COMMUNITY SUCCESSION FOLLOWING MASSIVE ICE-SCOUR ON AN EXPOSED ROCKY SHORE - EFFECTS OF FUCUS CANOPY ALGAE AND OF MUSSELS DURING LATE SUCCESSION

In April 1987, exposed rocky shores near Halifax, Nova Scotia, Canada, were severely scoured by rare sea-ice. These shores are normally dominated by a lush canopy of perennial fucoid rockweeds, with an associated understory of mussels, barnacles and crustose algae. Several observational and experimental studies have been carried out on the succession which has followed this scouring. This paper reports experimental tests of the effects of fucoid canopy algae and mussels on each other and on other species, during later succession. Previous work suggests that, in the absence of predation or disturbance, mussels Mytilus edulis (L.) would increase and outcompete Fucus vesiculosus L. as the dominant space occupant at the mid-shore level. However, during this experiment, 2-3 yr after the scour, abundance of mussels (as understory) decreased. This decrease was dependent on the presence of canopy and may be mediated by whelk carnivory. Mussels had no effect on canopy cover of Fucus vesiculosus L., and caused only a small decrease in Fucus density. Self-thinning appeared to affect Fucus density far more than mussels. Removal of canopy also affected Fucus recruitment, ephemeral algae, grazers and whelks, but did not affect crustose algae or barnacles significantly. In contrast, mussel removal was not found to affect any of these species, except to decrease whelk abundance. These results suggest that the interactions may be more variable than previous work implies. Although much of the Northwest Atlantic rocky seashore is dominated by rockweeds, and rockweeds are thought to be competitively inferior to mussels, we believe this study is the first to directly examine the effects of mussels on rockweeds.