Seagrass and macrophyte mediated CO2 and CH4 dynamics in shallow coastal waters.

Seagrass meadows are among the most important coastal/ marine ecosystems for long-term carbon storage and conditioning of coastal waters. A combined air-water flux of CO₂ and CH₄ from the seagrass meadows was studied for the first time from Asia’s largest brackish–water lagoon, Chilika, India. Ecosystem-based comparisons were carried out during two hydrologically different conditions of dry and wet seasons in the seagrass dominated southern sector (SS); macrophyte-dominated northern sector (NS); the largely un-vegetated central sector (CS) and the tidally active outer channel (OC) of the lagoon. The mean fluxes of CO₂ from SS, NS, CS and OC were 9.8, 146.6, 48.4 and 33.0mM m^-2d^-1, and that of CH₄ were 0.12, 0.11, 0.05 and 0.07mM m^-2d^-1, respectively. The net emissions (in terms of CO₂ equivalents), considering the global warming potential of CO₂ (GWP: 1) and CH₄ (GWP: 28) from seagrass meadows were over 14 times lower compared to the macrophyte-dominated sector of the lagoon. Contrasting emissivity characteristics of CO₂ and CH₄ were observed between macrophytes and seagrass, with the former being a persistent source of CO₂. It is inferred that although seagrass meadows act as a weak source of CH₄, they could be effective sinks of CO₂ if land-based pollution sources are minimized. [ABSTRACT FROM AUTHOR]