Efficient oxidation attenuates porewater‐derived methane fluxes in mangrove waters
Yvonne Y. Y. Yau, Alex Cabral, Gloria Reithmaier, Luiz C. Cotovicz, João Barreira, Gwénaël Abril, Cédric Morana, Alberto Borges, Wilson Machado, José Marcus Godoy, Stefano Bonaglia, Isaac R. Santos
Abstract
Abstract 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.