A detailed study has been carried out on Variability in forms and fluxes of carbon dioxide from the Arabian Sea mainly under the Joint Global Ocean Flux Studies Project (JGOFS, a core project of International Geosphere Biosphere Programme). Nine cruises have been undertaken on boards ORV Sagar Kanya and FORV Sagar Sampada in different seasons in the Arabian Sea, Bay of Bengal and Andaman Sea.

In this project, main emphasis was on undersanding the carbon cycling in the Arabian Sea through physical, chemical, biological and geological considerations. The consistent trends between pH_T and AOU, and deficit nitrate suggest that the former is mostly controlled by bacterial respiration in suboxic layers. Total alkalinity (TA) showed clear seasonal variability largely controlled by circulation and productivity, especially at surface. Well defined minima occurred at ~200 m because of intense remineralisation of organic matter. Large seasonal and spatial variabilities in TCO₂ were driven by physical and biological pumps. Mixing processes appear to be dominant in regulating TCO₂ over biological ones. Winter convective mixing and upwelling increased surface TCO₂ to 2120 and 2150 µM, respectively in the upper 20 m. pCO₂ values in surface waters were higher than in atmosphere (355 µatm) during all seasons. It mostly centered around ~420 µatm during NE monsoon but ranged from 360 to 420 µatm during other periods. Higher concentrations during NE- and SW monsoon seasons seem to be driven by physical mixing while by bacterial activity during inter-monsoon. Regeneration of CO₂ from soft tissue and the prevalence of reducing conditions result in a build up of pCO₂ that lowers the pH_T in sub-surface waters of the Arabian Sea. In general, pCO₂ in intermediate waters was higher during NE monsoon compared to that in other seasons. The pCO₂ peak (1100 - 1200 µatm) generally occurred in denitrifying zone and did not coincide with AOU maximum (1000-1200m). Very high surface pCO₂ of 682 µatm was recorded off the SW coast of India due to wind driven upwelling. This region seems to act like a chimney to the atmospheric CO₂. Low pCO₂ (<345 µatm) in surface layers at some places along the west coast of India during SW monsoon was due to discharge of fresh water from land.

To address the question as to which carbon pool supports the subsurface bacterial respiration, dissolved organic carbon and transparent exopolymer particles (TEPs) have been studied including their influence on denitrification. These results indicate that TEP significantly supports the bacterial respiration in the northern Arabian Sea.

14 day-time series observations were made at 21° N, 64° E during NE monsoon to examine diurnal variability in carbon components in relation to biological and physical processes. These results indicated that carbon components are highly influenced by biological processes during this season and the effect is discernible even upto 500 m.

Carbon budgets in the study region of the Arabian Sea showed that 70-75% of the carbon is remineralized within the surface mixed layer. Most importantly influx of carbon dioxide from atmosphere is 5 times higher than outfluxes.

The effect of soil-water interaction on pH in Mandovi-Zuari estuaries, Goa, India and its influence on the fluxes of carbon dioxide at air-water interface was studied. This study shows that if riverine sediments are acidic in nature it results in a decrease in river water pH. This would ultimately effect the carbonate equilibrium in the waters. This increased sea-to-air fluxes of carbon dioxide which varied from -20 to 200 mM m^-2 d^-1, which are among the highest observed so far.

Sarma, V.V.S.S. 1998. Variability in forms and fluxes of carbon dioxide in the Arabian Sea. Ph.D Thesis, Goa University, Goa, India.