Methane Emissions in Seagrass Meadows as a Small Offset to Carbon Sequestration

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. Seagrass meadows are hotspots for marine carbon storage in sediments. Part of the sediment carbon can be released as carbon dioxide and methane (CH 4 ). Methane has 45–96 times more powerful global warming effect than carbon dioxide. If seagrass meadows release CH 4 , the emissions counteract their climate mitigation potential. We measured greenhouse gas concentrations and fluxes in a seagrass‐dominated Mediterranean embayment. Low CH 4 coincided with oxygen produced from seagrass photosynthesis. Areas with dense seagrass meadows had lower CH 4 emissions. Overall, seagrass‐dominated coasts were a small source of CH 4 that offset only <2% of carbon buried in sediments on local and global scales. Hence, seagrass meadows remain an effective carbon sink. High‐resolution CH 4 observations reveal diel cycles linked to seagrass photosynthesis Dense seagrass meadow had lower sea‐air CH 4 flux than the area with patchy and dead seagrass CH 4 emissions were a small offset to seagrass C sequestration on local and global scales