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Different Apparent Gas Exchange Coefficients for CO2 and CH4: Comparing a Brown-Water and a Clear-Water Lake in the Boreal Zone during the Whole Growing Season

Rantakari, Miitta, Heiskanen, Jouni, Mammarella, Ivan, Tulonen, Tiina, Linnaluoma, Jessica, Kankaala, Paula, Ojala, Anne
Environmental Science & Technology 2015 v.49 no.19 pp. 11388-11394
air, carbon, carbon dioxide, gas exchange, growing season, lakes, landscapes, methane, surface water
The air–water exchange of carbon dioxide (CO₂) and methane (CH₄) is a central process during attempts to establish carbon budgets for lakes and landscapes containing lakes. Lake–atmosphere diffusive gas exchange is dependent on the concentration gradient between air and surface water and also on the gas transfer velocity, often described with the gas transfer coefficient k. We used the floating-chamber method in connection with surface water gas concentration measurements to estimate the gas transfer velocity of CO₂ (kCO₂) and CH₄ (kCH₄) weekly throughout the entire growing season in two contrasting boreal lakes, a humic oligotrophic lake and a clear-water productive lake, in order to investigate the earlier observed differences between kCO₂ and kCH₄. We found that the seasonally averaged gas transfer velocity of CH₄ was the same for both lakes. When the lakes were sources of CO₂, the gas transfer velocity of CO₂ was also similar between the two study lakes. The gas transfer velocity of CH₄ was constantly higher than that of CO₂ in both lakes, a result also found in other studies but for reasons not yet fully understood. We found no differences between the lakes, demonstrating that the difference between kCO₂ and kCH₄ is not dependent on season or the characteristics of the lake.