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.
ASJC Scopus subject areas
- Ecology, Evolution, Behavior and Systematics