Productivity and seasonal changes in selected reef areas in the Gulf of Aqaba (Red Sea)

Abstract

In the first detailed ecological description of a fringing reef (Mergner & Schuhmacher, 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 soft corals, especially xeniids, and some stony corals, mainlyStylophora 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, soft and stony corals and other coelenterates inhabited by zooxanthellae show a conspicuous surplus in O₂ production down to 40 m depth. In all species examined, maximum O₂ consumption and productivity per unit dry weight are found below 5 m, with a decrease of 25 to 50% at 1 m depth. O₂ consumption during the period of photosynthesis is 20 to 30% higher than during the night. There is a slight decrease in O₂ production after 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.