MEPS 179:175-186 (1999)  -  doi:10.3354/meps179175

ABSTRACT: Topographically controlled fronts are small-scale phenomena caused by the interaction between currents and complex reef topography, resulting in zones of convergence and eddies where debris and organisms are accumulated. Rocas is the only atoll in the South Atlantic (3°51'S, 33°49'W) and it is constructed predominantly by coralline red algae, vermetid gastropods and encrusting foraminiferans. The structure of meiobenthic and macrobenthic communities, particularly nematodes and polychaetes, over the sublittoral carbonate deposits was examined during May 1996. Univariate and multivariate analyses showed a gradual change in the meiobenthic and macrobenthic community structure from the windward to the leeward side of Rocas Atoll, which was significantly correlated with the measured environmental variables. The characteristics of the carbonate deposits, ranging from coarse sands to medium well-sorted sand, and their enrichment process towards the leeward, as shown by the organic content, suggest a strong linkage with the hydrodynamic regime. Even though both components of the benthos are influenced by this hydrodynamic heterogeneity, the meiobenthic and macrobenthic fauna over the sublittoral area of Rocas Atoll are affected in different ways. Changes in the hydrodynamic regime promote a significant increase in diversity and density of the meiofauna, particularly nematodes, but the macrobenthos showed a more complex response, with a decreasing number of taxa and a variable abundance towards the leeward side. The results of this study suggest that the significant changes in the sediment characteristics and benthic community structure along the atoll may be a result of topographically controlled fronts.

KEY WORDS: Meiofauna · Macrofauna · Rocas Atoll · Community structure · Sublittoral · Front · Brazil