| Summary: | 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 cool-temperate 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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