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

摘要

We measured the rate of advance of urchin (Strongylocentrotusdroebachiensis) feeding aggregations (fronts) as they destructively grazedkelp beds (Laminaria longicruris) at both a wave-exposed site and asheltered site in Nova Scotia over 3.5 years. The grazing fronts werecomposed of high densities of large adults (up to 98 and 70 per 0.25 m(2)at the exposed and sheltered sites, respectively). Urchins in the recentlyformed barrens, or in adjacent kelp beds, occurred at much lower densitiesand consisted mainly of juveniles. The fronts moved onshore into shallowerwater at each site, but their rate of advance varied markedly betweensites and over time at each site, ranging from 0 to 4 m.month(-1). Therate of advance of a front was related to the biomass of urchins; frontsdid not advance below a threshold biomass of similar to 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 inspring. A localized outbreak of disease decimated S. droebachiensis at theexposed site in 1993, which enabled kelp to recolonize the barrens.Surviving urchins gradually reaggregated and resumed destructive grazingafter similar to 1.5 years. A recurrence of disease in 1995 eliminatedurchins at both sites and terminated the transition from kelp beds tobarrens on a coastal scale. Our findings have important implications forthe management of the urchin fishery, which targets grazing fronts forharvesting. [References: 54]