Carbon dioxide and methane exchange at a cool-temperate freshwater marsh
Freshwater marshes have been shown to be strong sinks for carbon dioxide (CO2) on an annual basis relative to other wetland types; however it is likely that these ecosystems are also strong emitters of methane (CH4), 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 CO2, and four years of warm-season chamber measurements of CH4 at a cool-temperate Typha angustifolia marsh. Annual average cumulative net ecosystem exchange of CO2 (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 CH4 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 CH4 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 CH4 emission of 127 ± 19 g C m−2 yr−1. Combining CO2 and CH4, 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 CH4 when generating marsh C budgets.