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

Summary 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 (> 0.7 µm). Volumetric 15N uptake ranged from below detection (∼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 N2 fixation averaged ∼1 to 4 µmol N m−2 d−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 (0–0.04 nmol N l−1 d−1, integrated average 2.6 µmol N m−2 d−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.