Is dinitrogen fixation significant in the Levantine Basin, East Mediterranean Sea?

We report N₂ fixation rates measured from two stations monitored monthly off the Mediterranean coast of Israel during 2006 and 2007, and along a transect from Israel to Crete in September 2008. Analyses of time-series data revealed expression of nifH genes from diazotrophs in nifH clusters I and II, including cyanobacterial bloom-formers Trichodesmium and diatom-Richelia intracellularis associations. However, nifH gene abundance and rates of N₂ fixation were very low in all size fractions measured (>0.7μm). Volumetric ¹⁵N uptake ranged from below detection (~36% of >300 samples) to a high of 0.3nmolNl^-1d^-1 and did not vary distinctly with depth or season. Areal N₂ fixation averaged ~1 to 4μmolNm^-2d^-1 and contributed only ~1% and 2% of new production and ~0.25% and 0.5% of primary production for the mixed (winter) and stratified (spring-fall) periods respectively. N₂ fixation rates along the 2008 east-west transect were also extremely low (0-0.04nmolNl^-1d^-1, integrated average 2.6μmolNm^-2d^-1) with 37% of samples below detection and no discernable difference between stations. We demonstrate that diazotrophy and N₂ fixation contribute only a minor amount of new N to the P impoverished eastern Mediterranean Sea.