Variation of the surface water CO₂ concentration is likely to be the result of biological activity and physical processes 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 Merasjärvi 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 (PLS) models were used to link the logged CO₂ data to the multivariate dataset. On the longer-term time scale (analyzed with 24h 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 minute 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 PAR), 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.