[1] We present here the first comprehensive assessment of the carbon (C) footprint associated with the creation of a boreal hydroelectric reservoir (Eastmain-1 in northern Québec, Canada). This is the result of a large-scale, interdisciplinary study that spanned over a 7-years period (2003)(2004)(2005)(2006)(2007)(2008)(2009)), where we quantified the major C gas (CO 2 and CH 4 ) sources and sinks of the terrestrial and aquatic components of the pre-flood landscape, and also for the reservoir following the impoundment in 2006. The pre-flood landscape was roughly neutral in terms of C, and the balance between pre-and post-flood C sources/sinks indicates that the reservoir was initially (first year post-flood in 2006) a large net source of CO 2 (2270 mg C m À2 d À1) but a much smaller source of CH 4 (0.2 mg C m À2 d À1). While net CO 2 emissions declined steeply in subsequent years (down to 835 mg C m À2 d À1 in 2009), net CH 4 emissions remained constant or increased slightly relative to pre-flood emissions. Our results also suggest that the reservoir will continue to emit carbon gas over the long-term at rates exceeding the carbon footprint of the pre-flood landscape, although the sources of C supporting these emissions have yet to be determined. Extrapolation of these empirical trends over the projected life span (100 years) of the reservoir yields integrated long-term net C emissions per energy generation well below the range of the natural-gas combined-cycle, which is considered the current industry standard.

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The net ecosystem carbon dioxide (CO 2 ) exchange (NEE) between boreal bogs and the atmosphere and its environmental drivers remains understudied despite the large carbon store of these northern ecosystems. We present NEE measurements using the eddy covariance technique in a boreal ombrotrophic bog over five growing seasons and four winters. Interannual variability in CO 2 uptake was most pronounced in June-September (-4 to -122 g CO 2 -C m -2 ), less in March-May (-1 to -21 g CO 2 -C m -2 ) and very small in October-November (-2 to -4 g CO 2 -C m -2 ). Variability in NEE between years was linked primarily to changes in water table depth (WTD). Strong and significant relationships (r 2 [ 0.89, p B 0.05) were found between summer (JuneSeptember) maximum photosynthetic rate (A max ), net ecosystem productivity (NEP), gross ecosystem productivity and WTD. Adding air temperature through multiple regression analysis further increased correlation between summer A max , NEP, and WTD (r 2 = 0.96, p = 0.05). In contrast to previous studies examining controls on peatland CO 2 exchange, no relationships were found between productivity or cumulative exchange and early season temperature, timing of the snowmelt or growing season length.

Freshwater marshes have been shown to be strong sinks for carbon dioxide (CO 2 ) on an annual basis relative to other wetland types; however it is likely that these ecosystems are also strong emitters of methane (CH 4 ), reducing their carbon (C) sequestration potential. Multiyear C balances in these ecosystems are necessary therefore to determine their contribution to the global C cycle. Despite this, the number of multiyear studies in marshes is few, with, to the best of our knowledge, only one other Northern marsh C balance reported. This study presents five years of eddy covariance flux measurements of CO 2 , and four years of warm-season chamber measurements of CH 4 at a cooltemperate Typha angustifolia marsh. Annual average cumulative net ecosystem exchange of CO 2 (NEE) at the marsh was −224 ± 54 g C m −2 yr −1 (±SD) over the five-year period, ranging from −126 to −284 g C m −2 yr −1 . Enhancement of the ecosystem respiration during warmer spring, autumn and winter periods appeared the strongest determinant of annual NEE totals. Warm season fluxes of CH 4 from the Typha vegetation (avg. 1.0 ± 1.2 g C m −2 d −1 ) were significantly higher than fluxes from the water surface (0.5 ± 0.4 g C m −2 d −1 ) and unvegetated mats (0.2 ± 0.2 g C m −2 d −1 ). Air temperature was a primary driver of all CH 4 fluxes, while water table was not a significant correlate as water levels were always at or above the vegetative mat surfaces. Weighting by the surface cover proportion of water and vegetation yielded a net ecosystem CH 4 emission of 127 ± 19 g C m −2 yr −1 . Combining CO 2 and CH 4 , the annual C sink at the Mer Bleue marsh was reduced to −97 ± 57 g C m −2 yr −1 , illustrating the importance of accounting for CH 4 when generating marsh C budgets.

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