Vertical distribution of nitrogen sources for new and regenerated production in the western Bay of Bengal

Abstract

The study investigated the new and regenerated production using 15N tracers in the western Bay of Bengal, with observations were made in a vertical water column and sunlight gradients under stratified oligotrophic conditions, which are the major changes that distinguish the hydrobiological parameters associated with the deep chlorophyll maximum (DCM). In this study, new and regenerated production were measured in at different zones such as the surface, the DCM region and the euphotic depth (below 1% of light intensity) during the high stratification period. The results showed that new and regenerated productivity at the surface was low, while higher light intensity decreased productivity at the surface and productivity in the DCM layer increased significantly. Productivity at different location within regions is primarily controlled by the supply of nitrogen nutrients from deep within the region. In contrast, productivity at a given location with the available nutrient pool, at different depths is highly influenced by light availability. The study region exhibited potential for moderate export production during spring inter-monsoon, as is the case in other parts of the Indian Ocean. The f-ratio was estimated to be 0.2, which is a very low value in deep water and plays an important role in the Earth’s carbon cycle, as carbon transported to the deep ocean is absorbed from the atmosphere, and the ocean thus acts as a carbon reservoir. The results also found that the availability of nitrogen nutrients is capable of exporting a significant part of the total production to the deep ocean when conditions are favorable. Our results suggest that the Bay of Bengal may be equally important in its efficiency to export productivity to the deep sea and the DCM layer. Statistical analysis revealed a positive correlation between nitrogen productivity and chlorophyll-a, dissolved inorganic nitrogen and total nitrogen: phosphorous ratio.