Spatial variability of the infauna adjacent to intertidal rocky shores in a subtropical estuary

Negative responses of infauna close to rocky substrates are well known to subtidal bottoms, but there are few studies addressing similar intertidal habitats. We tested the hypothesis that the proximity to rocky shores negatively affects the density and richness of intertidal infauna of tidal flats by assessing infaunal variation across the increasing distances from rocky shores in two tidal flats (Pasto and Limoeiro) of a subtropical estuary in southern Brazil. Total density decreased significantly with the proximity to rocks only in Pasto due to density variations in the two numerically dominant species, the bivalve Anomalocardia flexuosa and the polychaete Armandia hossfeldi. Richness decreased significantly with the proximity to the rocky shores only in Limoeiro. Multivariate analyses revealed significant differences in species and functional groups composition between assemblages near and far from the rocky shores. Assemblage variability patterns were mostly explained by sediment variables. Our hypothesis was partially refuted because negative effects on the infauna were spatially inconsistent, due to differences in species composition between tidal flats, and to distinct responses of individual taxa. Even with the absence of consistent response patterns, the proximity to the rocky shores emerged as a relevant structuring factor of the surrounding intertidal infauna.