bays in carbon cycling is unclear, precluding an accurate assessment of the carbon budgets of reservoirs. To fill this knowledge gap, we take the Three Gorges Reservoir (TGR) as an example and assess the carbon budget of a typical tributary bay (Xiangxi River, XXR) of TGR. With laboratory experiments and numerical models, we demonstrate that XXR may be an important carbon sink. This study exhibits that primary productivity enhancement (e.g., algal bloom) in tributary bays plays a critical role in CO2 sequestration and contributes significantly to carbon burial. Moreover, the organic carbon burial flux of tributary bays in TGR is estimated to be equivalent to (22 +/- 9) % of that in the mainstream, demonstrating the crucial role of tributary bays in the carbon burial of TGR. Considering the scientific debate on whether the reservoir is a carbon sink or source, we suggest that the overlooking of tributary bays might lead to a significant error in assessing the reservoir carbon budget.