Patterns and changes of reef-coral communities at the Sanganeb-Atoll (Sudan, central Red Sea): 1980 to 1991

Abstract

Benthic coral reef communities were investigated on the Sanganeb-Atoll, 28 km off the coast of Port Sudan (Sudan, central Red Sea). Four test areas (5×5 m) were selected and marked in approx. 10 m depth along a transect running in SSW-NNE direction over the atoll and were mapped in 1980 and 1991. Detailed photographs were used for in situ verification of taxonomic composition of communities (comprising some 135 cnidarian species) and transferred to scale 1∶10 maps, used for the quantitative analyses of coral communities in the test areas. Shares of inanimate (unoccupied) substrate were 42–60% of the total test areas in 1991 (i.e. slightly larger than at the 1980 census). The living cover was mainly composed of scleractinian and alcyonacean species. Coral communities were analysed on the species level with regard to taxonomic composition, areal coverage, biophysiographic zonation, and changes in community structures during the investigation period of 11 years. The analyses revealed a general constancy in the overall composition and distribution of benthic taxa which reflect the different abiotic conditions along the transect across the leeward and windward sides of the atoll. However, the detailed view on species recruitment, growth, decrease and disappearance on the base of a decadal time span allows to detect life history traints of stony and soft corals and their significance for the dynamics of the respective communities. Especially soft corals (predominating in the two leeward test quadrats) exhibit various strategies to colonize and occupy space. Their presence, however, is hardly to be detected from the fossil record (except for spiculite of some Sinularia spp.). Exemplarity, single colonies' fates were reconstructed and extrapolated. Based on the data of species recruitment and disappearance a time estimate for development and turnover of reef communities is provided. Species turnover rates were calculated as approx. 15 species/10 years (Trel=2.63%/year) with estimated median community turnover periods of 416 (323–755) years from recruitment rates, or 338 (219–526) years from clearing rates.