ALBERT

Description: Studies on core samples of the genus Porites in the northern Gulf of Aqaba revealed marked short-term variations in the growth rate of this coral over the last 20 years, Individual growth rates show high variations, even among colonies growing next to each other, These variations might be attributed to locally restricted differences in the reef environment, or to endogenous differences between the individual colonies, A general increase in linear extension rate was observed for colonies situated in a relatively undisturbed reef area between 1 m and 15 m water depth. Over the time period 1972–1992, major variations in climate conditions that could have affected coral growth were not observed. Reduction of terrestrial sediment influx is a possible reason for the accelerated growth rates. The effect of intense sediment loading on the rate of coral growth within a stressed environment is demonstrated for one Porites-core taken from a colony growing in the vicinity of the phosphate loading berth at Aqaba. The drastic decrease in growth rate at this location during the observed time span stands in contrast with the general development of Aqaba's reefs and is most probably the result of sediment stress.

Description: The stable isotopic composition of scleractinian corals (Porites sp.), two recent and one fossil, from the Egyptian Red Sea coast was studied. The oxygen isotope record proves the assumption that recent sea surface temperatures are comparable to the time of last sea level highstand in Eemian (stage 5e, 125,000 yBP). Deposition of high-density and low-density bands in respect to season shows the sanre patterns as today with high-density band depositior in winter (low water temperatures) and low-density band deposition in summer (high water temperatures). 6180 is negatively correlated with 613C with a shift in phase of t to Z months. Thus, a coupling of carbon isotopes to light intensity and oxygen isotopes to water temperature is suggested. To get an overview on seasonal patterns of stable isotope composition a sampling technique with a resolution of four samples per year is of sufficient precision.

Description: At Aqaba in the Northern Gulf of Aqaba, Red Sea, the stable oxygen isotope record of a Por#es lutea coral colony was studied. From a vertical and a horizontal core two parts from the base of the colony (10 years of coral growth), and two parts from the living surface (12 years of coral growth) were analysed with a monthly sampling resolution. The results show that seasonality of sea surface temperatures (SST) in the northem GulfofAqaba has increased since the early 19th century, mainly due to higher summer temper- atures. 8~80-ratios indicate that SST increased by at least 1.3~ Horizontal and vertical cores display dif- ferent variations in the modern parts, probably caused by the very shallow water depth of the youngest part of the vertical core.

Description: This interim report deals with investigations on key factors controlling reef growth by zoophysiologists, ecologists, paleontologists and geologists. The different levels of emphasis are the coral animal and the reef community. The main study area is the Red Sea which reaches over 20°C latitude up to the northernmost margin of the global coral reef belt. Supplementary results on microborer ecology are provided from the Bahamas. The desert enclosed Red Sea, not influenced by land runoff and only minimally by anthropogenic (urban and touristic) nutrient inputs, is predestined for a study on the principal influence of light on calcification within bathymetrical and latitudinal gradients. Hence, on the level of the zooxanthellate scleractinian animal phototrophic and heterotrophic energy supply and its bearing on calcification are being measured in different coral species—in particular inPorites sp., one of the most important reef builders. The growth of 15 zooxanthellate scleractinians in the Gulf of Aqaba correlates with the annual light cycle. This correlation is observable down to 40 m depth. Other growth promoting factors seem to have less influence on coral extension. The availability of organically enriched sediments in shallow water probably yields nutritional value, in particular for filter feeding species, thus restricting their distribution to those areas. Zooxanthellae, when isolated fromMycedium elephantotus, are different in their dependence on depth in maximum rates of photosynthesis and photosynthetic efficiency (-slope). Increasing concentrations of pigments as a function of depth could be determined. Maximum rates of photosynthesis of zooxanthellae in vivo, collected at corresponding depth, have been 4 times higher. Structural and physiological adaptations improving heterotrophic and phototrophic energy intake are highlighted. Porites sp. was the subject of annual growth studies at locations extending from Aqaba in the North over the northern and southern Egyptian coast and islands, Sanganeb Atoll and Wingate reef offshore Sudan to the Gulf of Tadjoura in the Gulf of Aden (Djibouti). Mean growth rates in the shallow water zone increase with decreasing latitude and are highest at the southernmost studied reefs in the Gulf of Tadjoura. However, the observed latitutdinal growth reduction is restricted to the upper ca. 15 m of the water column. The upper limit of growth potential decreases with depth parallel to the decrease of light availability. Highest growth rates are recorded in shallow depth (10–2.9 mm yr−1). This zone reaches at Aqaba (29°30′N) to a depth of ca. 10 m. At the southern Egyptian reefs (24°30′N) this zone extends to ca. 15 m water depth. This effect is probably a result of the stronger reduction of winter light levels and water temperature in the northern regions. Compared to other oceans the decrease of growth with increasing latitude of Red SeaPorites corals is far less, and growth rates at Aqaba are the highest observed at these latttudes. On the level of the community of reef inhabitants four principal topics are addressed: The first one is the dynamics of the proportions of hermatypic and ahermatypic organisms and open space. The occurrence of stony and soft corals and the sharing of empty space in different reef sections at Aqaba and on Sanganeb Atoll were quantified. Soft corals, mainlySinularia- and xeniid species, occupy decreasing shares with depth. Among theXenia species a bathymetrical zonation pattern was detected. The next issue is the growth impeding role of soft corals and gastropod parasites and predators on scleractinians. Experimental and field observations showed xeniid soft corals to be opportunistic i.e. occupying rapidly open space rather than to attacking and outcompeting stony corals. An increasingly specialized behaviour was detected among corallivorous gastropods of the family Coralliophilidae to exploit their coral hosts. Whereas these snails are more or less sessile and depend for a long time on the surrounding host polyps the mobileDrupella cornus (Thaididae) forms feeding aggregations which denude mainly branching corals on shallow reef parts. Furthermore, the role counteracting reef growth of macro- and microbioeroders is investigated.Diadema setosum is a major destructive agent on reefs at Aqaba (not in the central Red Sea). The grazing sea urchins do not only keep potential colonization area free but also erode carbonate material (e. g. 1468 g/m2/year, 10 m depth). Demographic and bathymetric patterns in the sea urchin population are analyzed including their bearing on bioerosion of the reef. Investigations on microboring organisms in carbonate material have started in the Red Sea; initial results, however, are only available from similar studies near Lee Stocking Island, Bahamas. Three major environments have been identified based on the distribution of the different microborers. These are the intertidal environment dominated by boring cyanobacteria., reef sites from 2 to 30 m water depth dominated by a diverse assemblage of boring cyanobacteria and chlorophytes, and the deep reef slope from 100 to 300 m dominated by boring green algae and heterotrophs. The boring chlorophyte genusPhaeophila appears rapidly and dominates at sites from 2 to 30 m, but it leaves vacated borings and is replaced byOstreobium quekettii after 1 year. Different substrate types show very different rates of colonization by microborers. The greatest excavation rates (100 g/m2/3 months) occur in fine-grained limestone, while the slowest rates (0.5 g/m2/3 months) occur in calcite crystals. Molluscan shell material shows intermediate rates of excavation. Light conditions appear very important in determining the growth rate and distribution of different microborers between the sites, however, the interaction of light with other factors, such as substrate, time period of exposure, and water quality conditions may be involved.