The calculated flux of carbon dioxide between Lake Ontario and the atmosphere ranged from about 0.1 gC m−2 day−1 during the summer to about 3 during a winter storm. During the summer, the amount of carbon supplied from the atmosphere was from 5 to 30% of the estimated amount utilized in primary production. Also Lyman's dissociation constants for carbonic acid were found applicable to the carbonate system of the lake.