MEPS 239:241-250 (2002)  -  doi:10.3354/meps239241

Spatial and temporal distribution of Trichodesmium spp. in the stratified Gulf of Aqaba, Red Sea

A. F. Post1,*, Z. Dedej1, R. Gottlieb1, H. Li1, D. N. Thomas1,2,**, M. El-Absawi3, A. El-Naggar3, M. El-Gharabawi3, U. Sommer4

1Interuniversity Institute for Marine Science, PO Box 469, 88103 Eilat, Israel
2Centre for Marine Tropical Ecology‹ZMT, Fahrenheitstraße 6, 28359 Bremen, Germany
3National Institute for Oceanography and Fisheries, Kayet Bay, Alexandria, Egypt
4Institut für Meereskunde, Düsternbrooker Weg 20, 24105 Kiel, Germany
*E-mail: **Present address: School of Ocean Sciences, University of Wales Bangor, Menai Bridge LL59 5EY, Wales, UK

ABSTRACT: Phytoplankton (>100 µm) abundance was studied in the open waters of the Gulf of Aqaba during the summer stratification period of 1996. A succession took place among the major phytoplankton groups, with diatom numbers decreasing throughout the summer. The diazotrophic cyanobacteria Trichodesmium spp. became more prominent as the stratification period progressed; 5 Trichodesmium species were identified: T. thiebautii, T. erythraeum with tuft-shaped colonies and Trichodesmium sp. with puff-shaped colonies were common at ~102 colonies m-3 throughout the stratification period, whereas T. tenue and T. hildebrandtii were more rare. A bloom of T. thiebautii and T. erythraeum with >106 tuft colonies m-3 was observed in coastal waters of the Gulf during fall 1997. Tuft-shaped colonies were dominant near the surface, while puff-shaped colonies of Trichodesmium sp. were mainly found in the bottom half of the photic zone. These depth distributions were maintained for more than 2 mo, suggesting that the 2 colony types occupied distinct niches. Puff-shaped colonies were found to have higher chlorophyll a contents than tufts, but their photosynthetic activities were not significantly different. Fatty acid analysis of dominant plankton species yielded new trophic relationships for Trichodesmium spp. The Trichodesmium spp.-specific fatty acid C22:2 ω6 was found in Macrosetella gracilis (the sole copepod to graze on Trichodesmium spp.) and in chaetognaths, suggesting that these carnivorous zooplankton fed on M. gracilis. Furthermore, this fatty acid was observed in the filter-feeding Salpa maxima, which was abundantly present in the Gulf of Aqaba during June 1997.

KEY WORDS: Red Sea · Phytoplankton · Cyanobacteria · Trichodesmium · Nitrogen fixation

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