Sublittoral meiofauna and macrofauna of Rocas Atoll (NE Brazil): indirect evidence of a topographically controlled front

Topographically controlled fronts are small-scale phenomena caused by the i nteraction between currents and complex reef topography, resulting in zones of convergence and eddies where debris and organisms are accumulated. Roca s is the only atoll in the South Atlantic (3 degrees 51'S, 33 degrees 49'W) and it is constructed predominantly by coralline red algae, vermetid gastr opods and encrusting foraminiferans. The structure of meiobenthic and macro benthic communities, particularly nematodes and polychaetes, over the subli ttoral carbonate deposits was examined during May 1996. Univariate and mult ivariate analyses showed a gradual change in the meiobenthic and macrobenth ic community structure from the windward to the leeward side of Rocas Atoll , which was significantly correlated with the measured environmental variab les. The characteristics of the carbonate deposits, ranging from coarse san ds to medium well-sorted sand, and their enrichment process towards the lee ward, as shown by the organic content, suggest a strong linkage with the hy drodynamic regime. Even though both components of the benthos are influence d by this hydrodynamic heterogeneity, the meiobenthic and macrobenthic faun a over the sublittoral area of Rocas Atoll are affected in different ways. Changes in the hydrodynamic regime promote a significant increase in divers ity and density of the meiofauna, particularly nematodes, but the macrobent hos showed a more complex response, with a decreasing number of taxa and a variable abundance towards the leeward side. The results of this study sugg est that the significant changes in the sediment characteristics and benthi c community structure along the atoll may be a result of topographically co ntrolled fronts.