High rates of nitrogen fixation during an in-situ phosphate release experiment in the Eastern Mediterranean Sea

[1] Nitrogen fixation and d15N 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 L1d1 were measured at control stations in the absence of phosphate (<2 nmolL1), inferring the bioavailability of DOP to diazotrophs. In P-enriched waters, rates increased by 48 % to 197 nmol-N L1d1 five days after addition. d15N 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 no