Six hard‐water lakes were sampled May‐August for 14 yr in a 52,000 km 2 catchment to identify the mechanisms that regulate the spatial and temporal variability of net atmospheric exchange of CO 2 of lakes on the Northern Great Plains. Annual mean daily fluxes ranged from ‐100 to >200 mmol C m −2 d −1 , while p CO 2 values varied between 0.3 and 5500 Pa. We observed periods of net CO 2 2 uptake (1995, 2000) and release (1998, 2...