Intertidal communities differ between breakwaters and natural rocky areas on ice-scoured Northwest Atlantic coasts

Abstract

This is the first study of the ecological significance of rocky breakwaters as habitat for intertidal biota in marine environments that freeze in winter. Percent cover of intertidal seaweeds and invertebrates was quantified on exposed (high wave action and winter ice scour) and sheltered sides of 18 breakwaters (>5 yr old) and compared with 18 natural rocky intertidal areas along 430 km of the southern Gulf of St. Lawrence coast (Atlantic Canada) in the summer of 2010. Sheltered areas of breakwaters differed from natural rocky shores in having lower biotic richness and total abundance. However, these indices were not significantly different between habitat types for exposed areas. Multivariate analysis revealed significant differences in community composition between breakwaters and natural rocky shores in both sheltered and exposed areas. Ulva spp. (U. intestinalis and U. lactuca), Hildenbrandia rubra, and Mytilus edulis (exposed areas only) were more abundant on breakwaters than on natural rocky shores, while Semibalanus balanoides, Calothrix spp., Fucus spp., Chordaria flagelliformis (exposed areas only), and Ascophyllum nodosum (sheltered areas only) were less abundant on breakwaters. Our study shows that breakwaters from marine shores affected by winter sea ice support substantially different biotic communities than natural rocky intertidal areas. Thus, the findings of this study provide vital information for management decisions related to habitat loss and compensation when the coastal landscape is altered through the construction of breakwaters.