CO₂ is dominant greenhouse gas emitted from six hydropower reservoirs in southeastern United States during peak summer emissions

During August-September 2012, we sampled six hydropower reservoirs in southeastern United States for CO₂ and CH₄ emissions via three pathways: Diffusive emissions from water surface; ebullition in the water column; and losses from dam tailwaters during power generation. Estimates of average areal emission rates of CO₂ attributable to the six reservoirs (i.e., reservoir plus tailwater emissions) ranged from 994 to 2760 mg·m^-2. day^-1, which is low to moderate compared to CO₂ emissions rates reported for tropical hydropower reservoirs and boreal ponds and lakes, and similar to rates reported for other temperate reservoirs. Similar average rates for CH4 were also relatively low, ranging from 6 to 187 mg·m^-2. day^-1. On a whole-reservoir basis, estimates of total emissions of CO₂ ranged 10-fold, from 42,740 kg per day for Fontana to 501,151 kg per day for Guntersville, and total emissions of CH4 ranged over 30-fold, from 251 kg per day for Fontana to 9153 kg per day for Allatoona. Emissions through the tailwater pathway varied among reservoirs, comprising from 19% to 65% of total CO₂ emissions and 0% to 84% of CH₄ emissions, depending on the reservoir. Emission rates were significantly correlated with several reservoir morphological and water quality characteristics, including metrics related to vertical stratification (e.g., minimum water column temperature and maximum dissolved oxygen) and reservoir productivity (e.g., water transparency and chlorophyll a concentration).