Prairie pothole wetlands are important sources of methane to the atmosphere

Background/Question/Methods

Because many freshwater ecosystems are small, quantifying carbon cycling in wetlands is vital in understanding their role in regional carbon cycling.  Wetlands in the Prairie Pothole Region (PPR) of North America exist in either a clear-water state dominated by submerged macrophytes, or a turbid-water state dominated by phytoplankton. Methane emission through diffusive flux and ebullition has not been extensively analyzed in these systems, and may depend on state or other lake characteristics such as depth or productivity.  During July and August 2013, we measured methane fluxes in 6 wetlands in differing states in the southeastern PPR.  Measurements of water temperature, salinity, wind-speed, and concentrations of methane in the air and water were used to quantify the diffusive flux of methane.  To estimate ebullition, we placed four chambers on the surface of the water to measure methane emission from bubbling events during 24-hr deployments and corrected for the diffusive flux to estimate ebullition.  We then explored whether factors such as lake state and depth were associated with the rate of methane ebullition and diffusive flux. Finally, we compared methane emission to CO₂ emission and burial rates to assess their net effect on the carbon cycle.

Results/Conclusions

Daily methane emissions through diffusion and ebullition flux ranged from approximately 1 to 35 mmol CH₄ m^-2 d^-1 during the sampling period. Whether the majority of the flux was due to diffusion or ebullition greatly varied within and between wetlands.  The dominant plants in the wetlands (macrophytes vs. phytoplankton) did not seem to influence the rate of methane emissions.  Further work is being done to assess the effect of depth on methane emissions, as this variable correlates with anoxic sediments and therefore methane production. CO₂ fluxes ranged from -43 (influx into the wetland) to 153 mmol CO₂ m^-2 d^-1 out of the wetland but with mean values of ca. 20 mmol m^-2 d^-1.  These fluxes were similar and therefore methane represents a significant portion of the carbon emissions from these wetlands.  In future work, we will make measurements over the entire annual period because methane release after ice-out and in the fall are likely significant. In many cases, CH₄ and CO₂ fluxes exceed the carbon burial rate in the sediment, causing these systems to be carbon sources rather than sinks at the ecosystem scale.  Perhaps more importantly, these systems are a very large source of greenhouse gases to the atmosphere.