Al-Qutob (Kutub)^{1, 2}, Mutaz A., Clivia Häse^{3, 4}, Max M. Tilzer⁵, Noga Stambler², Zvy Dubinsky² and Boaz Lazar⁶

¹Department of Biology, Faculty of Science and Technology, Al-Quds University, West Bank, Tel: +972 51 794687, E-mail: qutob@planet.edu, ²Faculity of Life Sciences, Bar Ilan University, Ramat Gan, Israel, ³ZMT Bremen, D-28359 Bremen, Germany, ⁴DLR, German Aerospace Center, Oberpfaffenhofen, Germany, ⁵Aquatic Ecology, University of Constance, D–78457 Konstanz, Germany and ⁶Institute of Earth Sciences, Edmond Safra campus, The Hebrew University of Jerusalem, Jerusalem, Israel

Nutrient distribution and phytoplankton productivity in the oligotrophic Gulf of Aqaba, Red Sea

The observed depth-time distribution of ambient nutrient salt concentrations as well as nutrient stimulation bioassays have led to the conclusion that primary productivity of the sea is predominantly limited by nitrogen. Our field data show that, by contrast, observed ambient nitrogen to phosphorus ratios in the oligotrophic Gulf of Aqaba, northern Red Sea, are close to the Redfield Ratio, even at extremely low concentrations. This, as well as nutrient enrichment bioassays suggest that in the Gulf of Aqaba, nitrogen and phosphorus simultaneously control phytoplankton productivity. The growth response of algae was more rapid after the addition of nitrate as nitrogen source, than after the addition of ammonia. The fact that the nitrogen to phosphorus ratio in the Gulf of Aqaba is so close to the Redfield Ratio we attribute to a less pronounced denitrification compared to most other oligotrophic tropical and subtropical seas. Whereas nitrogen fixation is common during the stratification period in the Gulf of Aqaba, denitrification in open waters is supposed to play a minor role due to well oxygenised deep water layers as a consequence of deep winter mixing. This hypothesis was supported by a slightly but significant lower N:P ratio in the northern Red Sea than in the Gulf of Aqaba during spring 1999.