Carbon gas fluxes from a brown‐water and a clear‐water lake in the boreal zone during a summer with extreme rain events

We studied CO2 and CH4 fluxes from two boreal lakes with differing trophic status (chlorophyll a 17.8 vs. 48.7 mg m−2) and water color (100 vs. 20 mg Pt L−1) throughout an open‐water period when summer precipitation doubled, using both floating chambers and concentration gradients. Fluxes measured in chambers were higher, but irrespective of the method, both lakes were heterotrophic and were annual sources of carbon gases to the atmosphere. However, with the annual CO2 flux of 6.85 (chambers) or 5.43 mol m−2 (gradients), the humic lake had notably higher emissions than the clear‐water lake, where the fluxes were 3.97 and 3.38 mol m−2, respectively. The annual CH4 flux from the clear‐water lake was 28.5 (chambers) or 20.5 mmol m−2 (gradients) and from the humic lake 20.7 or 16.2 mmol m−2, respectively. There were interlake differences in seasonal patterns, but the most obvious changes in fluxes occurred during or just after the rains. In the humic lake, the resulting peak in CO2 and CH4 flux was responsible for 46% and 48% of the annual flux, respectively. Before the rains, the clear‐water lake was a small sink of CO2 or had near‐zero efflux but afterwards became a source of CO2. In the humic lake, biological mineralization explained the majority of the fluxes, whereas in the clear‐water lake the association between the biological processes and fluxes was less pronounced.