Nitrogen fixation and δ15N were measured within a warm-core eddy during an in-situ phosphate experiment (CYCLOPS) in the Eastern Mediterranean, with experimental procedures performed on unconcentrated, bulk water. Mean rates of 129 nmol-N L-1d-1 were measured at control stations in the absence of phosphate (<2 nmolL-1), inferring the bioavailability of DOP to diazotrophs. In P-enriched waters, rates increased by 48% to 197 nmol-N L-1d-1 five days after addition. δ15N of particulate material was homogenous throughout the upper mixed layer, but changed with time at both amended and control stations from +3.8? on day 1, to -0.6? on day 4--5 before returning to +3.1? after 9 days. This trend matched the observed response in other components of the biota and biogeochemistry in the P-enriched patch. In-vitro addition experiments indicated that diazotrophy was not limited by Fe availability.