Spatial and temporal methane emissions from urbanized subtropical estuaries in the northwest Gulf of Mexico

Coastal regions located in dynamic environments that are disturbed by extensive human activities have been recognized as long-term sources of atmospheric methane (CH₄). This study investigated dissolved CH₄ concentrations, sea-air CH₄ fluxes, and environmental factors in three urbanization-influenced subtropical estuaries in southern Texas, northwest of the Gulf of Mexico, from May 2018 to May 2021 to explore the interactions between natural processes and anthropogenic activities. The results revealed that high spatial-temporal variations in CH₄ emissions occurred because of differences in the distributions of environmental features, hydrological conditions, and anthropogenic influences in each estuary. Elevated CH₄ concentrations in channels were caused by waste discharge from maritime vessels and mechanical damage to seagrass meadows and sediment by dredging or boat grounding. Leakage from coastal oil and gas platforms was another anthropogenic source of atmospheric CH₄. Although sea-air CH₄ fluxes were lowest in bay water, annual regional CH₄ emissions evaluated using geometric mean sea-air CH₄ fluxes were much higher than other environmental characteristics because of the larger areas of bay water, suggesting that the contribution of bay water to atmospheric CH₄ cannot be ignored. Channels, as the primary anthropogenically disturbed areas, have not been found to input a significant amount of CH₄ into the atmosphere; however, their high sea-air CH₄ fluxes indicated a high risk of elevated CH₄ emissions if the disturbance areas are not controlled. These findings emphasize the importance of monitoring anthropogenically related CH₄ emissions from water to the atmosphere in the petroleum industry and maritime-influenced estuaries.