Is dinitrogen fixation significant in the Levantine Basin, East Mediterranean Sea
2011
Yogev, Tali | Rahav, Eyal | Bar-Zeev, Edo | Man-Aharonovich, Dikla | Stambler, Noga | Kress, Nurit | Béjà, Oded | Mulholland, Margaret R. | Herut, Barak | Berman-Frank, Ilana
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.
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