CO2and CH4 fluxes during spring and autumn mixing periods in a boreal lake (Pääjärvi, southern Finland)

Abstract

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 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 (ΔCO2 and ΔCH4), 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 ΔCO2 flux in autumn was 883 mmol m-2 and in spring, 666 mmol m-2, whereas total ΔCH4 fluxes were 0.60 mmol m-2 and 0.80 mmol m-2 in autumn and spring, respectively. We calculated gas transfer velocities (k600) to explain the near surface exchange mechanism and the difference between the results based on diffusion models and chambers. Wind speed and k600 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 CO 2 fluxes during the mixing periods, violating the primary assumption used in the storage method. Copyright 2009 by the American Geophysical Union.