Low Dinitrogen Fixation Rates in the Bay of Bengal during Summer Monsoon

Nitrogen is a staple element for every living organism in addition to carbon, since all the major cellular components (e.g., DNA and RNA), proteins, and energy carrier molecules (e.g., ATP) are stemmed from these elements. Biological dinitrogen (N₂) fixation exerts an important control on oceanic primary production by providing bioavailable form of nitrogen (such as NH₄⁺) to photosynthetic microorganisms. We hypothesized that the oligotrophic nature of the Bay of Bengal might create a suitable niche for N₂ fixing microorganisms.

In the Bay of Bengal, fresh water influx driven stratification prevent the vertical influx of nutrients to the sunlit layers. Most of the riverine nutrients are used within estuarine and coastal regions, and thus these have negligible contribution on open ocean biological productivity. Atmospheric deposition contribution to the nutrients supply is equally low (< 3%) in the Bay. Thus, the recently observed high new production rates in the Bay of Bengal suggests the higher probability of N₂ fixation in this basin than the Arabian Sea. In addition, nitrogen isotopic composition of sedimentary organic matter (low δ¹⁵N values) in the Bay of Bengal can also be alluded to the presence of diazotrophy in the Bay. Hence, we further strengthened our hypothesis that N₂ fixers play a crucial role for the primary production in the Bay.

We commenced the first N₂ fixation study in the sunlit layer of the Bay of Bengal using ¹⁵N₂ gas tracer incubation experiments on a cruise expedition during summer monsoon 2018. N₂ fixation rates varied from 4 to 124 μmol N m^-2 d^-1 – these rates were very low compared to that observed in the Bay’s western counterpart in the Indian Ocean, i.e., the Arabian Sea. The contribution of N₂ fixation to primary production was small (< 1%). Noteworthily, the upper bound of observed N₂ fixation rates in our study was still higher than that measured in other oceanic regimes such as Eastern Tropical South Pacific, Tropical Northwest Atlantic, and Equatorial and Southern Indian Ocean. Strong monsoonal winds, turbidity due to copious riverine discharge and cloud cover over the Bay of Bengal might have inhibited N₂ fixation. Therefore, a more detailed study covering all the seasons is needed to understand the role of N₂ fixation rates on primary productivity in the Bay of Bengal.