Methane Emissions in Seagrass Meadows as a Small Offset to Carbon Sequestration
Yvonne Y. Y. Yau, Gloria Reithmaier, Claudia Majtényi‐Hill, Óscar Serrano, Nerea Piñeiro‐Juncal, Martin Dahl, Miguel Á. Mateo, Stefano Bonaglia, Isaac R. Santos
Abstract
Abstract Seagrass meadows are effective carbon sinks due to high primary production and sequestration in sediments. However, methane (CH 4 ) emissions can partially counteract their carbon sink capacity. Here, we measured diffusive sediment‐water and sea‐air CO 2 and CH 4 fluxes in a coastal embayment dominated by Posidonia oceanica in the Mediterranean Sea. High‐resolution timeseries observations revealed large spatial and temporal variability in CH 4 concentrations (2–36 nM). Lower sea‐air CH 4 emissions were observed in an area with dense seagrass meadows compared to patchy seagrass. A 6%−40% decrease of CH 4 concentration in the surface water around noon indicates that photosynthesis likely limits CH 4 fluxes. Sediments were the major CH 4 source as implied from radon (a natural porewater tracer) observations and evidence for methanogenesis in deeper sediments. CH 4 sediment‐water fluxes (0.1 ± 0.1–0.4 ± 0.1 μmol m −2 d −1 ) were higher than average sea‐air CH 4 emissions (0.12 ± 0.10 μmol m −2 d −1 ), suggesting that dilution and CH 4 oxidation in the water column could reduce net CH 4 fluxes into the atmosphere. Overall, relatively low sea‐air CH 4 fluxes likely represent the net emissions from subtidal seagrass habitat not influenced by allochthonous CH 4 sources. The local CH 4 emissions in P. oceanica can offset less than 1% of the carbon burial in sediments (142 ± 69 g CO 2eq m −2 yr −1 ). Combining our results with earlier observations in other seagrass meadows worldwide reveals that global CH 4 emissions only offset a small fraction (<2%) of carbon sequestration in sediments from seagrass meadows.