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

P>We report N-2 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 N-2 fixation were very low in all size fractions measured (> 0.7 mu m). Volumetric 15N uptake ranged from below detection (similar to 36% of > 300 samples) to a high of 0.3 nmol N l-1 d-1 and did not vary distinctly with depth or season. Areal N-2 fixation averaged similar to 1 to 4 mu mol N m-2 d-1 and contributed only similar to 1% and 2% of new production and similar to 0.25% and 0.5% of primary production for the mixed (winter) and stratified (spring-fall) periods respectively. N-2 fixation rates along the 2008 east-west transect were also extremely low (0-0.04 nmol N l-1 d-1, integrated average 2.6 mu mol N m-2 d-1) with 37% of samples below detection and no discernable difference between stations. We demonstrate that diazotrophy and N-2 fixation contribute only a minor amount of new N to the P impoverished eastern Mediterranean Sea.