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

Nitrogen fixation and delta N-15 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-1 d(-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-1 d(-1) five days after addition. delta N-15 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 parts per thousand on day 4 5 before returning to + 3.1 parts per thousand 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.