Lakes as nitrous oxide sources in the boreal landscape.

Estimates of regional and global freshwater N2 O emissions have remained inaccurate due to scarce data and complexity of the multiple processes driving N2 O fluxes the focus predominantly being on summer time measurements from emission hot spots, agricultural streams. Here, we present four-season data of N2 O concentrations in the water columns of randomly selected boreal lakes covering a large variation in latitude, lake type, area, depth, water chemistry, and land use cover. Nitrate was the key driver for N2 O dynamics, explaining as much as 78% of the variation of the seasonal mean N2 O concentrations across all lakes. Nitrate concentrations varied among seasons being highest in winter and lowest in summer. Of the surface water samples, 71% were oversaturated with N2 O relative to the atmosphere. Largest oversaturation was measured in winter and lowest in summer stressing the importance to include full year N2 O measurements in annual emission estimates. Including winter data resulted in fourfold annual N2 O emission estimates compared to summer only measurements. Nutrient-rich calcareous and large humic lakes had the highest annual N2 O emissions. Our emission estimates for Finnish and boreal lakes are 0.6 and 29 Gg N2 O-N/year, respectively. The global warming potential of N2 O from lakes cannot be neglected in the boreal landscape, being 35% of that of diffusive CH4 emission in Finnish lakes.

Greenhouse Gas Emissions from Freshwater Reservoirs: What Does the Atmosphere See?

Freshwater reservoirs are a known source of greenhouse gas (GHG) to the atmosphere, but their quantitative significance is still only loosely con- strained. Although part of this uncertainty can be attributed to the difficulties in measuring highly variable fluxes, it is also the result of a lack of a clear accounting methodology, particularly about what constitutes new emissions and potential new sinks. In this paper, we review the main processes involved in the generation of GHG in reservoir systems and propose a simple approach to quantify the reservoir GHG footprint in terms of the net changes in GHG fluxes to the atmosphere induced by damming, that is, 'what the atmosphere sees.' The approach takes into account the pre-impoundment GHG balance of the landscape, the temporal evolution of reservoir GHG emission profile as well as the natural emissions that are displaced to or away from the reservoir site resulting from hydrological and other changes. It also clarifies the portion of the reservoir carbon burial that can potentially be considered an offset to GHG emissions.

Fluxes of methane, carbon dioxide and nitrous oxide in boreal lakes and potential anthropogenic effects on the aquatic greenhouse gas emissions.

We have examined how some major catchment disturbances may affect the aquatic greenhouse gas fluxes in the boreal zone, using gas flux data from studies made in 1994-1999 in the pelagic regions of seven lakes and two reservoirs in Finland. The highest pelagic seasonal average methane (CH(4)) emissions were up to 12 mmol x m(-2) x d(-1) from eutrophied lakes with agricultural catchments. Nutrient loading increases autochthonous primary production in lakes, promoting oxygen consumption and anaerobic decomposition in the sediments and this can lead to increased CH(4) release from lakes to the atmosphere. The carbon dioxide (CO(2)) fluxes were higher from reservoirs and lakes whose catchment areas were rich in peatlands or managed forests, and from eutrophied lakes in comparison to oligotrophic and mesotrophic sites. However, all these sites were net sources of CO(2) to the atmosphere. The pelagic CH(4) emissions were generally lower than those from the littoral zone. The fluxes of nitrous oxide (N(2)O) were negligible in the pelagic regions, apparently due to low nitrate inputs and/or low nitrification activity. However, the littoral zone, acting as a buffer for leached nitrogen, did release N(2)O. Anthropogenic disturbances of boreal lakes, such as increasing eutrophication, can change the aquatic greenhouse gas balance, but also the gas exchange in the littoral zone should be included in any assessment of the overall effect. It seems that autochthonous and allochthonous carbon sources, which contribute to the CH(4) and CO(2) production in lakes, also have importance in the greenhouse gas emissions from reservoirs.

Contribution of winter to the annual CH4 emission from a eutrophied boreal lake.

The springtime methane (CH4) emission from a small, eutrophied boreal lake was assessed during the winter ice-cover by measurement of gas ebullition and CH4 accumulation in the water column in association with the development of oxygen depletion after ice formation. The winter CH4 production was estimated to result in a loss of 3.6-7.9 g CH4 m(-2) from the lake to the atmosphere during the short period of ice melt. This could account for 22-48% of the annual CH4 emission from the pelagic zone of the lake. The contribution of winter to the annual CH4 release can be similar or even higher in seasonally ice-covered northern aquatic ecosystems than in northern terrestrial wetlands, thus winter must be considered in any studies into the aquatic CH4 emissions. The trophic state and wintertime oxygen conditions, linked to the changes in land-use in the catchments and climate, are important factors controlling the springtime lake CH4 emissions.

Seasonal variation in CH4 emissions and production and oxidation potentials at microsites on an oligotrophic pine fen.

Temporal and spatial variation in CH4 emissions was studied at hummock, Eriophorum lawn, flark and Carex lawn microsites in an oligotrophic pine fen over the growing season using a static chamber method, and CH4 production and oxidation potentials in peat profiles from hummock and flark were determined in laboratory incubation experiments. Emissions were lowest in the hummocks, and decreased with increasing hummock height, while in the lawns and flarks they increased with increasing sedge cover. Statistical response functions with water table and peat temperature as independent variables were calculated in order to reconstruct seasonal CH4 emissions by reference to the time series for peat temperature and water table specific to each microsite type. Mean CH4 emissions in the whole area in the snow-free period of 1993, weighted in terms of the proportions of the microsites, were 1.7 mol CH4 m-2. Potential CH4 production and oxidation rates were very low in the hummocks rising above the groundwater table, but were relatively similar when expressed per dry weight of peat both in the hummocks and flarks below the water table. The CH4 production potential increased in autumn at both microsites and CH4 oxidation potential seemed to decrease. The decrease in temperature in autumn certainly reduced in situ decomposition processes, possibly leaving unused substrates in the peat, which would explain the increase in CH4 production potential.