Spatial and temporal variability in the distribution of algae and invertebrates on rocky shores in the northwest Mediterranean

Abstract

This study examines the distribution of algae and invertebrates on rocky shores in the northwest Mediterranean. The main objectives were to: (1) examine the consistency of vertical patterns of distribution in space and time; and (2) determine the relative importance of vertical position on the shore with respect to other sources of spatial and temporal heterogeneity. These patterns were investigated by repeatedly sampling two shores (about 70 km apart) in four different areas (10's to 100's of m apart) at each of three different heights. These areas were sampled eight times between April 1995 and August 1996. At each time, the percentage cover of sessile organisms and the density of molluscan grazers were estimated in three replicate plots (50×10 cm) in each area. The patterns of distribution and abundance of mobile animals were also examined in relation to season and topography of the substratum. At each level on each shore, sampling was done inside crevices in two areas randomly chosen among the four available, and outside crevices in the remaining two areas. This sampling was repeated in summer and in winter, at two randomly chosen dates in each season. There were large spatial and temporal fluctuations in patterns of abundance and distribution. There were inconsistencies in temporal changes from one shore to the other and among areas within levels for several of the organisms considered. Differences from one level on the shore to another were usually large but, in most cases, were not consistent from shore to shore or from time to time. The magnitude of these spatial and temporal effects was at least as large as that due to vertical position on the shore. Season and topography of the substratum were related to differences in the distribution and abundance of molluscan grazers but, again, these effects were variable in space and time. The results were, however, consistent with the hypothesis that limpets may benefit from the presence of crevices in proximity to their upper limit of distribution. Overall, results indicate that the structure of these assemblages cannot be predicted reliably on the basis of vertical position on the shore alone.