Analysis of spatial and temporal variability in interactions among algae, limpets and mussels in low-shore habitats on the west coast of Italy

Abstract

Spatial and temporal variation in interactions between algae, limpets (Patella spp.) and mussels (Mytilus galloprovincialis) were examined in low-shore habitats south of Livorno, on the west coast of Italy (western Mediterranean), from August 1993 to February 1995. Experiments were set up to: (1) contrast the response of the length of fronds and coverage of the brown alga Cystoseira compressa to the removal of algal turfs and mussels in habitats dominated by these organisms; (2) test whether 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 limpets on the development of macroalgae in habitat dominated by mussels and (4 ) test for spatlal consistency in the temporal patterns of algal recovery in this hab~tat. Removal of the algal turfs had no effect on the length of fronds of C. compressa; no effect was observed on its coverage but this result was probably confounded by artifacts. In contrast, mussels had both positive and negatlve effects on C. compressa. The mean length of fronds of this alga was significantly reduced by the removal of mussels. this effect being particularly evident after a period of severe storms occurring soon after the initiation of the experiment. A possible mechanism to explain this could involve the stabilisation of the basal fronds of the plants, because mussels were often packed around them. Conversely, the coverage of C. compressa increased significantly after the removal of the mussels, this effect probably resulting from the lnhlbition of recruitment of the alga. The effects of mussels were consistent in space and almost independent of the presence of limpets. In contrast, the herbivores influenced the temporal changes in abundance of the coarsely-branched algae and articulated corallines, but not those of the filamentous and foliose algae or of C. compressa. 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 length. The results of this study suggest that the assemblage of species dominated by mussels on the rocky coasts south of Livorno was not a highly interactive system. Most of the biological processes investigated were consistent in space and/or in time.