Nitrogen fixation from bioassay experiments by stable-isotope mass spectrometry on UKSOLAS cruise D325.

A series of bioassay type experiments were determined to investigate the impact of iron, zinc, cobalt, copper and nitrogen on rates of Nitrogen fixation. Experiments were performed from near surface waters using 15N methodology at 6 different oceanographic stations in the vicinity of the Cape Verde Islands (north-east Atlantic) between 13 November and 18 December 2007 onboard RRS Discovery. Stations were selected from remotely sensed information on surface chlorophyll concentrations to give a gradient of conditions between 0.06 and 0.38 µg L^-1. The experiment was largely inconclusive, though at stations A there was evidence of limitation by Zn, Co and Cu. Interestingly Cu seemed to play a limiting role at other stations whilst nitrogen additions largely decreased rates below natural levels.