Biosphere-atmosphere coupling: A tropical mangrove system perspective

B regulation of climate depends on how the biosphere-atmosphere coupling is manifested or how the biosphere could provide an environment suitable for its own sustainability. Discovery of the oldest Gilboa fossil Eospermatopteris forest which was periodically affected by brutal episodes of sea-level rise indicates that little is known how changes in early terrestrial ecosystem influenced global processes. Mangroves evolved in the eastern Tethys Sea during the early Cretaceous followed by their westward dispersal via the Mediterranean route until about Miocene (18 Ma) and exhibited considerable speedy resilience to disturbance on a geological time frame. This coincides with the event of atmospheric CO2 fall from the Eocene level of 1400 ppmv to possibly as low as 200 ppmv during the Miocene. Now mangroves dominate the majority of the world’s tropical and subtropical coastline and are highly productive, fixing and storing considerable amount of carbon. Indian Sundarban mangrove forest at the land-ocean boundary of the Gangetic delta and the Bay of Bengal covers about 2.84 % of the global mangrove area (15x104 km2) and is a net sink for CO2. Model prediction showed enhancement of CO2 sequestration in response to the future atmospheric CO2 increase in spite of existing low nitrogen availability in the sediment. Mangrove adjusted the limited supply of nitrogen in the sediment through the stomatal uptake of atmospheric NOx, NH3, N2 fixation and sediment-water exchange of dissolved inorganic nitrogen. Non sea-sulphate aerosol sourced from anaerobic soil H2S efflux could counteract the extent of regional atmospheric warming effect by methane and sensible heat flux. The mangrove ecosystem is capable of resisting al least some of the anthropogenic perturbation and the crucial question is whether humanity’s actions can drive the system beyond any Gaia repair capability.