Benthic and pelagic sources of carbon dioxide in boreal lakes and a young reservoir (Eastmain‐1) in eastern Canada

Lakes and reservoirs typically emit CO 2 to the atmosphere as a result of respiration in excess of primary production. For reservoirs, this respiration is often assumed to be primarily of benthic origin, associated with drowned vegetation and soils. This study took measurements from a young hydroelectric reservoir (Eastmain‐1) in boreal Canada and nearby natural lakes to compare the rates of water column respiration and hypolimnetic CO 2 accumulation during a stratified period. Reservoir water column respiration rates were significantly higher than those in natural lakes, with surface means of 61.4 ± 4.6 mg C m −3 d −1 in Eastmain‐1 and 40.2 ± 3 mg C m −3 d −1 in lakes and profundal means of 58.1 ± 11 mg C m −3 d −1 in Eastmain‐1 and 17.6 ± 7.5 mg C m −3 d −1 in lakes. Reservoir hypolimnetic CO 2 accumulation rates were also higher than those in natural lakes, with means of 78.8 ± 16 mg C m −3 d −1 in Eastmain‐1 and 19.8 ± 2.7 mg C m −3 d −1 in lakes. Our findings further indicate that the relative contribution of benthic respiration (estimated as the difference between hypolimnetic CO 2 accumulation and water column respiration) toward total net CO 2 production during stratified periods was similar in both aquatic systems (23% in lakes, 27% in Eastmain‐1). By identifying the major role of the water column in the reservoir's CO 2 production, this study calls for a significant paradigm shift away from the common assumption that reservoir CO 2 emissions are predominantly the result of decaying vegetation in the benthic environment.