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Non-Redfieldian C:N:P ratio in the inorganic and organic pools of the Bay of Bengal during the summer monsoon

Deepika Sahoo*, Himanshu Saxena, Nidhi Tripathi, Mohammad Atif Khan, Abdur Rahman, Sanjeev Kumar, A. K. Sudheer, Arvind Singh*

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ABSTRACT: Nitrogen (N) and phosphorus (P) determine the strength of the ocean’s biological carbon (C) pump and variation in 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 surface to 2000 m. Overall, 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 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 nitrate + nitrite ~ 0.7 µmol l–1) with 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 significant to alter the observed C:N:P ratio. Eddies showed a mixed effect on C:N:P ratio. Our C:N:P ratios in particular organic matter are comparable to other tropical basins and supports the nutrient supply hypothesis for low latitude ecosystems.