ANALYSIS OF SPATIAL AND TEMPORAL VARIABILITY IN INTERACTIONS AMONG ALGAE, LIMPETS AND MUSSELS IN LOW-SHORE HABITATS ON THE WEST-COAST OF ITALY

Spatial and temporal variation in interactions between algae, limpets (Patella spp.) and mussels (Mytilus galloprovincialis) were examined i n low-shore habitats south of Livorno, on the west coast of Italy (wes tern Mediterranean), from August 1993 to February 1995. Experiments we re set up to: (1) contrast the response of the length of fronds and co verage of the brown alga Cystoseira compressa to the removal of algal turfs and mussels in habitats dominated by these organisms; (2) test w hether any effect of mussels on C. compressa was consistent in space ( at scales ranging from 1 to 1000 m) and as a function of the presence of limpets; (3) test for the temporal consistency in any effect of lim pets on the development of macroalgae in habitat dominated by mussels and (4) test for spatial consistency in the temporal patterns of algal recovery in this habitat. Removal of the algal turfs had no effect on the length of fronds of C. compressa; no effect was observed on its c overage but this result was probably confounded by artifacts. In contr ast, mussels had both positive and negative effects on C. compressa. T he mean length of fronds of this alga was significantly reduced by the removal of mussels, this effect being particularly evident after a pe riod of severe storms occurring soon after the initiation of the exper iment. A possible mechanism to explain this could involve the stabilis ation of the basal fronds of the plants, because mussels were often pa cked around them. Conversely, the coverage of C. compressa increased s ignificantly after the removal of the mussels, this effect probably re sulting from the inhibition of recruitment of the alga. The effects of mussels were consistent in space and almost independent of the presen ce of Limpets. In contrast, the herbivores influenced the temporal cha nges in abundance of the coarsely-branched algae and articulated coral lines, but not those of the filamentous and foliose algae or of C. com pressa. Finally, no space-time interactions were found in the analyses of variance, indicating that the course of succession was consistent among areas distributed along a stretch of shore about 1.5 km in lengt h. The results of this study suggest that the assemblage of species do minated by mussels on the rocky coasts south of Livorno was not a high ly interactive system. Most of the biological processes investigated w ere consistent in space and/or in time.