Long-term effects of elevated UV-B radiation on photosynthesis and ultrastructure of Eriophorum russeolum and Warnstorfia exannulata.

The depletion of stratospheric ozone above the Arctic regions may increase the amount of UV-B radiation to which the northern ecosystems are exposed. In this paper, we examine the hypothesis that supplemental UV-B radiation may affect the growth rate and photosynthesis of boreal peatland plants and could thereby affect the carbon uptake of these ecosystems. In this study, we report the effects of 3-year exposure to elevated UV-B radiation (46% above ambient) on the photosynthetic performance and ultrastructure of a boreal sedge Eriophorum russeolum and a moss Warnstorfia exannulata. The experiment was conducted on a natural fen ecosystem at Sodankylä in northern Finland. The effects of UV-B radiation on the light response of E. russeolum CO(2) assimilation and the maximal photochemical efficiency of photosystem II in a dark-adapted state (F(v)/F(m)) were measured in the field. In addition, the effect of supplemental UV-B radiation on organelles of photosynthetic cells was studied by electron microscopy. The UV-B treatment had no effect on the CO(2) assimilation rate of either species, nor did it affect the structure of the cell organelles. On chlorophyll fluorescence, the UV-B exposure had only a temporary effect during the third exposure year. Our results suggested that in a natural ecosystem, even long-term exposure to reasonably elevated UV-B radiation levels does not affect the photosynthesis of peatland plants.

Carbon dioxide and methane fluxes in boreal peatland microcosms with different vegetation cover--effects of ozone or ultraviolet-B exposure.

O(3) concentrations in the troposphere are rising and those in the stratosphere decreasing, the latter resulting in higher fluxes of solar ultraviolet-B (UV-B) radiation to the earth's surface. We assessed whether the fluxes of CO(2) and CH(4) are altered by enhanced UV-B radiation or elevated tropospheric O(3) concentrations in boreal peatland microcosms (core depth 40 cm, diameter 10.5 cm) with different vegetation cover. At the end of the UV-B experiment which lasted for a growing season, net CO(2) exchange (NEE) and dark ecosystem respiration ( R(TOT)) were sevenfold higher, and CH(4) efflux 12-fold higher, in microcosms with intact vegetation dominated by Eriophorum vaginatum L. and Sphagnum spp., compared to microcosms from which we removed E. vaginatum. Vegetation treatment had minor effects on CH(4) production and consumption potentials in the peat, suggesting that the large difference in CH(4) efflux is mainly due to efficient CH(4) transport via the aerenchyma of E. vaginatum. Ambient UV-B supplemented with 30% and elevated O(3) concentrations (100 and 200 ppb, for 7 weeks) significantly increased R(TOT) in both vegetation treatments. Elevated O(3) concentrations reduced NEE over time, while UV-B had no clear effects on the fluxes of CO(2) or CH(4) in the cloudy summer of the study. Field experiments are needed to assess the significance of increasing UV-B radiation and elevated tropospheric O(3) concentration on peatland gas exchange in the long-term.

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.

Ozone effects on Sphagnum mosses, carbon dioxide exchange and methane emission in boreal peatland microcosms.

Microcosms of a boreal peatland originating from an oligotrophic fen in Eastern Finland were fumigated under four ozone concentrations (0, 50, 100 and 150 ppb O3) in laboratory growth chambers during two separate experiments (autumn and summer) for 4 and 6 weeks, respectively. Ozone effects on Sphagnum mosses and the fluxes of carbon dioxide and methane were evaluated. In both experiments, the three Sphagnum species studied showed only a few significant responses to ozone. In the autumn experiment, membrane permeability of S. angustifolium, measured as conductivity and magnesium leakage, was significantly higher under ozone fumigation (P = 0.005 and < 0.001, respectively), and there was a distinct dose-dependence. S. magellanicum showed no clear responses, either for membrane leakage or pigment content. There were no substantial ozone responses in the gross photosynthesis or net CO2 exchange during the 6-week-long summer experiment, but dark ecosystem respiration was transiently increased by ozone concentration of 100 ppb after 14 days of exposure (P < 0.05). Fumigation with 100 ppb of ozone, however, more than doubled (P < 0.05) methane emission from the peatland monoliths. Our results suggest that increasing tropospheric ozone concentration may cause substantial changes in the carbon gas cycling of boreal peatlands, even though these changes are not closely associated with the changes in Sphagnum vegetation.

Responses of two Sphagnum moss species and Eriophorum vaginatum to enhanced UV-B in a summer of low UV intensity.

• The flux of ultraviolet (UV)-B radiation to the Earth's surface is increasing, particularly in high latitudes. We studied the sensitivity of some dominant plant species of boreal and subarctic peatlands to this increase. • Intact peat monoliths with the mosses Sphagnum balticum and Sphagnum papillosum, and cotton grass (Eriophorum vaginatum) were exposed to ambient solar UV-B or ambient solar UV-B supplemented by 30% in a field experiment in central Finland. • Although the UV-B dose was low during the growing season, owing to frequent cloudiness, both Sphagnum species showed significantly higher membrane permeability under enhanced UV-B. In S. balticum, UV-B tended to decrease the capitulum dry mass and induced a 30-40% increase in the concentration of chlorophyll and carotenoid pigments. Enhanced UV-B had no effects on leaf morphology, chlorophyll fluorescence or stomatal functioning in E. vaginatum. • The various UV-B responses in the Sphagnum species under investigation indicate that they may be sensitive even to small increases in solar UV-B radiation. By contrast, E. vaginatum appeared to tolerate the UV-B fluxes of the experiment.

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.

Reconstruction of the carbon balance for microsites in a boreal oligotrophic pine fen, Finland.

Carbon dioxide (CO2) exchange was studied at flark (minerotrophic hollow), lawn and hummock microsites in an oligotrophic boreal pine fen. Statistical response functions were constructed for the microsites in order to reconstruct the annual CO2 exchange balance from climate data. Carbon accumulation was estimated from the annual net CO2 exchange, methane (CH4) emissions and leaching of carbon. Due to high water tables in the year 1993, the average carbon accumulation at the flark, Eriophorum lawn, Carex lawn and hummock microsites was high, 2.91, 6.08, 2.83 and 2.66 mol C m-2, respectively, and for the whole peatland it was 5.66 mol m-2 year-1. During the maximum primary production period in midsummer, hummocks with low water tables emitted less methane than predicted from the average net ecosystem exchange (NEE), while the Carex lawns emitted slightly more. CH4 release during that period corresponded to 16% of the contemporary NEE. Annual C accumulation rate did not correlate with annual CH4 release in the microsites studied, but the total community CO2 release seemed to be related to CH4 emissions in the wet microsites, again excluding the hummocks. The dependence of CO2 exchange dynamics on weather events suggests that daily balances in C accumulation are labile and can change from net carbon uptake to net release, primarily in high hummocks on fens under warmer, drier climatic conditions.