Spatial and temporal variability in intertidal assemblages dominated by the mussel Brachidontes rodriguezii (d'Orbigny, 1846)

The mytilid Brachidontes rodriguezii (d'Orbigny, 1846) is the dominant organism on intertidal rocky substrata in warm-temperate shores of Argentina. Beds of this mussel were sampled at Quequen (Q) and Necochea (N) (Argentina), in order to test whether the structure of the associated assemblage shows significant changes among different times of the year, between horizontal and vertical surfaces and between localities situated in the vicinity or away from a source of sewage pollution. Twenty-three macrobenthic invertebrates and 3 species of macroalgae were found in association with B. rodriguezii. Temporal trends in diversity differed according to substrate slope: a significant minimum occurred in November only in horizontal, but not in vertical substrata. This decrease is coincident with massive bleaching and subsequent demise of intertidal algae, suggesting a strong temporal impact of desiccation stress. Significant differences in community structure were found between localities, and between vertical and horizontal samples. Differences in structure were much greater between localities than among replicates within each locality. The polychaete Boccardia polybranchia was extremely abundant at Q, but rare at N, probably reflecting the impact of organic enrichment due to the presence of a neighbouring sewage outfall. Production of B. rodriguezii was significantly higher on horizontal than on vertical substrata. On the other hand, production of the associated biota was an order of magnitude higher on vertical than on horizontal substrata. Organic enrichment, disturbance and competitive interactions are probably the main processes determining structure in this intertidal rocky shore community.