SEDIMENTARY METHANE EMISSION IN SUBTROPICAL ESTUARIES AT SOUTHERN TEXAS, USA

Methane (CH₄) is a greenhouse gas with an atmospheric warming potential 84-86 times over CO₂ through 20 years (IPCC, 2004). Coastal sediment is one of the crucial sources of atmospheric CH₄. To evaluate CH₄ contribution from sediment of subtropical estuaries to the atmosphere, we studied CH₄ concentration profiles in sediment porewater and CH₄ fluxes at the sediment-water interfaces in May and August 2019 at lagoonal estuaries of southern Texas, USA.

Results showed that CH₄ concentrations in sediment porewater were highest in mangrove sediment (1121.8~15418.7 nmol/L), then channel sediment (17.1~760.7 nmol/L) and least in seagrass sediment in May 2019 (11.3~259.8 nmol/L). However, porewater CH₄ concentrations decreased to similar levels from May to August (mangrove, channel, and seagrass: 20.1~85.9, 7.7~97.0, 30.6~56.2 nmol/L, respectively). Such variation corresponded to the increase of dissolved CH₄ concentration in the overlying water column from spring to summer. However, both calculations using the Fick’s First Law of diffusion and sediment core incubation experiments manifested CH₄ fluxes at the sediment-water interface were quite low, which indicated minor transport of diffusive CH₄ from sediment to water directly. Moreover, the increase of CH₄ concentration in overlying water during sediment incubation was higher in the seagrass site than in the mangrove site, indicating more CH₄ was released from seagrass sediment than from mangrove sediment. It implied the role of vegetation in transporting CH₄ from sediment to water column, e.g., tubing structure of seagrass. Another thing that we have noticed is that after incubation, porewater CH₄ concentrations in seagrass increased dramatically in comparison with those in channel and mangrove sediment. It further proved that seagrass vegetation could mediate sediment environments and explained the higher CH₄ concentration in nearby channel sediment.