Proceedings of the 11 th International Coral Reef Symposium, Ft. Lauderdale, Florida, 7-11 July 2008 Session number 23 Patterns of spatial variability in distribution of benthic invertebrates and algae at Sharm el Sheikh (South Sinai, Egypt) J. Giraldi, S. Acunto, L. Piazzi, F. Cinelli University of Pisa. Dept. of Biology - Marine Biology and Ecology Unit - Via Derna, 1 -56126, Pisa, Italy, Jessika.giraldi@gmail.com Abstract. A survey of benthic community was conducted as part of a project to construct an Underwater Observatory on the coast of Sharm el Sheikh (Egypt). To provide the first step of the EIA based on a “Beyond BACI” design, the spatial distribution of benthic invertebrates and algae was studied at a hierarchy of spatial scales: 5 sites, 3 areas within each site and 20 1m 2 frames within each area were randomly selected. Plots were sampled using the photo-quadrats method. A combination of univariate and multivariate analysis of the percentage cover was used to describe differences in the abundance of organisms identified at the genus level and, when necessary, grouped into broader categories. Results showed significant differences in the distribution and abundances of organisms among sites as well as among areas, indicating that abundances varied at the scales of hundreds and tens of meters along the coast investigated. These results are likely to be due to spatial differences of the hydrodynamic conditions of the studied sites, other than to stochastic events. One of the sites with the lowest percentage cover of living organisms was suggested as the most suitable to place the Underwater Observatory in order to minimize the impact of the structure. Key words: Spatial variability, Coral reef, Red Sea, Photo-quadrats, EIA Introduction This study is a part of the “Pilot project for the sustainable development of environmental sound management in South Sinai, Sharm el Sheikh, Ras Mohammed National Park” funded by the Italian Ministry of the Environment. Within this project, the construction of an Underwater Observatory close to the Visitor Center of the park was proposed. In response, the coast of Sharm el Sheikh was surveyed to provide information about spatial variability in distribution of benthic invertebrates and macroalgae. Variability in the distribution of natural species can occur at different spatial scales (Kennelly and Underwood 1992; Underwood and Petrait 1993; Underwood and Chapman 1996; Connell et al. 1997, Benedetti-Cecchi et al., 1998; Menconi et al. 1999; Ferdeghini et al. 2000; Balata et al. 2006). Documenting the spatial scales at which variability in the abundance of organisms occurs can help to focus attention on the relative importance of different ecological processes that can determine the patterns observed (Underwood and Chapman 1996). In this study, the sampling procedure was designed in order to be the first step of an Environmental Impact Assessment based on a “Beyond BACI” (Before-After, Control-Impact; Underwood 1991) design. The logic of this design is to separate the spatio- temporal natural fluctuations of the structure of benthic assemblages from those due to anthropogenic activities. Thus, an appropriate analysis would consist of properly replicated sampling several times before the development and several times after in potentially impacted and in reference locations. From these data, it is possible to ascertain interactions in impacted and control sites through time (Underwood 1991, 1992, 1993, 1994). Moreover, documenting the spatial distribution of living organisms was essential to suggest the most suitable stretch of coast to place the Underwater Observatory. Materials and Methods The study was carried out at Ras Mohammed National Park along the coast of Marsa Ghoslani bay (Fig. 1). Blocks of organic framework of dead and living corals characterized the substrate of the study area. A nested sampling design was used at a hierarchy of spatial scales: 5 sites (100s of m apart), 3 areas within each site (10s of m apart) and 20 replicates of 1m 2 frames within each area were randomly selected in the Northern part of Marsa Ghoslani bay (Fig. 2). Plots were sampled using the photo-quadrats method (English et al. 1997; Acunto et al. 2001; Hill 1161