Prior to the JGOFS, the conventional understanding about the Arabian Sea productivity, largely based on the International Indian Ocean Expedition (IIOE, 1960-65) and Indian Ocean Experiment (INDEX, 1979), was that upwelling occurring along the continental margins during summer (southwest) monsoon (June-September) leads to high production off Somalia, Arabia and to a lesser extent the southern parts of the west coast of India. It was thought that during the rest of the year Arabian Sea remains oligotrophic. The JGOFS - India field programme brought new insights regarding the biological productivity of the Arabian Sea over different seasons (winter, summer and inter-monsoon) and for the first time documented how tightly the biological processes are coupled to the physical forcings and associated chemical changes. High biological productivity (blooms) occurs in the northern Arabian Sea during winter (northeast) monsoon (November-February) due to cooling of surface waters. The process that drives this winter productivity is the physical forcing. Winter cooling and convection is a common phenomenon in the higher latitudes, but is unique to the Arabian Sea situated in tropics. In the tropical oceans cooling during winter is not usually sufficient to initiate convection. But in the Arabian Sea winter convection is possible only because of its high ambient salinity (about 36.0 PSU).

During winter the evaporative cooling and convection also lead to the formation of Arabian sea high-salinity water mass in the upper 150m which after formation spreads southwards along 24 sigma-t isopycnic surface.

The biomass of the mesozooplankton in the mixed layer of the Arabian Sea appears to be invariant over the year despite varying primary productivity regimes. During lean phytoplankton production periods, this would be facilitated through the microbial loop. The vast DOC pool that builds up as winter and summer blooms become senescent, leads to increased bacterial production. In the post-bloom scenario, the bacteria are consumed by heterotrophic flagellates and microzooplankton which in turn support the mesozooplankton. The mesozooplankton in the mixed layer of the Arabian sea is dominated by small filter - feeding herbivores. Adaptation to feed on smaller organisms other than phytoplankton by them seems to happen in all seasons and may be an evolutionary characteristic of all tropical basins.

The classical picture of high productivity in and low productivity outside the upwelling areas during summer is not to be necessarily true as the measurements showed that the central Arabian Sea remains productive during summer. This again is forced by a physical mechanism through lateral advection of upwelled waters and upward Ekman pumping in the open ocean (Prasanna Kumar et al, in press). With the onset of the southwesterly winds along the Somalia coast during late May/early June, upwelling starts off Somalia and propagates northward with time. By July upwelling is fully established along the coasts of Somalia as well as Arabia and enhances the nutrient levels in the coastal waters. But the swift Somali current, unlike the more sluggish currents off Arabia, reduces the residence time of the upwelled water and primary productivity in this region remains far below potential. The cold (17-22^oC), nutrient rich (nitrate 5 to 20 m M) upwelled waters from Somalia steadily move away from the coast under the influence of the prevailing easterly zonal currents till August. This fertilises the southern part of the central Arabian Sea . In the north, the high productivity results from a combination of advection of nutrient rich upwelled waters from the Arabian coast as well as the upward Ekman pumping in the open ocean under the influence of large-scale cyclonic (positive) wind stress curl.

Phytoplankton of the Arabian sea ( > 5 µM) is dominated by diatoms in all seasons. Cocolithopores and foraminifera are common at times. Bacteria and microzooplankton proliferate in seasons (inter-monsoons) when primary productivity is low.

Optical measurements revealed that the peak of photosynthetically available radiation in the eastern and central Arabian sea were in the range of 365-435 W/m² while the aerosol optical depth ranges were from 0.07-0.19. These measurements have great relevance to validation of algorithms for remote sensing through ocean colour.

Results indicate that the Arabian Sea acts as a perennial source of atmospheric carbondioxide at an annual flux of 45 TGCy^-1 from the study region. The fluxes are seasonally variable with higher values generally occuring during south-west monsoon due to intense winds. Inorganic carbon dioxide changes are found to depend on dissolved organic carbon and its oxidation. The particulate organic carbon is shown to occur in the significant quantities, in the form of Transparent Exopolymer Particles, even in denitrifying and sub-oxic layers in the Arabian Sea, thus supplying organic carbon to bacterial respiration.

Extensive measurements on nitrous oxide and methane revealed that the Arabian Sea emits 0.56 - 0.76 Tg N₂O y^-1 and 0.03 – 0.05 Tg CH₄ y^-1. Large seasonal variability in the abundance of these gases is also found. The role of atmospheric nitrate deposition was also studied for the first time and found to be 20 m mol m^-2d^-1. Further, ²³⁴Th - ²³⁸U disequilibria in water column led to the estimation of ²³⁴Th export fluxes of 1900-2350 dpm m^-2 d^-1, with higher flux occuring at 21^oN and 64^oE. Paleooceanographic studies revealed that CaCO₃ levels ranged between ~10 and 80% in sediment cores collected from continental margins of India that have accumulation rates of 0.35 – 0.40 mm y^-1.

Th234-U238 radioactive disequilibrium in the upper ~100m water column was used in conjunction with Th-234/C ratio in particulate matter to derive carbon export fluxes.The results yield very large export fluxes of carbon, raising concerns about the source of carbon in the water column and the underlying assumptions of the approach.

Export fluxes of organic carbon measured by sediment traps were not consistent with primary production measured at the same location because of the "swimmer problem". C.org/²³⁴Th ratios are also affected by swimmers. During February 1997 reliable separation of swimmers from fluxes could be done as most of the particulate matter fluxes were in the form of salp fecal pellets and a C.org/²³⁴Th ratio of 0.12 mg/dpm was obtained. This ratio was used to recalculate export fluxes of carbon. Annual export of carbon out of the euphotic zone is about 78 g C m^-2 y^-1 which is about 30 per cent of the annual primary productivity. A major portion of the exported carbon is remineralized within the oxygen minimum zone and only 2.5 g C m^-2 y^-1 reaches a depth of 1000 m.

The burial fluxes of organic carbon in the eastern margin sediments (water depth <600m) range between 13-95g/m2/y with a mean of ~37g/m2/y. This on an average corresponds to about a fifth of primary productivity.