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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: Dynamics of greenhouse gases, CH4, CO2 and N2O, and nutrients, NO2- + NO3-, NH4+ and P, were studied in the sediments of the eutrophic, boreal Lake Kevaton in Finland. Undisturbed sediment cores taken in the summer, autumn and winter from the deep and shallow profundal and from the littoral were incubated in laboratory microcosms under aerobic and anaerobic water flow conditions. An increase in the availability of oxygen in water overlying the sediments reduced the release of CH4, NH4+ and P, increased the flux of N2O and NO2- + NO3-, but did not affect CO2 production. The littoral sediments produced CO2 and CH4 at high rates, but released only negligible amounts of nutrients. The deep profundal sediments, with highest carbon content, possessed the greatest release rates of CO2, CH4, NH4+ and P. The higher fluxes of these gases in summer and autumn than in winter were probably due to the supply of fresh organic matter from primary production. From the shallow profundal sediments fluxes of CH4, NH4+ and P were low, but, in contrast, production of N2O was the highest among the different sampling sites. Due to the large areal extension, the littoral and shallow profundal zones had the greatest importance in the overall gas and nutrient budgets in the lake. Methane emissions, especially the ebullition of CH4 (up to 84% of the total flux), were closely related to the sediment P and NH4+ release. The high production and ebullition of CH4, enhances the internal loading of nutrients, lake eutrophication status and the impact of boreal lakes to trophospheric gas budgets.
BIOGEOCHEMISTRY. 65(1):83-103.
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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).
