Under-ice CO₂ and O₂ variability in a freshwater lake

Autonomous, in situ sensors for the partial pressure of CO₂ (pCO₂) and dissolved oxygen (DO) were deployed in a freshwater lake during the winters of 1997 and 1998 to evaluate magnitude and sources of variability during ice-covered periods. Gas variability on diel or shorter time scales was small or undetectable during most of the deployment periods, only becoming significant prior to ice-out when runoff and light penetration increased, promoting convective currents and biological production. A surprising 7.6d period oscillation, apparently driven by a baroclinic seiche, dominated the short-term variability during the first year. The gas trends associated with the seiche oscillations and periodic profile measurements revealed that ice formation led to gas gradients directly under the ice. Long-term variability was characterized by increasing CO₂ and decreasing DO as a consequence of biological oxidation of organic matter. The results suggest that both spatial and temporal variability can be significant over intervals which would not be resolved by traditional sampling-based studies.