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.

Seagrasses and mangroves are crucial sources of atmospheric methane (CH ₄ ) from coastal areas. To study the dynamics of CH ₄ cycling at subtropical seagrass and mangrove, we studied diurnal CH ₄ emissions at the sea-air and sediment-water interfaces and related environmental parameters in August 2019 at lagoonal estuaries of southern Texas, USA, northwest coast of the Gulf of Mexico. Although seagrass meadows and mangroves locate at closely connected subtropical estuaries, they displayed distinct mechanisms in CH ₄ cycling. Dissolved CH ₄ concentration at the seagrass meadow decreased in the daytime and increased overnight, expressing a tight relationship with photosynthesis and respiration of seagrass. Plant mediation of seagrass played a crucial role in CH ₄ production, oxidation, and transport from sediment to water column. In comparison, the diel variation of dissolved CH ₄ concentration at the mangrove creek was controlled by tidal progression. The maximum CH ₄ level occurred during ebb due to the export of CH ₄ from inside the mangrove to the outside bay. Tidal pumping and tidal inundation were essential conduits for dissolved CH ₄ exchange between water and porewater. In both areas, sea-air CH ₄ fluxes were significantly affected by wind speeds, which hid related diurnal variations caused by physiological or tidal cycles. Our study also revealed a more significant contribution from seagrass to the local CH ₄ budget than from mangroves, indicating CH ₄ released from subtropical seagrass needs further investigation.