Mechanisms of methane released from the subtropical seagrass and mangrove at southern estuaries of Texas, USA

Abstract:

Methane (CH₄) is a greenhouse gas with an atmospheric warming potential 84-86 times over CO₂ through 20 years (IPCC, 2004). Seagrass and mangrove are crucial sources of atmospheric CH₄ from coastal areas. To evaluate CH₄ contribution from subtropical seagrass and mangrove to the atmosphere, we studied the variation of CH₄ fluxes at the sea-air and sediment-water interfaces from spring to summer in 2019 at lagoonal estuaries of southern Texas, USA. Results showed that sea-air CH₄ fluxes from seagrass meadow were higher than from mangrove in spring and summer, indicating the contribution of seagrass was larger than mangrove to local atmospheric CH₄. Both in seagrass and mangrove areas, sea-air CH₄ fluxes were more massive in summer than in spring. Porewater CH₄ concentration in sediment cores was less in summer than in spring, suggesting the transport of CH₄ to water and atmosphere. However, diurnal observations showed mechanisms driving the emission of CH₄ from sediment are different in seagrass and mangrove water areas.

Diurnal variation of dissolved CH₄ concentration at a seagrass site manifested its tight relationship with photosynthetic and respiration processes of seagrass. Dissolved CH₄ concentration decreased at an average rate of 2.9 nmol/L·hr in the daytime (14 hrs) and increased at 5.1 nmol/L·hr at night (10 hrs). About 10 nmol/L dissolved CH₄ was left in daily CH₄ cycling in seagrass. It not only can explain the decrease of CH₄ concentration in sediment from spring to summer, but further emphasizes the role of seagrass in CH₄ emission from sediment since the sediment-water CH₄ flux was minor. Unlike in seagrass area, the diel variation of dissolved CH₄ concentration at a mangrove site coincided with the tidal process, with the highest level occurred in ebb. Dissolved CH₄ concentration increased at 4.1 nmol/L·hr during ebb, indicating the tidal pumping impact on transport CH₄ from sediment to water, since there was no other CH₄ input.

Although the seagrass meadow and mangrove locate at closely connected subtropical estuaries, they displayed various mechanisms in CH₄ emission. Our study revealed a more significant contribution from seagrass to the local CH₄ budget than from mangrove, indicating CH₄ released from subtropical seagrass need more concern.