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A system in balance? – Implications of deep vertical mixing for the nitrogen budget in the northern Red Sea, including the Gulf of Aqaba (Eilat)
1ZMT Bremen, 28359 Bremen, Germany
2Department of Life Sciences, Bar Ilan University, Ramat Gan, Israel
3Department of Biology, Faculty of Science and Technology, Al-Quds University, Palestinian Authority
4National Institute of Oceanography and Fisheries, Cairo, Egypt
5Institute of Earth Sciences and Moshe Shilo Centre for Biogeochemistry, The Hebrew University of Jerusalem, Jerusalem, Israel
6Aquatic Ecology, University of Constance, 78457 Konstanz, Germany
*now at: GKSS Research Center, 21502 Geesthacht, Germany
Abstract. We investigated the implications of deep winter mixing for the nitrogen budget in two adjacent systems, the northern Red Sea proper, and the Gulf of Aqaba. Both are subtropical oligotrophic water bodies. The main difference is that in the gulf deep winter mixing takes place regularly, whereas the northern Red Sea proper is permanently stratified. In the Gulf of Aqaba, we observed significantly lower nitrate deficits, i.e. deviations from the Redfield ratio, than in the northern Red Sea proper. Assuming that other external inputs and losses in N or P are very similar in both systems, the higher nitrate deficit can be explained by either lower nitrogen fixation in the (stratified) northern Red Sea, which seems unlikely. An alternative explanation would be higher rates of benthic denitrification than in the gulf. By comparing the two systems we have indirect evidence that benthic denitrification was much lower in the Gulf of Aqaba due to higher oxygen concentrations. This we attributed to the occurrence of deep winter mixing, and as a consequence, the nitrate deficit was close to zero (i.e. N:P ratio close to "Redfield"). If both nitrogen fixation and benthic denitrification take place, as in the northern Red Sea proper, the result was a positive nitrate deficit (i.e. a deficit in nitrate) in the ambient water. The nitrate deficit in the northern Red Sea was observed in spite of high iron deposition from the surrounding desert. Our results strongly support the concept of nitrogen as the proximate, and phosphate as the ultimate limiting nutrient for primary production in the sea. This must not be neglected in efforts for protecting the adjacent reefs against eutrophication.
Discussion Paper (PDF, 414 KB) Interactive Discussion (Final Response, 4 Comments)
Citation: Häse, C., Al-Qutob, M., Dubinsky, Z., Ibrahim, E. A., Lazar, B., Stambler, N., and Tilzer, M. M.: A system in balance? – Implications of deep vertical mixing for the nitrogen budget in the northern Red Sea, including the Gulf of Aqaba (Eilat), Biogeosciences Discuss., 3, 383-408, 2006. Bibtex EndNote Reference Manager
