Dynamics of methane emissions from northwestern Gulf of Mexico subtropical seagrass meadows

While seagrass meadows are perceived to be pertinent blue carbon reservoirs, they also potentially release methane (CH₄) into the atmosphere. Seasonal and diurnal variations in CH₄ emissions from a subtropical hypersaline lagoon dominated by Halodule wrightii in southern Texas, USA, on the northwest coast of the Gulf of Mexico were investigated. Dissolved CH₄ concentrations decreased in the daytime and increased overnight during the diel observation period, which could be explained by photosynthesis and respiration of seagrasses. Photosynthetic oxygen was found to significantly reduce CH₄ emissions from seagrass sediment. Diffusive transport contributed slightly to the release of CH₄ from the sediment to the water column, while plant mediation might be the primary mechanism. The diffusive CH₄ flux at the sea-air interface was 12.3–816.2 µmol/m² d, over the range of the sea-air fluxes previously reported from other seagrass meadows. This was related to relatively higher dissolved CH₄ concentrations (11.6–258.2 nmol/L) in a mostly closed lagoon with restricted water exchange. This study emphasizes seagrass meadows in the subtropical hypersaline lagoon as a source of atmospheric CH₄, providing insights into the interactions between seagrass ecosystems and methane dynamics, with potential implications for seagrass meadow management and conservation efforts.