Carbon dynamics of a freshwater marsh ecosystem

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Freshwater marshes are a wetland type that is defined by its tall, emergent vegetation and typically flooded environments and are found in ~16 million ha in Canada and ~95 million ha globally. Freshwater marshes provide many ecosystem goods and services from water filtration and flood control through habitats for avian and insect species. Despite freshwater marshes being an important wetland type, only a few studies exist that are focused on their carbon cycling. The net carbon exchange in a marsh is a balance between a large carbon dioxide uptake and a large methane release - both greenhouse gases. The drivers of these large fluxes are poorly understood but are likely highly sensitive to changes in the environment. Marsh methane emissions remain particularly understudied with very few continuous measurements at the ecosystem scale. Methane production is an anaerobic process and waterlogged marshes with highly available easily decomposed carbon provide an ideal environment for methane production. Three different pathways for transport exist: diffusion through the water column, ebullition (bubbling) and plant-mediated transport through hollow plant tissues. However, the balance between these and the controlling mechanisms remain poorly studied. We are thus left with limited knowledge about processes controlling methane production, transport and release in a system that that has been shown to potentially emit more methane than any other temperate or northern wetland type. Improved quantification and understanding of marsh methane emissions will, in turn, lead to better understanding of their role in the global carbon cycle and how they may affect climate. We cannot simply apply knowledge from other better-studied wetland types; marshes contain unique characteristics - shallow organic soil deposits, emergent aquatic plants, nutrient rich, and inundated environments - that lead to carbon flux dynamics that are distinct from those of other wetlands. The long-term fate of marshes are also of interest if we consider the carbon exchange potential of this ecosystem and how its ability to sequester carbon through time is dependent on the controls on methane release. This research will use a combination of field-based, laboratory and modelling approaches to improve upon our limited understanding of the ecosystem trace gas exchange in a temperate freshwater marsh ecosystem in eastern Ontario. This research will provide new insights into the role that dominant vegetative species and open water features play in the net carbon exchange of marshes, how disturbances modify annual and interannual carbon emissions and will reduce uncertainty about contemporary rates of carbon exchange with the atmosphere. Modelling efforts will thus be able to incorporate the effects of future change on the marsh net carbon balance.