Quantitative Analyse von Korallengemeinschaften des Sanganeb-Atolls (mittleres Rotes Meer). I. Die Besiedlungsstruktur hydrodynamisch unterschiedlich exponierter Außen- und Innenriffe

Abstract

Quantitative analysis of coral communities of Sanganeb-Atoll {central Red Sea). I.The community structure of outer and inner reefs exposed to different hydrodynamic regimes. The Sanganeb-Atoll off Port Sudan is an elongate annular reef which rests on a probably raised block in the fracture zone along the Red Sea-graben. Its gross morphology was most likely formed by subaerial erosion during low sealevel conditions. Features of its topography and hydrography are described. The prevailing wind waves are from NE. Hence, the outer and inner reef slopes are exposed to different hydrodynamic conditions. The sessile benthos was analysed using the quadrat method. Four test quadrats (5 × 5 m each) were selected on the outer and inner slopes at a depth of 10 m along a SSW-NNE transect across the atoll. Cnidaria were by far the most dominating group; coralline algae, Porifera, Bryozoa and Ascidia, however, counted for just under 3 % living cover. Light and temperature intensities did not differ significantly at the sites studied; water movement, however, decreased in the following order: TQ IV (outer NE side of the reef ring) was exposed to strong swell and surf; TQ II (inner side of the SW-ring) was met by a strong longreef current; TQ I was situated on the outer lee of the SW-atoll ring and TQ III in the inner lee of the NE-side. This hydrodynamic gradient correlates with the composition of the coral communities from predominantly branching Scleractinia (staghorn-like and other Acropora species and Pocillopora) in TQ IV, through a Lobophyttia, Porites and Xenia-dominated community in TQ II, and a mixed community with an increasing percentage of xeniid and alcyoniid soft corals in TQ I, to a community in TQ III which is dominated by the soft corals Sinularia and Dendronephthya. The cnidarian cover ranged between 42.4 and 56.6 % whereby the two exposed test quadrats had a higher living coverage than the protected ones. In total, 2649 colonies comprising 124 species of stony, soft and hydrocorals were recorded by an elaborate method of accurate in-situ mapping. The 90 scleractinian species found include 3 species new to the Red Sea and 11 hitherto unknown species from its central part. Only 7 cnidarian species covered large areas, forming a Xenia macrospiculata-zone in TC) I, a Lobophyllia corymbosa-zone in TQ II, a Sinularia-Dendronephthya-zone in TQ III and an Acropora-Pocillopora verrucosa-zone in TQ IV, Hydrodynamic exposition and ratio of genus abundance Acropora :Montipora are closely correlated: on the windward side (TQ IV) it is 943:4, but on the leeward side (TQ I) 18 : 208. Apart from the taxonomic and quantitative distribution of species and colonies, types of growth form and categories of the dead substrate are given. Since soft corals do not contribute to the reef framework, the distribution of Scleractinia and Alcyonaria indicates that at Sanganeb-Atoll reef substance is mainly generated on the windward side.

Red Sea Coral Reef Trajectories over 2 Decades Suggest Increasing Community Homogenization and Decline in Coral Size

“…This suggests that in our dataset, only specific environments but never entire reefs showed unique patterns based on variable species-abundances and presences. Clustering by untransformed proportional intercept of genera (all species summed to genus level), showed no difference among regions but suggested a gross community differentiation into windward Acropora slopes and leeward Porites slopes and coral carpets, as previously described from the region [9], [13], [16], [17], [18], [19], [20]. A mixed group of transects from semi-exposed Millepora -dominated sites and sites of mixed dominance had greater mean depth than the Acropora and Porites dominated clusters, but differences were not significant.…”

“…Previous studies in the Gulf of Aqaba [12], [30], [34] and Sudan [21], [35] showed higher coral population turnover in the N and more community constancy in the S (Sudan; [13], [21]). Thus, average colony size in Saudi Arabia should have been larger than in Egypt's Northern Islands, not smaller as recorded by us (Fig.…”

“…The Red Sea contains a typical Indo-Pacific fauna with many peripheral endemics [8] , [9] and a N-S gradient in species richness and community diversification was described [9] . It is a data rich environment, with reef studies dating to the 18 th century and assessment and monitoring work since the 1970s [5] , [9] , [10] , [11] , [12] , [13] , [14] , [15] , [16] , [17] , [18] , [19] , [20] , [21] , [22] that is useful for comparison.…”

“… Red stars = transect surveys 2006, 2008, 2009; blue crosses = transect surveys 1988, 1989, 1997; green crosses = squares of 1997. Sites in Egypt from the Gulf of Suez to Ras Banas from [5] , [18] , [20] ; Sites in Sudan (purple circles) from [13] , [17] , [26] were used to compare validity of results. …”

Quantitative Analyse von Korallengemeinschaften des Sanganeb-Atolls (mittleres Rotes Meer). II. Vergleich mit einem Riffareal bei Aqaba (nördliches Rotes Meer) am Nordrande des indopazifischen Riffgürtels

“…Die zum Vergleich herangezogenen Referenzareale aus dem mittleren Roten Meet sind die vier Testquadrate am Sanganeb-Atoll, die yon Mergner & Schuhmacher (1985) Eine ausfiihrliche Darstellung, wie die untersuchten Besiedlungsgemeinschaften in beiden Regionen erfal~t wurden, findet sich bei Mergner & Schuhmacher (1981, 1985 Wijsman-Best (1977Scheer & Pillai (1983) halten M. forskaelana ffir eine Wuchsform von Echinopora gemmacea.…”