Carbon Dioxide and Methane Emissions from Red Sea Mangrove Sediments

Abstract. Mangrove forests are highly productive tropical and subtropical coastal systems that provide a variety of ecosystem services, including the sequestration of carbon. While mangroves are reported to be the most intense carbon sinks among all forests, their sediments can also support large emissions of greenhouse gases (GHG), such as carbon dioxide (CO2) and methane (CH4), to the atmosphere. However, data derived from arid mangrove systems like the Red Sea are lacking. Here, we report emission rates of CO2 and CH4 from mangrove sediments along the Saudi Arabian coast of the Red Sea, and assess the relative role of these two gases in supporting total GHG emissions. Diel CO2 and CH4 emission rates in Red Sea mangrove sediments ranged from −3452 to 7500 µmol CO2 m−2 d−1 and from 0.9 to 13.3 µmol CH4 m−2 d−1, respectively. The rates reported here fall within previously reported ranges for both CO2 and CH4, but maximum CO2 and CH4 flux rates in the Red Sea are 10 to 100-fold below those previously reported for mangroves elsewhere. Based on the isotopic composition of the CO2 and CH4 produced by mangrove sediments, we identified the origin of the organic matter that supports GHG emissions. In most of the mangrove stands, GHG emissions were supported by organic matter from mixed sources while only in one mangrove stand the GHG emissions were supported by organic matter derived from mangrove tissues. Moreover, the organic matter derived from mangrove tissues reduced CO2 fluxes and enhanced CH4 production, pointing out the importance of the origin of the organic matter in GHG emissions. Methane was the main source of CO2-equivalents, despite the comparatively low emission rates, in most of the sampled mangroves, and therefore deserves careful monitoring in this region. Despite the mean net emission of CO2 and CH4 by Red Sea mangroves reported here, these forests become net organic carbon sinks when taking into account the existing carbon burial rates for the Red Sea mangroves. By further resolving GHG fluxes in arid mangroves, we will better ascertain the role of these forests in global carbon budgets.