139736eo.pdf (20MB) - Japan Oceanographic Data Center

This elevated productivity may result from thermal convection. During this season diatoms seem tooccur in increased numbers of cells. In summer, during the SW monsoon, higher chlorophyll valuesare measured south of 20°C. Dinoflagellates, which clearly are more abundant than diatomsthroughout the Red Sea, reach their climax in cell number during this season (HALIM, 1969).BENTHIC COMMUNITIESOur knowledge on the deep-sea faun2 and communities of the Red Sea is limited. The firstexpedition dredging in deep water was the Austrian “Pola” in 1897, and later some material wascollected by the “Mabahiss” in 1933. However, during these early years, abundance of organismswas not of interest. Further bottom samples in deep water have been taken by several expeditions, butno quantitative studies of epi- or endo-fauna have been conducted.CORAL REEF ECOSYSTEM IN THE RED SEAThe steep slopes of the Red Sea allow only relatively small fringing reefs along the coastline.However, in the central parts of the Red Sea (around Port Sudan) the reefs can extend some kilometersseawards, forming features very similar to barrier reefs.From the zoogeographical point of view, the Red Sea belongs to the Indo-Pacific region,although its unique hydrographic character means that some Red Sea species (fish and invertebrates)are endemic. Moreover, the fauna (particularly the hermatypic coral fauna) is more diverse than thefauna of the neighbouring Indian Ocean. Therefore some caution should be used when transferringresults of investigations on coral reefs from the Indo-Pacific to the Red Sea.Little is known about the reefs of the central parts of the Red Sea. Some investigations onfaunistic and geologic zonations have been done in the Gulf of Aqaba and the Farasan Archipelago(Sarsi Bebir) (e.g. CROSSLAND, 1907, 1939; FISHELSON, 1970, 1971; LOYA, 1972, 1976;LOYA and SLOBODKIN, 1971; MERGNER, 1967, 1971; SCHEER, 1967). The results of theseinvestigations demonstrate the diverse zonations in the coral reefs. Zonations are primarily dependenton the shape of the coastline, its exposure to wave action, on fluctuations of water level, and onregional geology and geologic history. In general, however, three typical ecological zones can bedistinguished: the reef flat, the reef slope, and the fore reef.As in other areas, corals in the Red Sea constitute the basic reef framework and substrate formany other organisms which attach to or penetrate the skeletal mass, (sponges, polychaetes,sipunculides, bivalves and gastropods). Corals also provide shelter for many fishes as well as variousinvertebrate species. Reef communities are resource-limited systems in which diversity is maintainedmainly by biotic specialization. Space is probably the major limiting factor in coral reef fishcommunities; many species have exploited biotic rather than physical habitats. Coral reef fishes exhibita particularly diverse range of feeding, some species consuming benthic algae growing and othersconsuming zooplankton or bottom fauna, including the corals themselves.Many reef building organisms harbor symbiotic algae which have long been thought to beboth an important carbon source and a catalyst for calcification of these organisms. Symbiotic algaehosts include corals, bivalves, ascidia and foraminifera. Corals are major consumers of primaryproduction on the coral reefs, and they (or their symbiotic algae) in turn provide a source of food formany other species on the reef. Organic material excreted by the corals provides nutrients forheterotrophic bacteria, which also constitute a prominent food source for other organisms.Algae provide food, substrate and shelter for numerous organisms and may be the main foodsource for various animals with economic importance, such as fish, turtles and, under certainconditions, dugongs. Although algae are not presently utilized by man in the region, some areas maybe suitable for the farming of economically important sea weeds. Distribution, abundance, seasonalvariation and relative importance of the various algae in different types of reefs and reef communities inthe region, however, are poorly understood, as are the factors influencing their distribution. In theRed Sea we are particularly ignorant of algae in deeper waters.Comparative floristic and ecological studies of the algal communities of different reef types,including aspects such as their relative importance in bioerosion, and their role in the “algal rim” on361