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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.

Abstract. This article provides an overview of the effects of land-use on the fluxes of carbon dioxide (CO 2 ), methane (CH 4 ) and nitrous oxide (N 2 O) and from peatlands in the Nordic countries based on the field data from about 100 studies. In addition, this review aims to identify the gaps in the present knowledge on the greenhouse gas (GHG) balances associated with the land-use of these northern ecosystems. Northern peatlands have accumulated, as peat, a vast amount of carbon from the atmosphere since the last glaciation. However, the past land-use and present climate have evidently changed their GHG balance. Unmanaged boreal peatlands may act as net sources or sinks for CO 2 and CH 4 depending on the weather conditions. Drainage for agriculture has turned peatlands to significant sources of GHGs (mainly N 2 O and CO 2 ). Annual mean GHG balances including net CH 4 , N 2 O and CO 2 emissions are 2260, 2280 and 3140 g CO 2 eq. m −2 (calculated using 100 year time horizon) for areas drained for grass swards, cereals or those left fallow, respectively. Even after cessetion of the cultivation practices, N 2 O and CO 2 emissions remain high. The mean net GHG emissions in abandoned and afforested agricultural peatlands have been 1580 and 500 g CO 2 eq. m −2 , respectively. Peat extraction sites are net sources of GHGs with an average emission rate of 770 g CO 2 eq. m −2 . Cultivation of a perennial grass (e.g., reed canary grass) on an abandoned peat extraction site has been shown to convert such a site into a net sink of GHGs (−330 g CO 2 eq. m −2 ). In contrast, despite restoration, such sites are known to emit GHGs (mean source of 480 g CO 2 eq. m −2 , mostly from high CH 4 emissions). Peatland forests, originally drained for forestry, may act as Correspondence to: M. Maljanen (marja.maljanen@uef.fi) net sinks (mean −780 g CO 2 eq. m −2 ). However, the studies where all three GHGs have been measured at an ecosystem level in the forested peatlands are lacking. The data for restored peatland forests (clear cut and rewetted) indicate that such sites are on average a net sink (190 g CO 2 eq. m −2 ). The mean emissions from drained peatlands presented here do not include emissions from ditches which form a part of the drainage network and can contribute significantly to the total GHG budget. Peat soils submerged under water reservoirs have acted as sources of CO 2 , CH 4 and N 2 O (mean annual emission 240 g CO 2 eq. m −2 ). However, we cannot yet predict accurately the overall greenhouse gas fluxes of organic soils based on the site characteristics and land-use practices alone because the data on many land-use options and our understanding of the biogeochemical cycling associated with the gas fluxes are limited.

[1] Concentrations and fluxes of greenhouse gases methane (CH 4 ), carbon dioxide (CO 2 ), and nitrous oxide (N 2 O) were measured during open water conditions in two hydroelectric reservoirs, Lokka and Porttipahta, in the northern boreal zone in Finland. These reservoirs were located on peat and forest soils and were built in 1967 and 1970, respectively. Over 20 years after their flooding, the reservoirs were still largely supersaturated with dissolved CH 4 and CO 2 . Measured with floating static chambers, the stations in Lokka released more CH 4 (means of 5. There was no clear association between the CH 4 emissions and the bottom type, including mineral soils and old peat deposits. The flooded vegetation, higher nutrient content, and primary production in the water column rather than old peat could account for the higher CH 4 emissions from the stations in Lokka. This conclusion is supported by the high content of modern carbon (C) in methane (percent modern C of 92-104%) that was extracted from the sediment of Lokka. The results suggested that if there is a good long-term availability of phosphorus and nitrogen, the intensive internal C cycle associated with the primary production could maintain high CH 4 and CO 2 production for decades, similar to the situation in eutrophied natural lakes.