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 (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.