Shore topography and spatial partitioning of crevice refuges by sessile epibenthos in an ice disturbed environment *

The effect of substratum heterogeneity on the distribution and abundance of sessile epibenthos was studied between May 1980 and September 1981, in a mid-intertidal shore subject to severe annual ice scouring during winter In summer, low abundances of organisms were associated with smooth regular surfaces whereas high abundances were associated with higher heterogeneity in the form of cracks and crevices in the substratum. The barnacle Semibalanus balanoides, the seaweed Fucus vesiculosus and the mussel Mytilus edulis CO-occured in crevice microhabitats. Effect of physical factors (crevice shape and size, exposure to ice erosion) on space partitioning within crevices was examined. The above species occured in distinct zones along the vertical gradient of crevice walls. M. edulis occupied the bottom of the crevices, while S. balanoides and F. vesiculosus colonized the intermediate zone immediately above mussels. The upper limit of all species varied with crevice width (angle formed by the sides) and depth. Upper limit of barnacles is determined by differential juvenile mortality during winter while their lower limit is determined by preemptive and probably interference competition for space with mussels. The near absence of barnacles outside crevices is due to lack of settlement there. The only true refuge for S. balanoidesis in a narrow zone along the crevice walls. The importance of spatial and temporal variations in settlement as well a s varying susceptibilities of different species to disturbances are discussed in relation to space partitioning. The relative importance of spatial heterogeneity on community structure in subarctic, temperate and tropical intertidal environments is compared.