Ecosystem Scale Measurements of Methyl Halide Fluxes from a Brackish Tidal Marsh Invaded with Perennial Pepperweed (Lepidium latifolium)

Natural methyl chloride (CH3Cl) and methyl bromide (CH3Br) emissions from
coastal marsh ecosystems may constitute a significant proportion of
stratospheric chlorine and bromine, which catalyze ozone depletion.
Current inventories involve substantial uncertainties associated with
up-scaling plot-scale footprints (i.e., ≤ 1m2).  Here we present
net ecosystem flux measurements of methyl halides from a brackish tidal
marsh on the west coast of the United States between April 2016 and June
2017 using the relaxed eddy accumulation method.  The measurement
footprint encompasses a large part of the studied tidal marsh, including
roughly 20 vascular plant species, open water and soil surfaces. 
On the annual scale, ecosystem methyl halide emissions showed the
strongest relationships to temperature and the growth cycle of halophyte
vegetation, whereas on diurnal timescales, fluxes correlated the most with
evapotranspiration. The maximum seasonal emissions occurred during the
flowering season of Lepidium latifolium (perennial pepperweed), one of the
most abundant halophytes on site.  The maximum hourly emissions
of 111 µg CH3Cl m-2 hr-1 and 38 µg CH3Br m-2 hr-1 were observed during a
heat wave in early June.  Annually integrated emissions were 135
mg m-2 for CH3Cl and 21 mg m-2 for CH3Br, scaling up to 621 kg and 96 kg
over the entire marsh. We provide a global salt marsh emission inventory
that takes into account the spatial distribution of salt marshes in
different climate zones, yielding a global salt marsh source of 31 Gg yr-1
CH3Cl (range: 10 to 77) and 3 Gg yr-1 CH3Br (range: 1 to 8 ).