Seagrass meadows are effective carbon sinks recognized for their potential role in climate change mitigation (Fourqurean et al., 2012;Lovelock & Duarte, 2019;Mcleod et al., 2011). Seagrass meadows sequester carbon dioxide (CO 2 ) through photosynthesis (Van Dam et al., 2021) and trap allochthonous particles within their canopy (Gacia et al., 2002). Part of this carbon is then stored as biomass and as organic carbon in sediments for centuries and even millennia (Serrano et al., 2021(Serrano et al., , 2016. Seagrass meadows account for 10%-18% of the total carbon burial (27 44 Tg C yr −1 ) in the ocean, even though they cover only 0.1% of the global ocean area (Kennedy et al., 2010). In addition, about 5% of the particulate organic carbon and dissolved organic carbon produced within seagrass habitats is exported beyond the meadows and stored in the deep ocean (Duarte & Krause-Jensen, 2017). Seagrass meadows are considered an important blue carbon ecosystem that should be protected and restored to mitigate anthropogenic CO 2 emissions.

Seagrass meadows are effective carbon sinks due to their high primary production and sequestration in sediments. However, methane (CH4) fluxes can partially counteract their carbon sink capacity. Here, we measured diffusive sediment-water and air-sea CO2 and CH4 fluxes in a coastal embayment dominated by Posidonia oceanica in the Mediterranean Sea. High resolution timeseries observations revealed large spatial and temporal variability in CH4 concentrations (2 to 36 nM). Higher emissions were observed in an area with dense seagrass meadows. A 6 − 40% decrease of CH4 concentration in the surface water around noon indicates that photosynthesis likely limits CH4 fluxes. Sediments were the major CH4 source as implied from radon (a natural porewater tracer) observations and evidence for methanogenesis in deeper sediments. CH4 sediment-water fluxes (0.1 ± 0.1 − 0.4 ± 0.1 µmol m-2 d-1) were higher than average water-air CH4 emissions (0.12 ± 0.10 µmol m-2 d-1), suggesting that dilution and CH4 oxidation in the water column could reduce net CH4 fluxes into the atmosphere. Overall, relatively low air-sea CH4 fluxes at this likely represent net emissions from subtidal seagrass habitats sites, which are not influenced by nearby allochthonous CH4 sources. The local CH4 emissions in P. oceanica offset less than 1% of the carbon burial in sediments (142 ± 69 g CO2eq m-2 yr-1). Combining our results with earlier observations in other seagrass meadows worldwide reveals that global CH4 emissions within seagrass meadows only offset a small fraction (<2%) of carbon sequestration in sediments.