Biosphere-Atmosphere coupling – a tropical mangrove system perspective

Biological 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. The Indian Sundarban mangrove forest (210 32/ and 220 40/ N; 880 05/ and 890 E) at the land-ocean boundary of the Gangetic delta and the Bay of Bengal comprises 9,630 km2 , out of which 4,264 km2 of intertidal area is subdivided as forest sub-ecosystem and 1,781 km2 of water area as aquatic sub-ecosystem. It contributes about 2.84 % of the global mangrove area (15x104 km2) and is a net sink for CO2. The net biosphere-atmosphere exchange of CO2 was found to be 3.05 Tg C a-1 and the surface waters exported 6.03 x 106 kg C, out of which 3.57 x 106 kg C was pumped out by the biological activity in the water annually. 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 and genetic DNA base heterogeneity. Mangrove adjusted the limited supply of nitrogen in the sediment through the stomatal uptake of atmospheric NOx and NH3. Carbon sequestration rate showing an increase with density varied between 0.088 and 0.171 µg C kg-1 AGB s-1, and Avicennia marina showed the maximum value and Bruguiera gymnorrhiza, the minimum. The changes in FTIR bands at 4000–2500 cm-1 and 1700–800 cm-1 were correlated to the variations in cellulose in mangrove woods and lignin to cellulose ratio ranged between 0.21 and 1.75. Thermal analyses of mangrove wood suggested that the fuel value index (985–3922) exhibited an increase with the decrease in maximum decomposition temperature and density. The mean annual incoming short wave radiation (435 ±32.8 Wm2 ) was partitioned into 29% sensible heat, 35% latent heat, 4% ground heat, 7% physical storage energy and 10% photosynthetic storage energy. The mean budget closing energy flux (68.96 ±24.6 Wm2 ) or, budget error was 15.8% of incoming short wave radiation. The extent of warming effect by CH4 and sensible heat flux was predominant over the resultant cooling effect due to the processes such as photosynthesis, evapotranspiration and albedo. 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. CO2 induced greater litter production could make sediment more anoxic leading to the occurrence of more non-sea sulphate aerosol and higher albedo. 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.