Impact of phosphate and iron fertilisation on rates of nitrogen fixation in surface waters by stable-isotope mass spectrometry during the FEEP programme cruise PO311.

Between the 24th April and 26th May 2004 a two-ship exercise using RRS Charles Darwin and RV Poseidon aimed to test the hypothesis that: The supply of, and the interaction between iron and phosphorous control biological activity and fluxes in the subtropical North Atlantic. An experimental area in international waters to the west of the Canary Islands was selected following an intense period of vertical and horizontal mapping by the Charles Darwin. Using SF6 as a tracer for amended waters, two separate experiments were performed: The first (5th - 15th May) involved the addition of 20 tonnes of anhydrous monosodium phosphate at 10 m depth over an area of approximately 25 km sq, centered at 27.8 N 23.3 W; The second experiment was conducted following a mid-cruise return by the Poseidon to Tenerife between 16th May and 22nd May, at 27.5 N 22.5 W. 5 tonnes of acidified iron sulphate were added over the first 12 hours, then following a brief recovery period, 20 tonnes of phosphate added over the top of the iron. Measurements of Nitrogen Fixation using 15N methodology were monitored prior to and after deployment (IN stations) of the fertilised patches relative to several (OUT) control stations. Following P addition nitrogen fixation rates increased progressively at IN stations relative to background, so that mean rates had increased by 3.3 times, 6 days after phosphate addition. At PRE and OUT stations, fixation rates were fairly uniform at all 3 depths sampled (0.30 +/- 0.12 nmol N/L/day, n=9). Following P and Fe additions rates of N fixation appeared unaffected 15 m and 40 m showing no significant difference to out stations at either depth. Relative to background rates of 0.3 +/- 0.1 nmol N/L/day at 25 m, N Fixation increased following Fe addition to 0.5 nmol N/L/day, and then after P addition rates increased to 3.3 times the background rate, reaching a maximum of 1.0 nmol N/L/day on the 20th May. By the 22th May rates at 25 m had fallen to 0.15 nmol N/L/day.