Nutrient accumulation and eutrophication restricted CO2 emissions in the two backwater bays in the Three Gorges Reservoir (TGR), China.

Reservoir is an important source of CO2 emissions. The backwater bay is a distinct hydrological unit formed in the open channel of canyon-reservoir as a result of impoundment. It is characterized by efficient nutrients retention and a propensity for frequent eutrophication. However, the spatio-temporal dynamics of CO2 emissions in these backwater bays remain unclear. This study investigated CO2 fluxes (fCO2) in two different backwater bays (Hanfeng Lake and Gaoyang Lake) within the Three Gorges Reservoir (TGR), to reveal the spatio-temporal dynamics and driving factors of CO2 emissions. The two backwater bays serve as minor sources of CO2 emissions, exhibiting lower fCO2 compared to other water areas within the TGR. In the subsiding and low water level periods of the TGR, the fCO2 in the two backwater bays were significantly lower, with some areas even converting into CO2 sinks due to extensive eutrophication. However, the water flow backward caused by the TGR water level elevation simultaneously enhances CO2 emissions by diluting algae density and constraining primary production. We highlighted that nutrients enrichment, eutrophication, and water level fluctuations co-dominate the temporal dynamics of CO2 emissions from backwater bays in the TGR. The CO2 fluxes decreased from upstream to downstream in the two backwater bays. The spatial distribution of nutrients and related algal density are critical factors driving this pattern. The key factors influencing CO2 emissions in the backwater bays diversified with water level fluctuations. Our findings contribute to a better understanding of CO2 emissions in large reservoirs with varying hydrological habitats.