Rates of nitrogen fixation in surface waters by stable-isotope mass spectrometry during the AMT programme cruise AMT15.

AMT15 departed Southampton UK on 17 September arriving Cape Town, South Africa on 29 October 2004. The main objective of the cruise was to determine the trophic state of the planktonic ecosystem with respect to large scale patterns in the ambient concentrations and supply to the surface waters of inorganic nutrients and of dissolved organic matter. Nitrogen fixation using 15N methodology was determined at all stations sampled though were generally very low in the northern hemisphere (<0.2 nmol L^-1d^-1) north of 21.4 N, though higher rates were determined associated with outer reaches of Mauritanian Upwelling and the equatorial upwelling between 21.2 N and 2.3 N (maximum at 18.8 nmol L^-1d^-1). South of this, between the equator and 17.6 S, rates ranged between 0.2 and 2.6 nmol L^-1d^-1.Between 21 S and 37 S unprecedented high rates of between 6.2 and 66.6 nmol L^-1d^-1 were determined, and whilst analytically these appear sound, they should be treated with caution.