Observation of diel patterns and episodic events in wetland methane efflux using automated chambers

Uncertainties in wetland methane flux characteristics stem from complex spatial and temporal heterogeneity driven, in part, by climate, vegetation, local hydrology and weather. One essential way of reducing the magnitude of these uncertainties is analysis of in situ measurements at higher temporal resolutions. We have instrumented an existing 10-chamber automated system in a 1.7 ha poor fen with a laser-based CH4 sensor capable of generating one concentration measurement per second. With this system we obtain eight fluxes per chamber per day. The ten chambers sample various vegetation classes that are common at the fen. To support these data, samples from the porewater profile were analyzed for dissolved methane content throughout one growing season and a suite of meteorological data was also collected. We have identified episodic ebullition (bubbling) events (Figure 1) and evaluated the magnitude, timing and controls on this emission pathway. The temporal coverage of flux measurements has also allowed for an investigation of the diel CH4 flux patterns at this site. Preliminary results show peak fluxes occurring between 02:00-04:00 contrary to most previous diel studies and the expected temperature response of methane production. Furthermore, the data suggest that the proportion of the total flux due to ebullition is higher during these early morning hours. Figure 1. Sudden jumps in methane concentration represent episodic release of bubbles within chambers. These are identified by setting a threshold for the standard deviation of the first difference of each flux time series