Net ecosystem exchange from a cattail marsh within the Mer Bleue wetland complex in Ontario, Canada

Although they occupy a minor portion of the global land area, wetlands are significant contributor to the global carbon budget. The future responses and contribution of wetlands to climate change are highly unpredictable since wetland processes are very sensitive to climatic variations. Temporal and spatial variability in wetland carbon fluxes is often associated with different trophic status, dominant vegetation, hydrology, substrate characteristics, microbial communities, etc. Organic wetlands, such as peatlands, are usually net carbon dioxide (CO2) sinks and net methane (CH4) emitters. Given that carbon processes in mineral wetlands, like marshes, are driven by different interactions of controlling factors, the direction and magnitude of carbon fluxes are likely to show unique patterns of biogeochemical cycling of CO2 and CH4. However, the greenhouse gas contribution from marshes is often overlooked because natural marshes are often hidden in forests and peatlands, and most constructed or restored wetlands, which tend to be marshes, are rarely studied. Very few studies on gas exchange in marshes have been done, and to the best of our knowledge, none have reported year-round measurement of net ecosystem CO2 exchange (NEE) for a temperate marsh.

In 2005-2006, the eddy covariance technique was used to measure the NEE in a cattail marsh located in the Mer Bleue wetland complex in Eastern Ontario, Canada, which is part of the Fluxnet-Canada Research Network. Preliminary analyses show that the marsh wetland was a net source of carbon to the atmosphere in May (0.91 g C m-2 d-1), October (1.10 g C m-2 d-1), November (0.58 g C m-2 d-1) and December (0.34 g C m-2 d-1), almost neutral in June (0.08 g C m-2 d-1), and a net carbon sink in July (-4.66 g C m-2 d-1), August (-3.98 g C m-2 d-1) and September (-1.66 g C m-2 d-1). NEE measurements are still being taken. Cattails started to grow at the end of May and were fully senesced by mid-October. Peak aboveground biomass (1156 g m-2) and peak green LAI (3.63) were reached in mid-August. Growing season CO2 fluxes are well correlated with live biomass (r2=0.83) and LAI (r2=0.95). Chamber measurements of CH4 fluxes ranged from 218 to 3356 mg CH4 m-2 d-1 (average of 948 mg CH4 m-2 d-1) in the open water portion of the marsh, and from 73 to 7456 mg CH4 m-2 d-1 (average of 1760 mg CH4 m-2 d-1) in the vegetated portion. The presence of aerenchyma tissue in cattail species and the high nutrient status of the marsh could explain the high fluxes of CH4.