Naveen Gandhi

First direct measurements of N2 fixation during a Trichodesmium bloom in the eastern Arabian Sea

We report the first direct estimates of N 2 fixation rates measured during the spring, 2009 using the 15N 2 gas tracer technique in the eastern Arabian Sea, which is well known for significant loss of nitrogen due to intense denitrification. Carbon uptake rates are also concurrently estimated using the 13C tracer technique. The N 2 fixation rates vary from 0.1 to 34 mmol N m -2d -1 after correcting for the isotopic under-equilibrium with dissolved air in the samples. These higher N 2 fixation rates are consistent with higher chlorophyll a and low I´ 15N of natural particulate organic nitrogen. Our estimates of N 2 fixation is a useful step toward reducing the uncertainty in the nitrogen budget.

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.

Primary and new production in the thermocline ridge region of the southern Indian Ocean during the summer monsoon

New and primary production, measured using 15N and 13C tracers, integrated over the euphotic zone (mostly upper 80-100 m) varied from 7.5 to 14.1 mmol N m-2d-1(average 10.8 mmol N m-2d-1) and 2.7 to 17.7 mmol C m-2d-1 (average 11.3 mmol C m-2d-1), respectively, in the tropical India Ocean (TIO). Shoaling of thermocline in the thermocline ridge located in the tropical Indian Ocean (TR) influences new and total productivity by supplying nutrients to the upper layers. At the same time, N-limited conditions prevail to the north of the TR region reducing biological productivity. While higher surface light intensity reduces the surface productivity, it enhances the overall depth-integrated productivity. Productivity at different locations within the region is primarily controlled by the supply of N-nutrients from below. In contrast, at a given location with the available nutrient pool, productivity at different depths is determined by light availability. The region has potential for moderate export production, as do the other parts of the Indian Ocean; the maximum possible f-ratio is found to be 0.56, suggesting that it is capable of exporting a significant part of the total production to the deep, under favorable conditions (i.e., availability of N-nutrients). A comparison with earlier data suggests no significant temporal trend in the productivity here over the past three decades.