Nitrogen uptake rates and new production in the Northern Indian Ocean

Abstract

Nitrogen-15 based new production measurements initiated during the last decade in the northern Indian Ocean are summarized. Two different biogeochemical provinces in eastern Arabian Sea have been recognized during the late winter monsoon: less productive southern (non-bloom) and more productive northern (Noctiluca scintillans bloom) regions. The southern sector is characterized by low column N-uptake and very low f-ratio (~5.5 mmol N m⁻²d⁻¹ and 0.4 respectively). The f ratio, although low, increased progressively towards north. This increase may be the effect of more intense winter cooling towards the north. The northern part is a highly productive zone, with very high N-uptake and significantly high f ratio (~19 mmol N m⁻²d⁻¹ and 0.8 respectively). New production during the pre-monsoon in the Bay of Bengal is higher than that in the post-monsoon. New production for the region during the pre-monsoon averages around 5 (±4) mmol N m⁻²d⁻¹, almost twice the average value observed during the post-monsoon (2.6 mmol N m⁻²d⁻¹). Average f ratio for the entire region increases to 0.70 (±0.1) during pre-monsoon from 0.5 during the post-monsoon.