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

The extensive presence of artificial reefs in marine coastal habitats deman ds a better understanding of the extent to which these structures can be co nsidered surrogates of natural rocky shores for populations of plants and a nimals. The primary aim of this study was to test the hypothesis that remov ing limpets from the midlittoral of artificial breakwaters in the northwest Mediterranean led to changes in assemblages similar to those observed on r ocky shores in the same area. Orthogonal combinations of the presence/absen ce 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 tha t the effects of limpets were variable at spatial scales comparable to thos e investigated on rocky shores, we repeated the experiment at two locations tens of kilometres apart, and on two reefs within each location a few kilo metres apart. The results revealed strong and relatively consistent negativ e 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 ab undant at one location but nearly absent at the other. This large-scale var iability in patterns of distribution generated inconsistencies in the effec ts of limpets between locations. Within locations, several effects of P. as pera and P. rustica were observed, ranging from independent effects on some organisms, to additive or interactive effects on others. Apparently, the r emoval of filamentous algae by limpets resulted in positive indirect effect s on Ralfsia and Rissoella. Collectively, these effects were comparable to those described for rocky shores in the northwest Mediterranean. The proces ses accounting for large-scale variation in grazing, however, appeared diff erent between the natural and the artificial habitat. (C) 2000 Elsevier Sci ence B.V. All rights reserved.