Efficient oxidation attenuates porewater‐derived methane fluxes in mangrove waters

AbstractMangroves 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 (δ13C‐CH4) observations collected over 14 tidal cycles in 2 Brazilian mangrove creeks with no river inputs. We found higher CH4 concentrations (~ 150 nM) more depleted in 13C (−75‰) during low tide than high tide at both creeks. Similar δ13C‐CH4 values between low tide surface waters and porewaters further suggest tidally driven porewater exchange as the main source of CH4. More 13C‐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 μmol 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 μmol 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.