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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 degrees 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 boreal 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.
BIOGEOSCIENCES. 01/2009; 6(2):209-223.
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ABSTRACT: Short-term (3–6 days) and long-term (27 days) laboratory experiments were carried out to determine the distribution of assimilated
C in the system Phragmites australis (common reed)-waterlogged fen soil after 14C pulse labelling. The investigated system of fen plants and anaerobic organic soil showed different patterns of assimilated
14C distribution when compared to systems with cultivated plants and aerobic mineral soil. Between 90% and 95% of the 14C in the system was found in the reed plants. A maximum of 2% of the assimilated plant 14C was released from the fen soil as CO2 and about 5–9% remained in the soil. The 14C remaining in the waterlogged fen soil of the reed plant had the same amount as that of a cultivated plant in mineral soil,
despite lower 14C-release (i.e. rhizodeposition and root respiration) from reed roots. Assuming that root respiration of fen plants is low,
this indicates that microbial C turnover in waterlogged fen soil is much slower than in mineral soil. The estimated quantity
of the assimilated C remaining in the soil was of an ecologically relevant order of magnitude.
Biology and Fertility of Soils 09/2000; 32(1):1-7. · 2.32 Impact Factor
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ABSTRACT: 1. The annual dynamics of methane (CH4) in a temporarily flooded meadow, mire bank, lacustrine sedge fen, temporarily and continuously inundated sedge (Carex sp.) and reed (Phragmites australis) marshes were studied from June to November in the humic mesoeutrophic Lake Mekrijarvi and in eutrophicated parts of the mesotrophic Lake Heposelka in the southern part of East Finland. The effects of water level and temperature on littoral CH4 fluxes were determined. Vegetation zonation along the moisture gradient, and associated CH4 fluxes, were evaluated. 2. The CH4 flux increased along the moisture gradient from -0.2 to 14.2 mg CH4 m(-2) h(-1), and was highest in the permanently inundated marshes. The duration of anoxia in the sediment caused differences in the CH4 flux. Estimated emissions for the period 1 June 30 September in continuously inundated sparse reed and sedge marshes, drying sedge marsh, and lacustrine sedge fen were 13, 11 and 6 g CH4 m(-2), respectively. 3. In continuously inundated vegetation, the fluxes were highest in late July/early August. The seasonal CH4 flux pattern suggested that the fluxes were regulated by the supply of organic matter during the course of the summer and the water level. In the temporarily flooded zone, the seasonal CH4 flux dynamics was greatly affected by changes in the lake water level, the fluxes being highest during the spring flood in early June.
FRESHWATER BIOLOGY. 46(7):855-869.
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ABSTRACT: Littoral zones at the interface of the lake and the catchment are intensive sites for mineralization of organic matter, but the contribution of vegetated littoral zone to winter fluxes of carbon dioxide (CO2) and methane (CH4) from lake ecosystems into the atmosphere is poorly known. We studied littoral carbon gas fluxes and their spatial controls at five boreal lakes of varying trophic state during three consequent winters with contrasting snow conditions and flooding regimes. Lake-wide estimates including littoral winter release and potential pelagic spring pulse of gases were calculated for three lakes. Large interannual and spatial differences in carbon gas fluxes were controlled by the interaction of climatic factors, ice and snow cover, on-site hydrology, and apparently substrate supply from biomass production of the previous growing season. Littoral CO2 fluxes ranged from 0.9 to 7.5 mol m(-2) winter(-1), and the CH4 fluxes ranged from 0.04 to 0.38 mol m(-2) winter(-1), the latter being highest in eutrophic lakes. The vegetated littoral contributed the most (66-78%) to winter CH4 emissions from two lakes; in the smallest and most productive lake, pelagic accumulation exceeded littoral release. The large variation in littoral CO2 release could contribute to between-winter differences of 82% in lake-wide carbon gas emissions. The water level of the preceding summer and precipitation during early winter were found to be useful predictors for littoral carbon gas fluxes in winter. This suggests that the carbon gas exchange of a shallow boreal lake can be highly sensitive to changes in snowfall and subsequent flooding.
JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES. 109(D19).
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ABSTRACT: [1] The surface-atmospheric exchange of nitrous oxide (N2O) was investigated in the vegetated littoral zone of a eutrophied midboreal lake (Lake Kevaton, Finland) with a static chamber technique. During a dry summer (three to six samplings per site), the meadow site and two marsh sites in the temporarily flooded eulittoral zone and the Phragmites australis-dominated site in the continuously flooded infralittoral zone had mean daytime N2O-N emissions from 11 +/- 7 to 22 +/- 7 mug m(-2) h(-1), whereas the Nuphar lutea-dominated site in the infralittoral zone had a mean N2O flux close to zero. During a wet summer (13-14 samplings per site), the mean daytime N2O-N fluxes ranged from 4 +/- 1 to 15 +/- 5 mg m(-2) h(-1) at the three eulittoral sites and were negligible at the two infralittoral sites. The littoral zone occupied 26% of the lake area but was estimated to account for most of the N2O emissions from the lake. The studied eulittoral zone, which did not have adjacent nitrogen fertilization, exhibited higher N2O emissions during the summer than seen in northern natural ecosystems in general, including peatlands, forests, and the pelagic regions of lakes. Thus in lake-rich landscapes the littoral zone and other lake-associated wetlands must be considered as potential sources of atmospheric N2O. An assessment of their atmospheric importance requires further data on the N2O fluxes and their regulation in different littoral areas and on the total littoral coverage, neither of which is yet available.
JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES. 108(D14).
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ABSTRACT: [1] Diurnal variation in methane fluxes can cause systematic errors in flux estimates, particularly at places with aquatic vegetation. Closed chamber measurements were made during the ice-free period in central Finland. Methane fluxes were quantified at an interval of a few hours for several boreal lakeshore communities consisting of Phragmites australis (Cav.) Trin. ex Steud. and of wetland sedges, grasses, and herbs. The automated measurement system was operated for 4 months in a P. australis stand and for 2 months in a site with mixed wetland vegetation. Additional manual measurements lasted for a few days. In P. australis stands the CH4 fluxes were typically highest around noon. On the average, efflux of CH4 measured during office hours (0800 - 1600 local time (LT)) should be corrected using a factor 0.68 to obtain a more reliable estimate of daily CH4 release. The fluctuation magnitude varied between different locations and over the study period, being the greatest in the most productive parts of the stands in August. A harmonic regression model, scaled with temperature and stand biomass, was successfully applied to reconstruct CH4 flux in P. australis stand. In the sedge, grass and herb stands the diurnal fluctuation in CH4 flux was generally small. Commonly, daily maximum in efflux occurred during evening or night and large ( threefold) diurnal differences were occasionally found. Daytime measurements alone can result in a slight or moderate underestimate of the total flux for the wetland communities. These results recommend that spatial and seasonal differences in diurnal CH4 flux dynamics should be considered when planning measuring or modeling CH4 fluxes.
JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES. 109(D19).
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ABSTRACT: 1. In order to study the dynamics of primary production and decomposition in the lake littoral, an interface zone between the pelagial, the catchment and the atmosphere, we measured ecosystem/atmosphere carbon dioxide (CO2) exchange in the littoral zone of an eutrophic boreal lake in Finland during two open water periods (1998-1999). We reconstructed the seasonal net CO2 exchange and identified the key factors controlling CO2 dynamics. The seasonal net ecosystem exchange (NEE) was related to the amount of carbon accumulated in plant biomass. 2. In the continuously inundated zones, spatial and temporal variation in the density of aerial shoots controlled CO2 fluxes, but seasonal net exchange was in most cases close to zero. The lower flooded zone had a net CO2 uptake of 1.8-6.2 mol m(-2) per open water period, but the upper flooded zone with the highest photosynthetic capacity and above-ground plant biomass, had a net CO2 loss of 1.1-7.1 mol m(-2) per open water period as a result of the high respiration rate. The excess of respiration can be explained by decomposition of organic matter produced on site in previous years or leached from the catchment. 3. Our results from the two study years suggest that changes in phenology and water level were the prime cause of the large interannual difference in NEE in the littoral zone. Thus, the littoral is a dynamic buffer and source for the load of allochthonous and autochthonous carbon to small lakes.
FRESHWATER BIOLOGY. 48(8):1295-1310.
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ABSTRACT: Littoral wetlands comprise a terrestrial to aquatic continuum along which carbon dioxide is exchanged with the atmosphere and organic carbon is transferred to lakes. Net ecosystem productivity-the difference between atmospheric CO2 uptake and total ecosystem respiration-in these shore areas depends partly on the extent and duration of spring flooding. Ecosystem-atmosphere CO2 fluxes were studied at a boreal lake in Finland to analyze how flooding affects the dynamics of littoral net ecosystem productivity. Two shore transects with different hydrological conditions and vegetation distributions were studied during consecutive ice-free periods with contrasting flooding patterns. Net ecosystem productivity in different vegetation zones did not respond consistently to extended flooding; the response depended on the phenology of plant emergence during the flood and on the decrease in the water level after flooding. The decrease in the water level was steeper in silt-mud sediment than in fen peat, With an exceptionally high water level relative to the height of the vegetation, net ecosystem productivity decreased by 50-100% (net loss, 0.4-7.4 mol m(-2) of CO2 during the wetter open-water period). However, the wetter season could also have a 60-140% higher net ecosystem productivity (net CO2 gain of 0.7 Mol m(-2) to net loss of 6.8 Mol m(-2)) because of the decreased decomposition rate. An extended flooding period greatly reduces the amount of litter produced in a specific year. In both flooding patterns, the littoral zone was an overall net CO2 emitter, but the large variation in the decomposition rate suggests that there are differences in the load of organic matter from the littoral to the pelagic zone.
LIMNOLOGY AND OCEANOGRAPHY. 49(5):1896-1906.
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ABSTRACT: [1] Diurnal variation in methane fluxes can cause systematic errors in flux estimates, particularly at places with aquatic vegetation. Closed chamber measurements were made during the ice-free period in central Finland. Methane fluxes were quantified at an interval of a few hours for several boreal lakeshore communities consisting of Phragmites australis (Cav.) Trin. ex Steud. and of wetland sedges, grasses, and herbs. The automated measurement system was operated for 4 months in a P. australis stand and for 2 months in a site with mixed wetland vegetation. Additional manual measurements lasted for a few days. In P. australis stands the CH4 fluxes were typically highest around noon. On the average, efflux of CH4 measured during office hours (0800 - 1600 local time (LT)) should be corrected using a factor 0.68 to obtain a more reliable estimate of daily CH4 release. The fluctuation magnitude varied between different locations and over the study period, being the greatest in the most productive parts of the stands in August. A harmonic regression model, scaled with temperature and stand biomass, was successfully applied to reconstruct CH4 flux in P. australis stand. In the sedge, grass and herb stands the diurnal fluctuation in CH4 flux was generally small. Commonly, daily maximum in efflux occurred during evening or night and large ( threefold) diurnal differences were occasionally found. Daytime measurements alone can result in a slight or moderate underestimate of the total flux for the wetland communities. These results recommend that spatial and seasonal differences in diurnal CH4 flux dynamics should be considered when planning measuring or modeling CH4 fluxes.
JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES. 109(D19).
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ABSTRACT: Lake littoral zones have a transitional nature and dynamic conditions, which are reflected in their CH4 emissions. Thus, detailed studies are needed to assess the littoral CH4 emissions in a regional scale. In this study, CH4 fluxes were followed during the ice-free seasons in 1998 and 1999 by using the static chamber method in the littoral zone of two lakes in Finland. An exceptionally high water level in 1998 caused an unusually long inundation in otherwise ephemerally flooded zone. The flooding was normal in year 1999. The factors controlling CH4 emissions were examined and statistical response functions were constructed. Further, the effect of extended flooding on the littoral CH4 budged was estimated. The methane flux was primarily regulated by the water level in grass and sedge dominated eulittoral zone, but not in infralittoral reed and water lily stands. Methane emissions in the sedge dominated zone decreased significantly, when the flood was high enough to submerge the venting structures of the plants. Besides water level, sediment temperature determined CH4 emission. The cumulative CH4 emissions from the whole littoral wetlands in wet year were 1.1 times (L. Kevaton), or 0.61 and 0.79 times (L. Mekrijarvi) those in dry year. The crucial factor was the discrepancy between the exceptional and the average water level. The extension of inundated area does not necessarily increase CH4 emissions if the flood reaches infrequently inundated areas, which apparently have low CH4 production potential. This is the case especially, if the emissions in lower zones simultaneously decrease due to high water level. Our study analyses these complex responses between CH4 emissions and water level.
GLOBAL CHANGE BIOLOGY. 9(3):413-424.
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ABSTRACT: [ 1] Transitions between aquatic and terrestrial environments can be recognized as biogeochemically active ecotones that support high CH4 release. We studied the links between littoral CH4 fluxes and aquatic vegetation, hydrologic conditions, and sediment quality, and integrated the CH4 fluxes into a whole-lake assessment. Methane fluxes were measured using a closed chamber method in the littoral and pelagic zones of three Finnish mid-boreal lakes from May to October. The cumulative CH4 fluxes were spatially integrated over the lake relative to the vegetation coverage in the littoral, and to depth zones in the pelagic regions. During the ice-free period, 66-77% of the CH4 was released from the littoral zone, and the mean CH4 effluxes from these lakes were 0.08-0.42 mol m(-2) ice-free season(-1). Littoral and pelagic productivity was reflected in CH4 release from the lakes. Our results show that estimates of lake CH4 release should include an assessment of the vegetated littoral zone.
GLOBAL BIOGEOCHEMICAL CYCLES. 17(4).
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ABSTRACT: [1] Diurnal variation in methane fluxes can cause systematic errors in flux estimates, particularly at places with aquatic vegetation. Closed chamber measurements were made during the ice-free period in central Finland. Methane fluxes were quantified at an interval of a few hours for several boreal lakeshore communities consisting of Phragmites australis (Cav.) Trin. ex Steud. and of wetland sedges, grasses, and herbs. The automated measurement system was operated for 4 months in a P. australis stand and for 2 months in a site with mixed wetland vegetation. Additional manual measurements lasted for a few days. In P. australis stands the CH4 fluxes were typically highest around noon. On the average, efflux of CH4 measured during office hours (0800 - 1600 local time (LT)) should be corrected using a factor 0.68 to obtain a more reliable estimate of daily CH4 release. The fluctuation magnitude varied between different locations and over the study period, being the greatest in the most productive parts of the stands in August. A harmonic regression model, scaled with temperature and stand biomass, was successfully applied to reconstruct CH4 flux in P. australis stand. In the sedge, grass and herb stands the diurnal fluctuation in CH4 flux was generally small. Commonly, daily maximum in efflux occurred during evening or night and large ( threefold) diurnal differences were occasionally found. Daytime measurements alone can result in a slight or moderate underestimate of the total flux for the wetland communities. These results recommend that spatial and seasonal differences in diurnal CH4 flux dynamics should be considered when planning measuring or modeling CH4 fluxes.
JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES. 109(D19).
