Source of methane released from the subtropical coastal area of southern Texas, USA: mangrove or seagrass?

Methane (CH₄) is a greenhouse gas with an atmospheric warming potential roughly 25 times that of carbon dioxide. Long-term observation data show that atmospheric CH₄ levels over the Gulf of Mexico off the coast of Corpus Christi, Texas, are always higher than global averages. To evaluate the flux and source of CH₄ released from Corpus Christi coastal area, diffusive CH₄ fluxes at the sea-air and sediment-water interfaces were analyzed at the local lagoonal estuaries (Corpus Christi Bay, Nueces Bay, Aransas Bay, and Upper Laguna Madre) from May 2018 to June 2019. Results showed that diffusive CH₄ released from the estuaries of Corpus Christi was much higher than the previous studies at the northern Gulf of Mexico, indicating the significant contribution of the coastal area to the local and global atmospheric CH₄ budget.

Moreover, higher diffusive CH₄ fluxes were found at seagrass area of Upper Laguna Madre instead of the saltmarsh/mangrove areas and the bays. Floating chamber experiments showed that more CH₄ was released from seagrass area than from mangrove area, although much higher diffusive CH₄ concentration was found in porewater of mangrove sediments than seagrass sediments. In-situ sediment incubation experiments further proved that sediment in shallow water was a significant source of local CH₄, and more CH₄ was released from seagrass sediment than mangrove during incubation. Our results suggested that seagrass probably played a crucial role in transporting CH₄ in sediment to the water body and atmosphere because of its tubing structure. Relative to seagrass, mangrove seemed to contribute more to CH₄ storage in sediment, rather than emitting CH₄. The contribution of seagrass to the increase of atmospheric CH₄ should be given more attention.