Modeling of the Ecological Succession of Encrusting Organisms in Recent Coralline-Algal Frameworks from Atol Das Rocas, Brazil

Models of the ecological succession of encrusting organisms in the recent coralline-algal reefs from Atol das Rocas (South Atlantic, Brazil)are presented for a windward and a leeward transect. These models are constructed on probability calculations that use the relative frequencies of upward (overgrowing)transitions (succession)of observed pairings of reef- framework builders. These are taken from blocks excavated across a reef-front / reef-crest / reef-flat section in the windward transect, and a reef-crest / reef-flat section in the leeward transect. The main substrate available for encrustation is the dominant primary framework - building coralline alga Porolithon cf. pachydermum. The diversity (number)and complexity of competitive interactions among encrusters increase from reef-front and reef-crest to reef-flat in both transects. Vermetid gastropods and homotrematid and acervulinid foraminiferans are the most important secondary framework builders in the surveyed reefs. The proposed successional models present an account of the operation of competitive processes over time and space, providing further evidence of a non-hierarchical pattern of competition for space. Changes in reef-framework succession along the reef transects are controlled by extrinsic factors. However, intrinsic factors are likely to be the main control of the succession when windward and leeward reef-flat environments are compared. Finally, reef-framework successional models derived from transition-probability matrices can be used to investigate paleoenvironmental gradients (such as wave energy)in preserved frameworks from ancient reefs and to understand reef-community structure based on the succession of encrusting epibionts.