Recovery of an intertidal assemblage following a rare occurrence of scouring by Sea Ice in Nova Scotia, Canada

The recovery of an intertidal assemblage of macroalgae and sessile macrofauna was monitored following a rare occurrence of scouring by sea ice on an exposed rocky shore near Halifax, Nova Scotia, Canada. The ice-scour denuded the substratum at all intertidal levels, and the objective of this study was to determine whether the assemblage would return to its original structure. Sampling was done along a 10 m transect at about yearly intervals from 7 months before to 5 years and 7 months after the ice-scour. Ephemeral algae blanketed the shore soon after the ice-scour, but these were replaced by perennial macroalgae in less than two years. The characteristic zonation of the macroalgae on the shore before the ice-scour, with a successive replacement of Fucus spp. from the high to mid-shore regions and Chondrus crispus low on the shore, was restored in only two years. The cover of macroalgae at mid-intertidal heights (Fucus vesiculosus, F. evanescens) was re-established more rapidly than the cover high (F. spiralis) or low (Chondrus crispus) on the shore. The barnacle Semibalanus balanoides covered less than 10% of the shore before and soon after the ice-scour, and except in the year following the ice-scour when barnacles settled in large densities low on the shore, they were restricted to the mid- and high shore regions. Mussels (Mytilus edulis and/or M. trossulus) covered more than 50% of the shore before the ice-scour, and their rate of recolonisation was inversely related to height on the shore. After about four years, the cover of mussels in the low and mid-shore regions was similar to that before the ice-scour, but even after almost six years the cover of mussels high on the shore remained less than that before the ice-scour. Many of the observations made during the recovery of this assemblage, such as the rate and sequence of replacements of species at particular heights on the shore, paralleled the results of experimental studies done at much smaller spatial scales within similar, established assemblages.