Wetland ecosystems, the largest natural source of methane (CH4), are facing great risks from climate change and sea level rise. While the area covered by wetland ecosystems has been declining, the global atmospheric CH4 concentration has increased, doubling since the beginning of the industrial revolution. Although we understand that hydrology has a significant impact on the development of conditions required for CH4 production, we don't fully understand the physical and biological controls on CH4 exchange. In this study, we compare two freshwater marshes with different hydroperiods (short versus long) in the Florida Everglades, the largest subtropical wetland in the United States. The Everglades’ historic range has been reduced by ~50% due to urbanization and water management activities. We address three questions using approximately five years (2015 to 2019) of CH4 fluxes collected using the eddy covariance method: 1) What are the temporal patterns in CH4 emissions? 2) What are the environmental drivers of CH4 in freshwater marshes? And 3) Are patterns and drivers similar between the two sites? We used a combination of mixed effects linear models and time series methods to examine the complex relationships between CH4 and a suite of micrometeorological variables by site.
Results/Conclusions
The two sites showed similar temporal patterns over diel and seasonal scales. In both sites, CH4 showed a diurnal pattern, with an early morning peak. Hourly CH4 release was at its lowest rate around solar noon for the short-hydroperiod site and in the late evening for the long-hydroperiod site. Daily rates of CH4 flux peaked in September, near the end of the wet season and daily emissions were lowest in December-January in both sites, after the start of the dry season. Furthermore, the long-hydroperiod site had greater mean daily CH4 emissions. While we found significant relationships between CH4 emission and changes in water level, analyses of cross-correlations reveal a complex pattern whereby CH4 emissions are correlated to water level changes at daily to weekly lags. As water levels continue to change throughout the Everglades region, it is imperative that research is targeted to reduce uncertainties in predictions of CH4 release to improve our understanding of the carbon cost of water management decisions.