Importance of water temperature and thermal stratification dynamics for temporal variation of surface water CO 2 in a boreal lake

Abstract

Variation of the surface water CO 2 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 CO 2 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 CO 2 throughout the study period. There were, however, considerable diurnal and longer‐term temporal variations of the surface water CO 2 concentrations. Partial least squares models were used to link the logged CO 2 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 CO 2 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 CO 2 stores within this layer. Variables related to CO 2 emission and photosynthesis (wind and photosynthetically active radiation) showed only weak correlations to variations of the surface water CO 2 concentration. Accordingly, the CO 2 flux, measured with the eddy‐covariance technique, was not correlated to the surface water CO 2 concentration.