dc.description.abstract | We tracked changes in community composition that occurred with state shifts in the rocky
subtidal ecosystem in Nova Scotia, Canada, from 1992 to 2008. At the beginning of our study, a dense
aggregation of sea urchins was destructively grazing a kelp bed, leaving coralline-algae- dominated
barrens in its wake. In subsequent years, the system underwent a series of state shifts mediated by
mass mortality of sea urchins due to amoebic disease, defoliation of kelp by an invasive bryozoan
Membranipora membranacea, invasion of the green alga Codium fragile ssp. fragile, and finally
decline of C. fragile and recolonisation by kelps. Using multidimensional scaling (MDS), we found 4
macroalgal assemblage types (dominated by kelp, coralline algae, C. fragile, or a transitional
mixture of species), each associated with a distinct invertebrate assemblage. Sea stars and
kelp-grazing gastropods were associated with the kelp-dominated state; sea urchins, chitons and
scale worms were most abundant in the barrens state; and small bivalves and amphipods were most
abundant in the C. fragile-dominated state. Invertebrate diversity remained relatively constant
despite dramatic shifts in the algal assemblage. Although the system was vulnerable to multiple
perturbations between 1992 and 2002, the partial recovery of kelp beds and associated fauna by 2008
demonstrates some resilience in the longer term. | en_US |