The interplay of physical and biological factors in maintaining mid-shore and low-shore assemblages on rocky coasts in the north-west Mediterranean

This study examined the interactive effects of grazing by limpets and incli nation of the substratum in maintaining differences between mid-shore and l ow-shore assemblages of algae in the northwest Mediterranean, at different scales of space and through time. Alternative models leading to different p redictions about these effects were proposed and tested, Limpets were exclu ded by fences from areas of the substratum at mid levels on the shore. The response of algal assemblages to this manipulation was compared with contro l and enclosure plots at the same level, and with unmanipulated plots in th e low shore where limpets are less abundant, The effects of limpets were ex amined at several replicated sites (0.1-4 km apart) for each slope of the s ubstratum (nearly horizontal vs vertical), at different locations (hundreds of kilometres apart) and at different times. Individual taxa responded dif ferently to limpet exclusion. The percentage cover of the coarsely branched and filamentous algae increased significantly in exclosure plots, in some loser reaching values found on the low shore. These patterns, however, vari ed greatly from shore to shore and significant effects were found both on h orizontal and vertical substrata. Multivariate analyses indicated that graz ing by limpets accounted for about 20% of the differences between mid-shore and low-shore assemblages. This effect was independent of substratum incli nation and was consistent in space and time, suggesting that physical condi tions were not as stressful for macroalgae on vertical substrata as initial ly supposed, Variable recruitment of algae is proposed as a possible explan ation for the lack of consistency in the effects of limpets at the scale of the shore. The results of this study emphasize the need for multiple-scale analyses of the interactive effects of physical and biological factors to understand the organization of natural assemblages.