Importance du régime de dénudation sur la structure et la succession des communautés intertidales de substrat rocheux en milieu subarctique

Abstract

Developmental patterns were studied following natural (abrasion by ice) and experimental disturbances (total or partial clearings) in intertidal rocky communities. Four semi-exposed communities along the St. Lawrence Estuary and Gulf were studied from May 1977 to May 1979. In the spring, when free space becomes available, annual algae, mainly Ulothricales and Ulvales, rapidly colonize areas denuded by ice abrasion. This early stage is soon followed by an abundant fucoid growth, largely Fucus vesiculosus but also some Ascophyllum nodosum, and a large number of Balanus balanoides, along with some Mytilus edulis. Opportunists, F. vesiculosus and B. balanoides, dominate where ice disturbances are severe and regular but where the community is not severely disturbed, initial differences in the abundance of F. vesiculosus and A. nodosum are reduced, and the community becomes closer to the mature one dominated by A. nodosum. The period of disturbance influences the species' settling and their abundance. The earlier the disturbance in the spring, the greater will be the colonization by the opportunists Ulothricales, Ulvales, and B. balanoides, but when the disturbance occurs later, F. vesiculosus and M. edulis are the main settlers. The tidal level influences mainly the establishment of dominance. In the mid zone, few species are present, densities are low, competition is not an important controlling factor, and Balanus dominate. In the low zone, barnacles are replaced by mussels, the number of species and densities are high, and competition is greater. Exclosure and enclosure experiments using cages have shown that littorinid grazers have a negligible influence on the reduction of the dominant annual alga Ulothrix flacca. Its reduction in midsummer appears to be related to its life cycle. Predators were rarely observed at the study sites. Structural and developmental patterns of intertidal rocky communities were slow and the monopolization of space by A. nodosum and M. edulis, as observed in the mature, less disturbed community, must result from a long period [Formula: see text] without major disturbances.