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.

Model-based estimates suggest that lake sediments may be a significant, long-term sink for organic carbon (C) at regional to global scales. These models have used various approaches to predict sediment storage at broad scales from very limited data sets. Here, we report a large-scale direct assessment of the standing stock and sedimentation rate of C for a representative set of lakes in Finland. The 122 lakes were selected from the statistically selected Nordic Lake Survey database, they cover the entire country and the water quality represents the average lake water quality in Finland. Unlike all prior estimates, these data use sediment cores that comprise the entire sediment record. The data show that within Finland, aquatic ecosystems contain the second largest areal C stocks (19 kg C m À2 ) after peatlands (72 kg C m À2 ), and exceed by significant amounts stocks in the forest soil (uppermost 75 cm; 7.2 kg C m À2 ) and woody biomass (3.4 kg C m À2 ). Kauppi et al. (1997). The Finnish estimate extrapolated over the boreal region gives a total C pool in lakes 19-27 Pg C, significantly lower than the previous model-based estimates.

Carbon dioxide (CO2) accumulates under lake ice in winter and degasses to the atmosphere after ice melt. This large springtime CO2 pulse is not typically considered in surface‐atmosphere flux estimates, because most field studies have not sampled through ice during late winter. Measured CO2 partial pressure (pCO2) of lake surface water ranged from 8.6 to 4,290 Pa (85–4,230 µatm) in 234 north temperate and boreal lakes prior to ice melt during 1998 and 1999. Only four lakes had surface pCO2 less than or equal to atmospheric pCO2, whereas 75% had pCO2 >5 times atmospheric. The δ13CDIC (DIC = ΣCO2) of 142 of the lakes ranged from –26.28‰ to +0.95‰. Lakes with the greatest pCO2 also had the lightest δ13CDIC, which indicates respiration as their primary CO2 source. Finnish lakes that received large amounts of dissolved organic carbon from surrounding peatlands had the greatest pCO2. Lakes set in noncarbonate till and bedrock in Minnesota and Wisconsin had the smallest pCO2 and the heaviest d13CDIC, which indicates atmospheric and/or mineral sources of C for those lakes. Potential emissions for the period after ice melt were 2.36 ± 1.44 mol CO2 m−2 for lakes with average pCO2 values and were as large as 13.7 ± 8.4 mol CO2 m−2 for lakes with high pCO2 values.

We studied the interannual variation of surface water partial pressure of CO 2 (pCO 2 ) and the CO 2 emissions from the 37 large Finnish lakes linking them to the water quality, catchment and climate attributes in 1996-2001. The lake water CO 2 was measured three times a year in the study lakes in 1998 and 1999 and for the rest of the years the CO 2 was modeled by measured alkalinity. The median annual CO 2 emission to the atmosphere ranged between 1.49 and 2.29 mol m À2 a À1 . The annual CO 2 emission followed closely the annual precipitation pattern with the highest emission during the years when the precipitation was highest (r 2 5 0.81-0.97, Po0.05). There was a strong negative correlation (r 2 5 0.50-0.82, Po0.001) between O 2 and CO 2 saturation in the lake water during stratification suggesting effective decomposition of organic matter in the lakes. Furthermore, total phosphorus and the proportion of agricultural land in the catchment had significant positive correlations with CO 2 saturation.