Spatial-temporal characterization of carbon dioxide and methane emissions from four electric power-producing reservoirs in the southeastern U.S.

Background/Question/Methods

Recent studies have shown significant emissions of greenhouse gases (GHG) from tropical hydropower reservoirs. However, fewer data have been reported for GHG emissions from temperate reservoirs. We sampled four reservoirs in eastern Tennessee (Douglas Lake, Cherokee Lake, and Watts Bar Lake) and northern Georgia (Allatoona Lake), known to have summer hypolimnetic hypoxia, to assess CO₂ and CH₄ emissions through diffusion and ebullition fluxes at various sites along the main channel, in coves, and along the tailwaters. Emissions from nearby, free-flowing tributary river sites also were measured to estimate emissions representative of pre-reservoir conditions.  

Results/Conclusions

GHG emissions at all sites increased from spring to summer, as expected, due to higher water temperatures, stronger thermal stratification, and higher rates of  respiration. Emissions of CO₂ were much greater than emissions of CH₄ at all sampling dates and locations. Diffusive emissions of CH₄ were generally low at all sites (< 10 mg/m²/day); however, they were greater at shallow sites in late spring and summer. CH₄ emission rates also were higher at tailwater sites than at tributary river sites. These results suggest that CH₄ emissions may be substantial only at shallow reservoir sites where CH₄, produced under anoxic conditions in bottom waters or sediments, can reach the water surface fast enough to avoid being oxidized to CO₂. Diffusive emissions from the reservoir surface were similar to values reported by the Intergovernmental Panel on Climate Change for other reservoirs in humid temperate regions.