Observations of Greenhouse Gas Fluxes from Restored Wetlands in the Sacramento - San Joaquin Delta, California, USA

Monitoring the effects of water management on carbon and energy fluxes from two 3–hectare restored wetlands on Twitchell Island in the Sacramento–San Joaquin Delta (Delta), California, USA, is part of an ongoing, longer-term study designed by the U.S. Geological Survey (USGS), investigating mitigation techniques for island subsidence through atmospheric carbon sequestration and soil carbon storage. In 1997, we established two experimental wetlands: a western wetland managed at a water depth of 25 centimeters (cm) and an eastern wetland managed at a depth of 55 cm. Over the past 14 years, the western wetland has developed into a dense canopy of emergent marsh species with some floating vegetation. The eastern wetland is similar to the western wetland, but also includes areas of open water, submerged vegetation, and algae. For this study, we used atmospheric gas flux measurements to gage the effects of water management on energy and carbon budgets in two designed wetlands.

The Delta region is classified as a Mediterranean type climate and is characterized by cool, moist winters and warm, dry summers. The clear summer skies are conducive to the generation of maximum daily carbon dioxide uptake rates of approximately 30 µmol m-2 s-1 or higher. Elevated rates of carbon dioxide uptake were measured in the eastern wetland during 2002 through 2004, using the eddy covariance method, but in 2010, the maximum carbon dioxide uptake rates were reduced by one-fourth, approximately 10 µmol m-2 s-1. We hypothesize that large mats of accumulating senescent material within the flux footprint may have contributed to this discrepancy in flux rates by either significantly slowing or stopping the growth of the emergent marsh species. In contrast to carbon dioxide uptake, anaerobic conditions created by permanent flooding have resulted in methane flux measurements in excess of 250 nmol m-2 s-1. These methane fluxes are some of the highest values observed when compared to other Delta flux studies from rice, pasture, and natural wetlands, measured by the Biometeorology group at the University of California, Berkeley, which typically range from 10 to 100 nmol m-2 s-1. For land management decisions, it is important to determine which practice will yield the highest rates of carbon dioxide uptake, while minimizing carbon loss through plant respired carbon dioxide or methanogenesis.

We tested our hypothesis that residual senescent plant material reduces carbon dioxide uptake rates during the growing season by moving the eddy covariance tower to the western wetland, where emergent marsh species are more dense and the presence of residual senescent material is reduced. Here we present results showing diurnal and seasonal trends of carbon dioxide fluxes for the years 2002 through 2004 and 2010 through 2011, and methane fluxes for years 2010 through 2011. We also focus on the influence of seasonal variability by normalizing flux measurements with various abiotic and biotic conditions, such as air, leaf, and water temperatures, differences in humidity, and changes in daily and seasonal variations in solar radiation. Additionally, we present the results from a footprint algorithm designed to examine seasonal variances from the 2010 through 2011 flux measurements. Finally, we compare our results with other ongoing flux studies in the Delta.