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The Bay of Bengal (BoB) is known to experience low productivity but high sinking carbon fluxes to the bottom, and this paradox is attributed to mineral ballast of organic matter. We found for the first time that primary production in the BoB is higher, and it is supported by dissolved organic nitrogen (DON) and phosphorus (DOP) transported from the shelf regions through eddies. Both DON and DOP contribute up to 70–99% to the total dissolved nutrients in the waters above the thermocline. DON and DOP displayed positive relationship with primary production in the upper 25 m of water column, suggesting that organic nutrients significantly support primary producers. Primary production and export production in the BoB is comparable to that in the Arabian Sea, in contrast to the earlier belief that it has low production due to lack of inorganic nutrients caused by strong salinity stratification.

In order to understand the major sources of particulate organic carbon (POC) in the frontal zones and to examine their variability with space and time, a total of five temperature fronts of different ages was sampled in the northeastern Arabian Sea during winter. Compared to the nonfrontal regions, POC and chlorophyll‐a were higher within the coastal fronts, whereas chlorophyll‐a was lower within the open ocean front (T1). The variation of POC between coastal and open ocean fronts is attributed to the combined influence of variable vertical mixing, heterotrophic transformation and age of the front. Relatively depleted δ13CPOC and δ15NPN were observed within the fronts, suggesting that POC pool is contributed by in situ production supported by upwelling of nutrient‐rich water and zooplankton biomass. Elemental C:N ratios, POC:Chl‐a, δ13CPOC, and δ15NPN suggest that POC is mainly contributed from primary producers and heterotrophs in the study region. However, relative contributions from these two sources vary spatially from coastal to open ocean and with the age of the front. Stable Isotope Analysis in R (SIAR) model revealed that zooplankton biomass largely contributed to POC in the open ocean (60–80%) than phytoplankton (20–40%) and nearly equal contribution was observed in the coastal fronts (50–60% and 40–50%, respectively). This study, thus, demonstrates that dominant heterotrophy and autotrophy in the open ocean and coastal fronts and it is consistent with their biomasses. Predominant heterotrophy in the open ocean is attributed to deeper mixed layer resulting in upwelling of bacteria‐rich and phytoplankton‐poor water to surface leading to existence of microbial loop.