Long-term CH₃Br and CH₃Cl flux measurements in temperate salt marshes

Abstract. Fluxes of CH₃Br and CH₃Cl and their relationship with potential drivers such as sunlight, temperature and soil moisture, were monitored at fortnightly to monthly intervals for more than two years at two contrasting temperate salt marsh sites in Scotland. Manipulation experiments were conducted to further investigate possible links between drivers and fluxes. Fluxes followed both seasonal and diurnal trends with highest fluxes during summer days and lowest (negative) fluxes during winter nights. Mean (± 1 sd) annually and diurnally-weighted net emissions from the two sites were found to be 300 ± 44 ng m⁻² h⁻¹ for CH₃Br and 662 ± 266 ng m⁻² h⁻¹ for CH₃Cl. The fluxes from this work are similar to findings from this and other research groups for salt marshes in cooler, higher latitude climates, but lower than values from salt marshes in the Mediterranean climate of southern California. Statistical analysis generally did not demonstrate a strong link between temperature or sunlight levels and methyl halide fluxes, although it is likely that temperatures have a weak direct influence on emissions, and both certainly have indirect influence via the annual and daily cycles of the vegetation. CH₃Cl flux magnitudes from different measurement locations depended on the plant species enclosed whereas such dependency was not discernible for CH₃Br fluxes. In 14 out of 18 collars with vegetation CH₃Br and CH₃Cl net fluxes were significantly positively correlated. The CH₃Cl/CH₃Br net-emission mass ratio was 2.2, a magnitude lower than mass ratios of global methyl halide budgets (~22) or emissions from tropical rainforests (~60). This is likely due to preference for CH₃Br production by the relatively high bromine content in the salt marsh plant material. Extrapolation based solely on data from this study yields salt marsh contributions of 0.5–3.2% and 0.05–0.33%, respectively, of currently-estimated total global production of CH₃Br and CH₃Cl, but actual global contributions likely lie between these values and those derived from southern California.