Ecosystem engineers can modify habitat, creating structural microhabitats. This structural complexity can affect species richness. Marine ecosystem engineers are able to produce local effects in combination with environmental variables (e.g. to create more humid habitat during low tides). We tested the hypotheses that if there is a relationship between mussel morphology and environmental factors, mussels would be larger at warmer than at cooler locations, and in areas where 2 species of mussels overlap, size and biomass will decrease and density will increase. At a smaller scale, we predicted that there is a relationship between the assemblage structure and hardness of the substratum and sediment content of a mussel bed. Using a nested design, we measured density, biomass and size of 2 species of mussels, Brachidontes rodriguezii and Perumytilus (Brachidontes) purpuratus, and diversity of species associated with mussel beds at 2 rocky intertidal sites at each of 4 shores along the northern Argentinean coast. These variables were correlated with oceanographic conditions and local characteristics. Significant correlations were found between intertidal assemblages and local factors. The largest specimens of B. rodriguezii and P. purpuratus were found at the warmer shores. In areas where they overlapped, size, biomass and density of P. purpuratus were lower, although B. rodriguezii did not change. The mean abundance of invertebrates associated with a mussel bed showed significant differences among shores. These 2 species of mytilids coexist over a small area, and although these species are very similar in their biological and ecological function, the fauna associated with their matrices are very different.
KEY WORDS: Spatial variations · Rocky shores · Ecosystem engineers · Environmental factors · Benthos · Southwest AtlanticResale or republication not permitted without written consent of the publisher Aquat Biol 18: 91-103, 2013 physically harsh conditions (Murray et al. 2002). In addition, local meteorological conditions such as wind, precipitation and climate anomalies can cause large impacts on ecosystems (Mantua & Hare 2002). The species that live in these habitats survive in variable and sometimes extreme physical conditions. One factor that correlates well with the abundance and diversity of species is water temperature, particularly over latitudinal gradients (Rohde 1992). Many other factors also correlate with species distributions and patterns of abundance (e.g. physical interactions between organisms and their habitats, behavioural and physiological adaptations; Burrows & Hughes 1989).Changes in the structure of assemblages of intertidal species at the scales of hundreds of kilometres and hundreds of metres may be attributed to differences in topography, substrate type, hydrodynamic conditions, intertidal elevation, wave exposure, climatic differences or variation in primary productivity (Araujo et al. 2005, Liuzzi & López Gappa 2008, Burrows et al. 2009. Spatial variability at the scale of t...