FACTORS CONTROLLING HOLOCENE REEF GROWTH - AN INTERDISCIPLINARY APPROACH

This interim report deals with investigations on key factors controlli ng reef growth by zoophysiologists, ecologists, paleontologists and ge ologists. The different levels of emphasis are the coral animal and th e reef community. The main study area is the Red Sea which reaches ove r 20 degrees latitude up to the northernmost margin of the global cora l 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) nutri ent inputs, is predestined for a study on the principal influence of l ight on calcification within bathymetrical and latitudinal gradients. Hence, on the level of the zooxanthellate scleractinian animal phototr ophic and heterotrophic energy supply and its bearing on calcification are being measured in different coral species - in particular in Pori tes sp., one of the most important reef builders. The growth of 15 zoo xanthellate scleractinians in the Gulf of Aqaba correlates with the an nual light cycle. This correlation is observable down to 40 m depth. O ther growth promoting factors seem to have less influence on coral ext ension. The availability of organically enriched sediments in shallow water probably yields nutritional value, in particular for filter feed ing species, thus restricting their distribution to those areas. Zooxa nthellae, when isolated from Mycedium elephantotus, are different in t heir dependence on depth in maximum rates of photosynthesis and photos ynthetic efficiency ( -slope). Increasing concentrations of pigments a s a function of depth could be determined. Maximum rates of photosynth esis of zooxanthellae in vivo, collected at corresponding depth, have been 4 times higher. Structural and physiological adaptations improvin g heterotrophic and phototrophic energy intake are highlighted. Porite s 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 Gul f 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, th e observed latitudinal growth reduction is restricted to the upper ca. 15 m of the water column. The upper limit of growth potential decreas es with depth parallel to the decrease of light availability. Highest growth rates are recorded in shallow depth (10-12.9 mm yr(-1)). This z one reaches at Aqaba (29 degrees 30'N) to a depth of ca. 10 m. At the southern Egyptian reefs (24 degrees 30'N) this zone extends to ca. 15 m water depth. This effect is probably a result of the stronger reduct ion of winter light levels and water temperature in the northern regio ns. Compared to other oceans the decrease of growth with increasing la titude of Red Sea Porites corals is far less, and growth rates at Aqab a are the highest observed at these latitudes. On the level of the com munity of reef inhabitants four principal topics are addressed: The fi rst one is the dynamics of the proportions of hermatypic and ahermatyp ic organisms and open space. The occurrence of stony and soft corals a nd the sharing of empty space in different reef sections at Aqaba and on Sanganeb Atoll were quantified. Soft corals, mainly Sinularia- and xeniid species, occupy decreasing shares with depth. Among the Xenia s pecies a bathymetrical zonation pattern was detected. The next issue i s the growth impeding role of soft corals and gastropod parasites and predators on scleractinians. Experimental and field observations showe d xeniid soft corals to be opportunistic i.e. occupying rapidly open s pace rather than to attacking and outcompeting stony corals. An increa singly specialized behaviour was detected among corallivorous gastropo ds of the family Corallophilidae to exploit their coral hosts. Whereas these snails are more or less sessile and depend for a long time on t he surrounding host polyps the mobile Drupella cornus (Thaididae) form s 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 destr uctive agent on reefs at Aqaba (not in the central Red Sea). The grazi ng sea urchins do not only keep potential colonization area free but a lso erode carbonate material (e. g. 1468 g/m(2)/year, 10 m depth). Dem ographic and bathymetric patterns in the sea urchin population are ana lyzed including their bearing on bioerosion of the reef. Investigation s on microboring organisms in carbonate material have started in the R ed Sea; initial results, however, are only available from similar stud ies near Lee Stocking island, Bahamas. Three major environments have b een identified based on the distribution of the different microborers. These are 1) the intertidal environment dominated by boring cyanobact eria, 2) reef sites from 2 to 30 m water depth dominated by a diverse assemblage of boring cyanobacteria and chlorophytes, and 3) the deep r eef slope from 100 to 300 m dominated by boring green algae and hetero trophs. The boring chlorophyte genus Phaeophila appears rapidly and do minates at sites from 2 to 30 m, but it leaves vacated borings and is replaced by Ostreobium quekettii after 1 year. Different substrate typ es show very different rates of colonization by microborers. The great est excavation rates (100 g /m(2)/3 months) occur in fine-grained lime stone, while the slowest rates (0.5 g/m(2)/3 months) occur in calcite crystals. Molluscan shell material shows intermediate rates of excavat ion. Light conditions appear very important in determining the growth rate and distribution of different microborers between the sites, howe ver, the interaction of light with other factors, such as substrate, t ime period of exposure, and water quality conditions may be involved.