We studied CO 2 and CH 4 fluxes from two boreal lakes with differing trophic status (chlorophyll a 17.8 vs. 48.7 mg m 22 ) and water color (100 vs. 20 mg Pt L 21 ) 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 CO 2 flux of 6.85 (chambers) or 5.43 mol m 22 (gradients), the humic lake had notably higher emissions than the clear-water lake, where the fluxes were 3.97 and 3.38 mol m 22 , respectively. The annual CH 4 flux from the clear-water lake was 28.5 (chambers) or 20.5 mmol m 22 (gradients) and from the humic lake 20.7 or 16.2 mmol m 22 , 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 CO 2 and CH 4 flux was responsible for 46% and 48% of the annual flux, respectively. Before the rains, the clear-water lake was a small sink of CO 2 or had near-zero efflux but afterwards became a source of CO 2 . 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.

[1] The greatest gas loss from dimictic lakes occur during spring and autumn mixing periods. Thus, we measured daily concentration gradients of carbon gases (CO 2 and CH 4 ) in mesohumic Lake Pääjärvi during the mixing periods in autumn 2004 and spring 2005 and calculated and compared the fluxes using three different methods: the boundary layer diffusion model (DCO 2 and DCH 4 ), floating static chambers (FC), and changes in gas storage. CO 2 fluxes were higher in autumn than in spring, whereas CH 4 fluxes were lower in autumn than in spring. The method based on changes in storage underestimated the fluxes whereas the floating chambers and the boundary layer diffusion models resulted in similar estimates. However, the chambers always yielded somewhat higher fluxes. Total DCO 2 flux in autumn was 883 mmol m À2 and in spring, 666 mmol m À2, whereas total DCH 4 fluxes were 0.60 mmol m À2 and 0.80 mmol m À2 in autumn and spring, respectively. We calculated gas transfer velocities (k 600 ) to explain the near surface exchange mechanism and the difference between the results based on diffusion models and chambers. Wind speed and k 600 showed significant correlation. In spring the transfer velocity at similar wind speed was higher compared to the autumn. Weekly measurements of algal primary production and community respiration revealed that the lake was net heterotrophic in autumn as well as in spring. Our study showed that the excess CO 2 from the lake metabolism contributed significantly to the CO 2 fluxes during the mixing periods, violating the primary assumption used in the storage method.Citation: López Bellido, J., T. Tulonen, P. Kankaala, and A. Ojala (2009), CO 2 and CH 4 fluxes during spring and autumn mixing periods in a boreal lake (Pääjärvi, southern Finland),

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