Importance of water temperature and thermal stratification dynamics for temporal variation of surface water CO2 in a boreal lake

Variation of the surface water CO2 concentration is likely to be the result of biological activity and physical processes such as water mixing and gas exchange with the atmosphere. Here we have studied the variations in surface water CO2 during the ice-free period in the humic Lake Merasjarvi in northern Sweden. Meteorological, hydrological, and limnological data were collected using data logging equipment permitting high time resolution. The surface water of the lake was supersaturated with respect to CO2 throughout the study period. There were, however, considerable diurnal and longer-term temporal variations of the surface water CO2 concentrations. Partial least squares models were used to link the logged CO2 data to the multivariate data set. On the longer-term timescale (analyzed with 24 h means of the logged data), high concentrations of surface water CO2 were best related to the depth and temperature of the upper warmer layer (epilimnion), and to erosion of the underlying colder layer (hypolimnion). The diurnal variation (analyzed with 30 min means of the logged data) was best related to the thermal dynamics within the epilimnion, which regulated the surface water access to CO2 stores within this layer. Variables related to CO2 emission and photosynthesis (wind and photosynthetically active radiation) showed only weak correlations to variations of the surface water CO2 concentration. Accordingly, the CO2 flux, measured with the eddy-covariance technique, was not correlated to the surface water CO2 concentration.