Three Gorges Dam Operations Affect the Carbon Dioxide Budget of a Large Downstream Connected Lake

The effects of dams on carbon dioxide (CO₂) fluxes in downstream lakes remain elusive. Here we combined eddy covariance observations and random forest models to examine multi-decadal variations in CO₂ fluxes in the Poyang Lake, the largest freshwater lake in China, and quantified the contribution of the Three Gorges Dam (TGD), the world's largest hydraulic project. We found the lake fluctuated between CO₂ source and sink in 1961–2016, and tended to be CO₂ sink in the post-TGD period (2003–2016) when vegetation expanded early and spatially due to declining water level. TGD can explain approximately 6% of the total differences in annual CO₂ fluxes, with major contributions in the impoundment period (up to 22% in middle September to October). The results show a positive side of operational major hydraulic projects on lake carbon sink, and probably caution the negative side of carbon release after dam removal.