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Uruguayan mussel beds are located along the estuarine gradient caused by the interaction of the Río de la Plata run-off and the Atlantic Ocean, changing in species composition across this gradient. In the oceanic portion of the gradient, the exotic Perna perna showed a sequence of local extinction–colonization episodes since at least the second half of the 20th Century, thus altering the dominance relationships within the mussel beds. Owing to its larger size, this species may have caused profound effects on the spatial structure of the mussel beds and consequently on its associated macrofauna. In this vein, we analysed patterns in zonation, abundance and dominance of mussel species in a mussel bed located in the oceanic portion of the Uruguayan coast in relation to tidal and exposure level. We also evaluated if these factors could affect the dominance patterns between mussel species. The main results showed that the dominance of the native mussel Brachidontes rodriguezii was consistent across all tidal levels in all sites in terms of absolute and relative abundance, with the exception of the shallow subtidal at the intermediate site, where P. perna dominated over this species in terms of relative abundance. This suggests that the effect of an invasive mussel is highly dependent on the receptive assemblage, and that the outcome of interspecific competition can also be modulated by small-scale factors. [ABSTRACT FROM PUBLISHER]

Abstract: We evaluated scale-dependent patterns of distribution of sandflat macroinfauna of a coastal lagoon, using different analytical approaches. The sampled area was divided into three sectors (outer, inner-north, inner-south) containing small promontories. At each side of the promontories we defined stations in a line transect across the sandflat. Thus, we evaluated sediment characteristics and macroinfaunal responses to sectors, orientations and intertidal levels; animal–sediment relationships were also studied. At a large scale, there was a clear pattern of sediment composition and macroinfaunal abundance. While the outer sector had medium to coarse sands, reflecting the high hydrodynamic conditions existing near the lagoon entrance, the inner sectors showed sandy and muddy sediments. Most species were in low abundances at the outer sector. At small scale, macroinfaunal abundance and species richness decreased sharply towards the upper sandflat level. Also at small scale, sediment composition limited the maximal densities reached by all species, with exception of the deposit-feeding polychaetes. Thus, the macroinfaunal community at muddy sediments was dominated by burrowing deposit-feeders, while all species peaked in sandy sediments. Our results suggest that physical factors shaping macroinfaunal communities operate at different scales and are better detected using several analytical approaches. Large scale patterns, associated with along-shore variations in disturbance by currents, were detected as changes in the mean abundance of macroinfauna. Small scale patterns, related to sediment characteristics, were observed as changes in maximal densities of macroinfauna. Small scale patterns, associated with the level of inundation of the sandflats, were detected through changes in the abundance and presence of macroinfauna. The evaluation of the role of the physical conditions on communities must involve the use of several sampling and analytical approaches. [Copyright &y& Elsevier]