This study was conducted during 1999 to 2002 and addresses the community structure and some ecological aspects of the benthic reef flat assemblages of Atol das Rocas, located offshore the NE brazilian coast. It corresponds to the sole atoll of the SW Atlantic, which characterized by a shallow topography and is almost completely built by coralline algae. The turf forming red macroalgae Digenea simplex and the crustose coralline Hydrolithon pachydermum were the dominant species of the reef flat. The crustose green macroalgae Dictyosphaeria ocellata and the turf forming red macroalgae Gelidiella acerosa were the subdominant species. Biomass values of D. simplex were about twice higher than the other species, pointing out to its relevance in the community structure of this reef zone. Biodiversity indices indicated a high equitability within the few species observed and a relative temporal stability of the community structure. Some local spatial variations were found in the community structure of the reef flat zone, enabling the definition of three subhabitats. The patterns of distribution and abundance of the benthic organisms seem to be related to the environmental conditions of the reef flat, such as low water turbulence, lengthy periods of aerial exposure, and low herbivore pressure. 1. Introduction Reefs of tropical atolls have been described as harboring well-defined geomorphic zones, including a fore-reef, a reef crest, a reef flat, and a lagoonal reef zone [1–3]. Theoretically, each zone is subject to intrinsic environmental conditions which circumscribe unique habitat types and communities. Benthic communities within each zone were thought to exhibit a great similarity between each other. However, local small scale variability of physical and biotic factors in a well-defined geomorphic zone may harbor microcosms, each supporting unique benthic communities [4–6]. The reef flat zone is a complex area with gradients of environmental factors such as temperature, turbidity, and tidal related exposure of communities [3]. These gradients coupled with differences in depth and substrate type provide a great number of habitats that have resulted in subdivisions of this zone [7–10]. For further understanding of the similarities and differences within this geomorphic zone, it becomes essential to obtain more information on the organization of the benthic community assemblages and how they operate in accordance to the physical-geomorphological features of their habitats [11]. The Marine Biological Reserve of Atol das Rocas, NE Brazil, is a pristine reef area
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