Biological dinitrogen (N 2 ) fixation exerts an important control on oceanic primary production by providing bioavailable form of nitrogen (such as ammonium) to photosynthetic microorganisms. N 2 fixation is dominant in nutrient poor and warm surface waters. The Bay of Bengal is one such region where no measurements of phototrophic N 2 fixation rates exist. The surface water of the Bay of Bengal is generally nitrate-poor and warm due to prevailing stratification and thus, could favour N 2 fixation. We commenced the first N 2 fixation study in the photic zone of the Bay of Bengal using 15 N 2 gas tracer incubation experiment during summer monsoon 2018. We collected seawater samples from four depths (covering the mixed layer depth of up to 75 m) at eight stations. N 2 fixation rates varied from 4 to 75 μmol N m −2 d −1 . The contribution of N 2 fixation to primary production was negligible (<1%). However, the upper bound of observed N 2 fixation rates is higher than the rates measured in other oceanic regimes, such as the Eastern Tropical South Pacific, the Tropical Northwest Atlantic, and the Equatorial and Southern Indian Ocean.

Nitrogen (N) and phosphorus (P) determine the strength of the ocean’s biological carbon (C) pump, and variation in the N:P ratio is key to phytoplankton growth. A fixed C:N:P ratio (106:16:1) in organic matter and deep-water nutrients was observed by Alfred C. Redfield. However, recent studies have challenged the concept of the Redfield Ratio, and its veracity remains to be examined in oceanic basins like the Bay of Bengal. For this purpose, we sampled the water in the Bay of Bengal for C, N, and P content in the organic and inorganic pools from the surface to 2000 m. Overall, the C:N:P ratio deviated greatly from the Redfield Ratio. The C:N:P ratio in particulate organic matter varied from 232:25:1 in the top layer (surface to the depth of the chlorophyll maximum) to 966:72:1 in the deep water (300-2000 m). In dissolved organic matter, the ratio varied from 357:30:1 in the top layer to 245:66:1 in the deep water. The N:P ratio in nutrients varied from 3 in the top layer to 12 in the deep water. The nutrient-depleted top layer (average NO3- + NO2- ~ 0.7 µmol l-1) with a low N:P ratio coupled with reported low primary production rates in the Bay suggested that the production was N limited. Concurrent N2 fixation rates were not sufficient to alter the observed C:N:P ratio. Eddies showed a mixed effect on the C:N:P ratio. Our C:N:P ratios in particulate organic matter are comparable to other tropical basins and supports the nutrient supply hypothesis for low latitude ecosystems.

Dark carbon (C) fixation in the ocean twilight zone plays a crucial role toward C sink, but its potential has not been tested sufficiently in experiments. Here we analyzed dark C fixation in the twilight zone of the Arabian Sea along with the primary production measurements in the euphotic zone. The average dark C fixation rates in the suboxic oxygen minimum zone (OMZ) waters were higher than that in the hypoxic OMZ waters, which could be explained by the preferential existence of chemoautotrophic ammonium oxidizers and anammox bacteria owing to NO2− maxima in the suboxic OMZ waters. This study supports a previous hypothesis of significant contribution of dark C fixation to sinking C fluxes in the OMZ of the Arabian Sea. Extrapolation of the measured dark C fixation rates to the global ocean ranged up to 7.4 Pg C y−1; amounting to ∼15% of the global ocean primary production.

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