Efficient oxidation attenuates porewater‐derived methane fluxes in mangrove waters

Mangroves store significant amounts of carbon in both sediment and water. Methane (CH 4 ) is often produced in anoxic, organic‐rich sediments during carbon degradation and released to overlying waters via porewater exchange. Yet, a portion of CH 4 can be oxidized to CO 2 before emission. Here, we investigate whether CH 4 oxidation impacts its emissions using high‐temporal resolution CH 4 concentration and stable isotope (δ 13 C‐CH 4 ) observations collected over 14 tidal cycles in 2 Brazilian mangrove creeks with no river inputs. We found higher CH 4 concentrations (~ 150 nM) more depleted in 13 C (−75‰) during low tide than high tide at both creeks. Similar δ 13 C‐CH 4 values between low tide surface waters and porewaters further suggest tidally driven porewater exchange as the main source of CH 4 . More 13 C‐enriched CH 4 in surface waters and surface sediments than deep sediments indicate partial CH 4 oxidation prior to exchange with the atmosphere. A stable isotope mass balance revealed that 17–58% of CH 4 was oxidized at rates of 3–25 μ mol m −2 d −1 in the water column of tidal creeks. A larger portion of deep porewater CH 4 (45–61%) was oxidized in sediments prior to porewater exchange with surface creek waters. The two mangrove creeks had average water–air CH 4 fluxes of 51–109 μ mol m −2 d −1 over spring‐neap tidal cycles. These aquatic CH 4 emissions offset only < 3% of the mangroves' soil carbon sequestration. Overall, CH 4 oxidation in both surface water and sediment attenuated CH 4 emissions to the atmosphere.