Spatial and temporal variability of patterns of colonization by mussels (Mytilus trossulus, M-edulis) on a wave-exposed rocky shore

Colonization rates of mussels (Mytilus trossulus and M. edulis) were measured on natural substrata in tidepools and on emergent rock in recently ice-scoured and non-scoured regions of a rocky shore near Halifax, Nova Scotia, Canada. The relative importance of initial settlement/colonization, compared to subsequent dispersal and mortality, in determining the distribution and abundance of mussels was examined by comparing patterns and rates of mussel colonization at sampling intervals of days to months over a 17 mo period. Less than 4% of mussels which colonized the quadrats sampled at short (2 to 7 d) intervals were settling larvae (<0.5 mm shell length), indicating the importance of postsettlement dispersal of mussels on this shore. Most colonists were >2 mm, too large to be dispersed by byssal drifting, suggesting they were redistributed by wave dislodgment and deposition. At both short (2 to 7 d) and long (5 to 16 mo) sampling intervals, colonists were most abundant in ice-scoured tidepools and least abundant on ice-scoured emergent rock, probably reflecting differences in the macrobenthic assemblage, the substratum for colonization. In addition, the long term abundance of colonists was linearly related to the cumulative short term abundance during all but one of the intervals. Therefore, our results indicate that, over time scales up to 16 mo, patterns of initial colonization by settlers and larger post-larval mussels were more important than post-colonization mortality and dispersal in determining patterns of distribution and abundance of mussels on this shore.