Nitrogen Uptake Rates during Spring in the NE Arabian Sea

We present new data on N uptake rates and f-ratios in the north-eastern (NE) Arabian Sea, where significant amounts of Trichodesmium were present in spring, 2006. The measured total nitrogen uptake rates ranged from 0.34 to 1.58 mmol N m−2d−1. N2 fixation associated with Trichodesmium varied from 0.002 to 0.54 mmol N m−2d−1 estimated from the abundance of Trichodesmium and specific N2 fixation rates of 1.5 pmol N trichome−1h−1. Inclusion of N2 fixation rates significantly changes f-ratios particularly in the coastal stations. Nitrogen isotopic data of surface suspended particles suggest that recently fixed nitrogen contributes as high as ~79% of the nitrogen in surface suspended particles. In addition, water column gained ~30 mmol N m−2 in the form of nitrate, likely due to nitrification of ammonium released by Trichodesmium. For better estimations, direct measurement of N2 fixation is recommended