Morphology, ecology and zonation of coral reefs at Aqaba (Gulf of Aqaba, Red Sea). The coral reefs of the Gulf of Aqaba are among the most northern ones of the world. This study, the first concerning the east coast of this topographically and hydrographically peculiar sea, considers relationships of biophysiographical and structural reef zones to fundamental abiotic environmental factors. An introduction to paleogeography, geology, petrography, topography, climate and hydrography is followed by terminological definitions used to describe the different reef areas. The investigations were carried out on two transects crossing fringing reefs of different shape. Each transect was 20 m wide and run from the shore over nearly 200 m to the fore reef in about 30 m depth. One reef, a "coastal-fringing reef", represents an unaltered straight reef flat from shore to the reef edge 60 m away; two large pinnacles reach the surface some 125 m off the shore. The other reef, a "lagoon-fringing reef", is divided into a 100 m wide lagoon of 0.5-2.3 m depth and a reef crest separated from the former by a rear reef. The reef platform of the lagoon-fringing reef is cut by a system of channels and tunnels; the reef edge is about 135 m off shore. Such water depth, substrate, temperature, illumination and water movement were recorded, about 200 common or dominant species (plants and animals) were collected, their distribution plotted and, together with other data and structural items, charted. Indicator species characterize the biophysiographical zones. Their variation as well as that of the structural and substrate zones depend on different zones of water movement. This basic factor also controls other ecological parameters such as food and oxygen supply as well as temperature and salinity gradients between fore reef and shore. From this point of view the ecological requirements of some indicator and other species and conversely the ecological settings of different reef areas are discussed. The different shapes of both reefs are explained on the basis of a "reef development cycle" -a hypothesis applicable to fringing reefs at unchanging sea level and based on the fact that only a small surf-influenced area of "living reef" is able to compensate for reef destruction: While a young coastal fringing reef is growing outwards, its back reef is gradually altered to a reef lagoon by erosion. Aflcer stillstand of seaward expansion the reef crest, too, is cut by a channel system eroded by rip currents. This stage is represented by the lagoon-fringing reef. Isolated pinnacles remain as remnants of the former reef crest; young coastal-fringing reefs develop from the shore. This stage is examplified by the first reef studied. Extension, growth intensity, dominant frame building corals, and the number of species of the Aqaba reefs are compared with those of Eilat and with reefs of the middle Red Sea, South India, Southwest-Pacific and Jamaica.

Quantitative analysis of the coral community of a fore reef area near Aqaba (Red Sea). Previous descriptions of the ecology and zonation of Aqaba reefs (Mergner & Schuhmacher, 197,1) are supplemented by this quantitative study of a test quadrat (5 x 5 m in size), randomly chosen in some 10 m depth in the middle fore reef of a coastal fringing reef. Of the total surface of 25 m 2 Cnidaria represent 42.31%, sponges 0.17 %, calcareous algae 0.20 %, dead coral rock and pebble 30.27 % and sand and coral debris 26.15 %. The cnidarian cover is roughly equally contributed by 50.86 % Scleractinia and 48.61% Alcyonaria, mainly Xeniidae (35.81%). For each species the percentage of the total cover (measured as vertical projection), colony number, average and maximal colony size are given. A total number of 104 cnidarian species was recorded, among which the 78 scleractinian species represent 34 of the 55 coral genera known from the Red Sea. The well balanced regime of moderate light and current conditions which are tolerated both by shallow and deep water species may account for the high species number. Disturbances such as occasional sedimentation, grazing of sea urchins (Diadema setosum) and overgrowth of stony corals by xeniids result in continuous fluctuations of the coral community, in small colony size and in high colony number. Abiotic factors and biotic interactions maintain a diversity (H = 3.67) which ranks among the greatest ever found in reef communities. The data obtained from the fore reef square are compared with those of a similar test square in the lagoon of the same reef and with results from transect zonations on the opposite coast of the Gulf of Aqaba. These comparisons indicate that the fore reef harbours the richest coral fauna in the reef. The inventory of coral species at the northern end of the Gulf of Aqaba, one of the northernmost outposts of the coral reef belt, is only little reduced when compared with that of the central Red Sea; this great species diversity is in contrast to the worldwide decrease of species number towards the periphery of the reef belt. EINLEITUNG Artenspektrum und Artenvielfalt von Korallen und anderen sessilen Siedlern im Riff haben 6kologischen und tiergeographischen Aussagewert: So charakterisieren Vergesellschaftungen, die ftir bestimmte Riffzonen typisch sind, und ihre zeitliche und 6rtliche Abfolge auch bestimmte Regime abiotischer Faktoren. In Verfolg dieser Fragestellung werden seit dem Jahre 1972 bei Aqaba am Roten Meer kiistennahe Rifle untersucht. Diese Riffe stellen-als geographische Besonderheit-die n6rdlichsten Rifle im Roten Meer und im Indischen Ozean dar, und zugleich-nach den Riffen der Ryukyu-Inseln (Pazifik) und Bermudas (Atlantik)-die n6rdlichsten Riffvorkommen iiberhaupt.

In the first detailed ecological description of a fringing reef (Mergner & Schuhreacher, 1974) two parts of the reefs south of Aqaba (Red Sea) were considered. These investigations have been extended to an eco-functional analysis of the seasonal fluctuations of algal growth, sessile and mobile fauna, and productivity of biocoenoses. Two test areas were observed from October 1975 to June 1976; one is situated in the reef lagoon with strong influences of temperature, light intensity, currents and sedimentation; the other, in the middle fore reef with reduced illumination, water movement and sedimentation. A precise quantitative analysis of the stock of benthic reef algae was made by elaborating the percentual coverage ratio of each species within a well-defined reef area. At the beginning of the year, the algal vegetation with 23 species appeared in different successions, abundance and dominance. The phytal disappeared, in several steps, from early spring to late summer (e.g. due to grazing, epiphytes, loss of algal dyes). Such rhythms in phytal population dynamics lead to fundamental changes in mobile fauna living conditions. Molluscs and echinoderms serve as good examples (about 140 species were identified). The increase in some species, caused by rapid algal growth is remarkable, as is the high percentage of juvenile individuals. Grazing by sea-urchins constitutes an important factor controlling seasonal changes in benthic algae. Some soi~ corals, especially xeniids, and some stony corals, mainly Stylophora pistillata, undergo similar seasonal fluctuations within the same period. The fluctuations are partly characterized (1) by the disappearance of colonies caused by feeding and natural collapse following the breeding season, and (2) by recolonization or new settling. During 24 h in situ measurements, soflc and stony corals and other coelenterates inhabited by zooxanthellae show a conspicuous surplus in O-~ production down to 40 m depth. In all species examined, maximum 02 consumption and productivity per unit dry weight are found below 5 m, with a decrease of 25 to 50 % at 1 m depth. Oe consumption during the period of photosynthesis is 20 to 30 ~ higher than during the night. There is a slight decrease in O~ production afler reaching morning light saturation values; this might be caused by the storage of metabolic products. Xeniids show a stepwise increase in oxygen output, caused by diurnal rhythmic colony contraction. INTRODUCTION AND PROBLEMS

Quantitative ecological analysis of a reef lagoon area near Aqaba (Gulf of Aqaba, Red Sea). Following previous investigations on a fringing reef in the Red Sea near Aqaba (Mergner & Schuhmacher, 1974; Mergner & Svoboda, 1977) this paper presents a quantitative analysis of the faunal and floral stock in one of the selected test areas (5 x 5 m) located in the reef lagoon studied. During the observation period 237 macrobenthic species of 192 genera were found. Species numbers were 22 (algae), 18 (cnidarians), 22 (crustaceans), 113 (molluscs), 27 (echinoderms), 14 (fishes) and 21 (smaller animal taxa including foraminiferans, sponges, turbellarians, polychetes and ascidians). These numbers, though derived from a small and relatively monotonous reef area, exceed by far all data known from comparable reef zones. The ecological conditions as well as habitat preferences, times of occurrence, seasonal abundances, species diversities and dominances of the inhabitants observed in the test area are discussed. Comparisons of the population data of October, 1975 and March, 1976 indicate the influence of the succession, abundance and disappearance of the phytal on the environmental conditions of the mobile fauna, for example molluscs, echinoderms and fishes. The increase of some of its species, especially of juvenile individuals, is remarkable. Within the same period juvenile colonies of some xeniids and the stony coral Stylophora pistillata undergo similar seasonal fluctuations; colonies eliminated by grazers or other, unknown circumstances, are replaced by new settlers. Altogether, the faunal and algal stock of this lagoon area can be interpreted as a climax community; its impressive density is apparently the result of a high number of various microhabitats. Each of these ecological niches provides manyfold living spaces for specific communities with different adaptive strategies. The highest number of species plus the highest diversity indices rank mostly among the mobile groups. EINLEITUNG

Quantitative analysis of coral communities of Sanganeb-Atoll (central Red Sea). lI.Comparison with a reef area near Aqaba (northern Red Sea) at the northern margin of the Indopacific reef-belt. Quantitative studies of coral communities in the central and northern Red Sea were designed for comparison of the community structure in both areas. The central Red Sea provides reef-building Scleractinia and reef-inhabiting Alcyonaria with optimal temperature conditions, whereas the north tip of the Gulf of Aqaba (29 ° 30' N) represents the northernmost outpost of coral reefs in the Indian Ocean. It is generally assumed that coral diversity decreases towards the margins of the global reef-belt. In the Red Sea, generic diversity of hermatypic Scleractinia slightly decreases from the central to the northern part (51 : 48 genera); but cnidarian species abundance (species number per 25 m 2 area) was found to increase from 62 to 98 species and the ShannonWiener diversity index increased from 2.58 to 3.67 with regard to colony number. The mean colony size was 189 cm 2 at Sanganeb-Atoll, but only 52 cm 2 at Aqaba. The mean numbers of colonies were inversely related: 662 per 25 m 2 at Sanganeb-Atoll and 2028 at Aqaba. Uninhabited parts of the studied areas amounted to 47 % at Sanganeb-Atoll and to 56 % at Aqaba. The community structure of the studied areas indicates that occasional perturbations prevent the progress of the community towards a low-diversity equilibrium state. Since severe hydrodynamic damage is extremely rare in 10 m depth, major disturbances may occur by sedimentation, by the interference of grazers (e. g.Diadema setosum) and due to overgrowth by space-competitors (mainly soft corals). These events are to be regarded as throwbacks in the process of monopolization of the area by well adapted species. Recovery from such perturbations (i.e. recolonization of dead areas) obviously takes place at different velocities in the northern and central Red Sea, for the mean water temperature at Aqaba is 5 °C lower than in the central Red Sea. Hence the process of taking over a given space by a few species proceeds further in the central Red Sea than at its northern end. The increase in diversity per area towards high latitudes is comparable to that with depth. It is concluded from the great number of species at Aqaba that these reefs mark the northernmost outpost of the Indian Ocean only geographically but not ecophysiologically; they would occur at even higher latitudes, if the Gulf of Aqaba extended farther north.