Coastal wetlands have recently been lauded for their carbon sequestration capacity, but many also emit compounds with deleterious atmospheric effects. For example, brackish tidal marshes—where fresh and salt water mix—release significant amounts of methyl chloride and methyl bromide into the atmosphere. (Compounds derived from a methyl group and halogens, including methyl chloride and bromide, are collectively referred to as methyl halides.) Both compounds contribute to stratospheric ozone destruction because they carry substantial quantities of chlorine and bromine into the stratosphere, where they catalyze ozone loss reactions.
Tidal marshes are known to be terrestrial methyl halide hot spots. The estimated magnitude of their emissions is highly variable, however, and reported fluxes vary by 2 to 3 orders of magnitude. This uncertainty is one reason why the global methyl halide budgets remain out of balance, despite decades of research. A missing source exists, and it is not evident if tidal marshes can account for the imbalance.
To more accurately account for terrestrial methyl halide emissions, Deventer et al. conducted the first ecosystem-scale evaluation of these emissions from a tidal marsh. The researchers employed a nonintrusive technique known as relaxed eddy accumulation to measure the exchange of gases between the Suisun Marsh—the largest surviving tidal marsh in the San Francisco Estuary in California—and the atmosphere. The innovative method allowed the researchers to measure multiple ground cover types, including vegetation, bare soil, and open water. They also used static flux chambers and soil cores to monitor smaller spatial scales and more accurately estimate emissions from specific ground cover. The study spanned a 14-month period in 2016 and 2017.
The results show that the marsh is constantly emitting methyl chloride and methyl bromide throughout the year, but the emission rates varied seasonally. Warm summer months drove more emissions than cool winters, with the highest emissions occurring in June. The seasonality of the results suggests that methyl halide emissions track ambient air temperature and the vegetative growth cycle. A warming climate portends larger methyl halide releases, which may be accentuated during heat waves.
The small-scale measurements revealed that emissions were not equal across plant types. The invasive halophyte pepperweed (Lepidium latifolium) occupied large, densely packed patches across the marsh and belched more methyl halides into the atmosphere than its native neighbors. To date, pepperweed is the third-strongest emitting wetland species identified outside of the tropics. Its high invasion potential and recent rapid expansion could increase methyl halide emissions from salt-tolerant wetlands in the future.
Using previous estimates of total global tidal marsh area, the study authors statistically upscaled their results to project methyl halide emissions around the world. This numerical model estimates that tidal marshes contribute up to 1.3% and 5.0% of global methyl chloride and methyl bromide emissions, respectively.
The study indicates that, cumulatively, brackish marshes are minor sources of stratospheric chlorine but notable sources of stratospheric bromine. The novel experimental technique provides a framework for future emissions estimates and helps to reduce the variability of terrestrial methyl halide estimates. (Journal of Geophysical Research: Biogeosciences, https://doi.org/10.1029/2018JG004536, 2018)
—Aaron Sidder, Freelance Writer