First direct measurements of N 2 fixation during a Trichodesmium bloom in the eastern Arabian Sea

Abstract

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 δ 15N of natural particulate organic nitrogen. Our estimates of N 2 fixation is a useful step toward reducing the uncertainty in the nitrogen budget. Copyright 2011 by the American Geophysical Union.