Ecosystem-Scale Measurements of Methyl Halide Fluxes From a Brackish Tidal Marsh Invaded With Perennial Pepperweed (Lepidium latifolium)

Abstract

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 upscaling plot-scale footprints (i.e., ≤1 m2). 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 time scales, 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 s 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/m2 for CH3Cl and 21 mg/m2 for CH3Br, scaling up to 621 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/year CH3Cl (range 10 to 77) and 3 Gg/year CH3Br (range 1 to 8).