Methane oxidation in surface water reduces emissions to the atmosphere in mangroves

Mangroves store significant amounts of organic carbon in sediments. During carbon burial, methane (CH4) is produced in anoxic, organic-rich sediments and released to the surface waters via porewater exchange and ebullition. Yet, highly variable CH4 emissions have been reported in mangroves because of high uncertainty of methane production and oxidation rates. Combining the stable isotopic composition of methane (δ13C-CH4) in porewater and surface water can reveal the fraction of CH4 oxidized or emitted to the atmosphere. Here, we report high-temporal resolution CH4 concentrations from creek waters and porewater at two mangrove creeks in Brazil along with measurements of δ13C of CH4. Enriched δ13C-CH4 in top surface layer sediments indicates CH4 oxidation in the surface sediment before porewaters reach the water column. Tidal pumping in mangrove facilitates CH4 oxidation in the water column. Surface waters at low tide exhibited a lighter δ13C-CH4 than at high tide. A similar δ13C-CH4 signature between low tide surface water (-70 ± 2 ‰) and porewater (-74 ± 4 ‰) imply that sediment is the source of CH4. A stable isotope mass balance showed that 30 – 66% of was oxidized within water column, with the rate of 19-121 µmol m-2 d-1. Air-sea CH4 emissions were estimated at both mangroves (68 ± 65 in a pristine and 179 ± 205 µmol m-2 d-1 in an urbanized mangrove), on the same order of magnitude as the CH4 oxidation rate. Overall, our results suggested that CH4 oxidation in mangrove surface water and sediments partially reduce CH4 emissions to the atmosphere.