Destructive grazing, epiphytism, and disease: the dynamics of sea urchin - kelp interactions in Nova Scotia

We measured the rate of advance of urchin (Strongylocentrotus droebachiensis) feeding aggregations (fronts) as they destructively grazed kelp beds (Laminaria longicruris) at both a wave-exposed site and a sheltered site in Nova Scotia over 3.5 years. The grazing fronts were composed of high densities of large adults (up to 98 and 70 per 0.25 m2 at the exposed and sheltered sites, respectively). Urchins in the recently formed barrens, or in adjacent kelp beds, occurred at much lower densities and consisted mainly of juveniles. The fronts moved onshore into shallower water at each site, but their rate of advance varied markedly between sites and over time at each site, ranging from 0 to 4 m·month-1. The rate of advance of a front was related to the biomass of urchins; fronts did not advance below a threshold biomass of ~2 kg·m-2. Infestations of kelp by an epiphytic bryozoan (Membranipora membranacea) caused marked reductions in kelp canopy cover and biomass during winter, but the canopy regenerated through recruitment of juvenile sporophytes in spring. A localized outbreak of disease decimated S. droebachiensis at the exposed site in 1993, which enabled kelp to recolonize the barrens. Surviving urchins gradually reaggregated and resumed destructive grazing after ~1.5 years. A recurrence of disease in 1995 eliminated urchins at both sites and terminated the transition from kelp beds to barrens on a coastal scale. Our findings have important implications for the management of the urchin fishery, which targets grazing fronts for harvesting.