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Abstract. This study explores the variability in concentrations of dissolved CH 4 and annual flux estimates in the pelagic zone in a statistically defined sample of 207 lakes in Finland. The lakes were situated in the boreal zone, in an area where the mean annual air temperature ranges from −2.8 to 5.9 • C. We examined how lake CH 4 dynamics related to regional lake types assessed according to the EU water framework directive. Ten lake types were defined on the basis of water chemistry, color, and size. Lakes were sampled for dissolved CH 4 concentrations four times per year, at four different depths at the deepest point of each lake. We found that CH 4 concentrations and fluxes to the atmosphere tended to be high in nutrient rich calcareous lakes, and that the shallow lakes had the greatest surface water concentrations. Methane concentration in the hypolimnion was related to oxygen and nutrient concentrations, and to lake depth or lake area. The surface water CH 4 concentration was related to the depth or area of lake. Methane concentration close to the bottom can be viewed as proxy of lake status in terms of frequency of anoxia and nutrient levels. The mean pelagic CH 4 release from randomly selected lakes was 49 mmol m −2 a −1 . The sum CH 4 flux (storage and diffusion) correlated with lake depth, area and nutrient content, and CH 4 release was greatest from the shallow nutrient rich and humic lakes. Our results support earlier lake studies regarding the regulating factors and also the magnitude of global emission estimate. These results propose that in boCorrespondence to: S. Juutinen (sjuutine@mtholyoke.edu) real region small lakes have higher CH 4 fluxes per unit area than larger lakes, and that the small lakes have a disproportionate significance regarding to the CH 4 release.

We show that sediment respiration is one of the key factors contributing to the high CO 2 supersaturation in and evasion from Finnish lakes, and evidently also over large areas in the boreal landscape, where the majority of the lakes are small and shallow. A subpopulation of 177 randomly selected lakes (o100 km 2 ) and 32 lakes with the highest total phosphorus (P tot ) concentrations in the Nordic Lake Survey (NLS) data base were sampled during four seasons and at four depths. Patterns of CO 2 concentrations plotted against depth and time demonstrate strong CO 2 accumulation in hypolimnetic waters during the stratification periods. The relationship between O 2 departure from the saturation and CO 2 departure from the saturation was strong in the entire data set (r 2 5 0.79, n 5 2 740, Po0.0001). CO 2 concentrations were positively associated with lake trophic state and the proportion of agricultural land in the catchment. In contrast, CO 2 concentrations negatively correlated with the peatland percentage indicating that either input of easily degraded organic matter and/or nutrient load from agricultural land enhance degradation. The average lake-area-weighted annual CO 2 evasion based on our 177 randomly selected lakes and all Finnish lakes 4100 km 2 (Rantakari & Kortelainen, 2005) was 42 g C m À2 LA (lake area), approximately 20% of the average annual C accumulation in Finnish forest soils and tree biomass (covering 51% of the total area of Finland) in the 1990s. Extrapolating our estimate from Finland to all lakes of the boreal region suggests a total annual CO 2 evasion of about 50 TgC, a value upto 40% of current estimates for lakes of the entire globe, emphasizing the role of small boreal lakes as conduits for transferring terrestrially fixed C into the atmosphere.

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