Efficient oxidation attenuates porewater-derived methane fluxes in mangrove waters

Mangroves store significant amounts of carbon in both sediment and water. Methane (CH4) is often produced in anoxic, organic-rich sediments during carbon degradation and released to overlying waters via porewater exchange. Yet, a portion of CH4 can be oxidized to CO2 before emission. Here, we investigate whether CH4 oxidation impacts its emissions using high-temporal resolution CH4 concentration and stable isotope (delta C-13-CH4) observations collected over 14 tidal cycles in 2 Brazilian mangrove creeks with no river inputs. We found higher CH4 concentrations (similar to 150 nM) more depleted in C-13 (-75 parts per thousand) during low tide than high tide at both creeks. Similar delta C-13-CH4 values between low tide surface waters and porewaters further suggest tidally driven porewater exchange as the main source of CH4. More C-13-enriched CH4 in surface waters and surface sediments than deep sediments indicate partial CH4 oxidation prior to exchange with the atmosphere. A stable isotope mass balance revealed that 17-58% of CH4 was oxidized at rates of 3-25 mu mol m(-2) d(-1) in the water column of tidal creeks. A larger portion of deep porewater CH4 (45-61%) was oxidized in sediments prior to porewater exchange with surface creek waters. The two mangrove creeks had average water-air CH4 fluxes of 51-109 mu mol m(-2) d(-1) over spring-neap tidal cycles. These aquatic CH4 emissions offset only < 3% of the mangroves' soil carbon sequestration. Overall, CH4 oxidation in both surface water and sediment attenuated CH4 emissions to the atmosphere.