Grazing by two species of limpets on artificial reefs in the northwest Mediterranean.

The extensive presence of artificial reefs in marine coastal habitats demands a better understanding of the extent to which these structures can be considered surrogates of natural rocky shores for populations of plants and animals. The primary aim of this study was to test the hypothesis that removing limpets from the midlittoral of artificial breakwaters in the northwest Mediterranean led to changes in assemblages similar to those observed on rocky shores in the same area. Orthogonal combinations of the presence/absence of two species of limpets, P. aspera and P rustica, were produced using manual removals from June 1997 to February 1998. To test the hypothesis that the effects of limpets were variable at spatial scales comparable to those investigated on rocky shores, we repeated the experiment at two locations tens of kilometres apart, and on two reefs within each location a few kilometres apart. The results revealed strong and relatively consistent negative effects of limpets on filamentous algae, whereas interactions with other members of assemblages were complex and variable. Several taxa (Cyanophyta, encrusting and articulated coralline algae, Ralfsia and Rissoella) were abundant at one location but nearly absent at the other. This large-scale variability in patterns of distribution generated inconsistencies in the effects of limpets between locations. Within locations, several effects of P. aspera and P. rustica were observed, ranging from independent effects on some organisms, to additive or interactive effects on others. Apparently, the removal of filamentous algae by limpets resulted in positive indirect effects on Ralfsia and Rissoella. Collectively, these effects were comparable to those described for rocky shores in the northwest Mediterranean. The processes accounting for large-scale variation in grazing, however, appeared different between the natural and the artificial habitat.