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

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.

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