[1] In this study, we examine the magnitude and temporal variability of surface water pCO 2 in a set of lakes in boreal Québec, and explore the links between lake and catchment properties. The study lakes were consistently supersaturated in CO 2 , with the mean lake pCO 2 ranging from 400 to over 1800 matm. There was significant interannual variability in pCO 2 , apparently driven by regional patterns in precipitation. The best multivariate model of average pCO 2 included dissolved organic carbon (DOC), lake area and chlorophyll as independent variables, suggesting that external carbon (C) loading to lakes plays a central role in lake CO 2 dynamics and that lake trophic status may modulate the influence of external C loading. We show that even if the key drivers of lake pCO 2 are similar, they interact differently among regions and the resulting models may be dramatically different. In particular, we show that although pCO 2 is invariably correlated to DOC, the shape of this relationship varies greatly among regions, suggesting large-scale regional differences in C delivery, quality, and in-lake processing. As a consequence, current models cannot be extrapolated across regions unless we apply region-specific variables.

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[1] Permafrost degradation can result in the loss of significant amounts of carbon, through release to the atmosphere in the form of carbon dioxide and/or methane and through export downstream to lakes and rivers. The fate of this carbon in lake ecosystems is poorly understood. We investigated the capacity of lake bacteria to utilize carbon from different soils from an adjacent mire. Dark bioassays were undertaken to measure the dynamics of the bioavailability and chemical character of dissolved organic carbon (DOC). The soils ranged from already degraded minerotrophic fens to ombrotrophic active layer and soils from the permafrost layer. Our study shows that soil DOC was rapidly consumed by bacteria collected from lake water, particularly within the first 48 h (about 85% of the total consumed DOC). The mean DOC consumption by lake bacteria was 0.087 mg L À1 d À1 when supplied with lake water DOC and varied between 0.382 mg L À1 d À1 (permafrost soil) and 0.491 mg L À1 d À1 (degraded fen soil) when supplied with terrestrial DOC. Thus, the data suggest that export of DOC from degrading permafrost mires at any stage of degradation can potentially increase rates of respiration by fourfold to sevenfold and can have pronounced effects both on receiving lake ecosystems and on the land-atmosphere carbon balance. In this study we also propose simple predictive models, incorporating weight-averaged molecular weight and specific UV absorption in combination with other simple qualitative parameters for the estimation of potential bioavailability of soil DOC in aquatic ecosystems.Citation: Roehm, C. L., R. Giesler, and J. Karlsson (2009), Bioavailability of terrestrial organic carbon to lake bacteria: The case of a degrading subarctic permafrost mire complex,

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[1] Climate change and thawing of permafrost will likely result in increased decomposition of terrestrial organic carbon and subsequent carbon emissions to the atmosphere from terrestrial and aquatic systems. The quantitative importance of mineralization of terrestrial organic carbon in lakes in relation to terrestrial carbon fluxes is poorly understood and a serious drawback for the understanding of carbon budgets. We studied a subarctic lake in an area of discontinuous permafrost to assess the quantitative importance of lake carbon emission for the catchment carbon balance. Estimates of net ecosystem production and stable carbon-isotope composition of dissolved organic carbon in the lake water suggest substantial input and respiration of terrestrial organic carbon in the lake. The lake was a net source of CO 2 and CH 4 to the atmosphere at ice breakup in spring and during the whole ice-free period. The carbon emission from the lake was similar in magnitude to the terrestrial net release of carbon to the atmosphere. The results indicate that lakes are important sources of catchment carbon emission, potentially increasing the positive feedback from permafrost thawing on global warming.Citation: Karlsson, J., T. R. Christensen, P. Crill, J. Förster, D. Hammarlund, M. Jackowicz-Korczynski, U. Kokfelt, C. Roehm, and P. Rosén (2010), Quantifying the relative importance of lake emissions in the carbon budget of a subarctic catchment,

[1] Peatlands are sinks for carbon dioxide (CO 2 ) because net primary production exceeds decomposition. The contribution of non-growing-season fluxes to the annual C budget of a peatland is, to date, little studied. We therefore measured the changes in the pattern of carbon exchange with seasons in a bog located in the cool temperate climate region. The growing season CO 2 -C uptake was of À113 g m À2. During the non-growing season, 36 g C m À2 was lost to the atmosphere, resulting in an estimated net ecosystem production of À76 g C m À2 . Despite the non-growing-season loss equaling 33 to 40% of the summer uptake, the net annual accumulation of was 3 times the long-term average net accumulation rate usually cited in the literature. The high rate of non-growing-season efflux could be supported directly by temporal concurrent respiration and the release of stored CO 2 from prior production. These results indicate the need to revise current models to address peat thermal properties inducing CO 2 production at lower temperature ranges.

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