CO2 and CH4 fluxes during spring and autumn mixing periods in a boreal lake (Paajarvi, southern Finland)

The greatest gas loss from dimictic lakes occur during spring and autumn mixing periods. Thus, we measured daily concentration gradients of carbon gases (CO2 and CH4) in mesohumic Lake Paajarvi 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 (Delta CO2 and Delta CH4), floating static chambers (FC), and changes in gas storage. CO2 fluxes were higher in autumn than in spring, whereas CH4 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 Delta CO2 flux in autumn was 883 mmol m(-2) and in spring, 666 mmol m(-2), whereas total Delta CH4 fluxes were 0.60 mmol m(-2) and 0.80 mmol m(-2) 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 CO2 from the lake metabolism contributed significantly to the CO2 fluxes during the mixing periods, violating the primary assumption used in the storage method.