Rapid succession in subtidal understorey seaweeds during recovery from overgrazing by sea urchins in eastern Canada

Cover and biomass of understorey seaweeds were followed over two growing seasons at 10 m depth at two sites in St Margaret's Bay in eastern Canada during a period of recovery from destructive grazing by sea urchins. At one site a complete canopy of the kelp Laminaria longicruris de la Pylaie had been re-established while at the other there was none. Recolonization by understorey species was rapid. Moreover, the same understorey species occurred at both sites in both years and there was no evidence of a seral replacement of macroalgal species in the development of these communities. Thus, in the first growing season after major disturbances, succession can proceed rapidly from microalgae to a community of macroalgae that is similar in structure to long standing pre-disturbance assemblages. There were consistent broad scale trends in seasonal abundances but there was also considerable variation in the quantitative abundances of many species among years within sites and among sites within years. An experimental introduction of sea urchins within an enclosed area led to the elimination of understorey species within 3 months. When this population of urchins began to decline as a result of infection with a paramoeboid pathogen, macroalgae began to recolonize even while urchins were still present and grazing actively, so that diversity and cover of seaweeds had recovered to ca. two-thirds of that of ungrazed controls by the time the urchin population reached zero. There was no evidence that the grazing of high densities of limpets [Notoacmaea testudinalis (Linnaeus)] and chitons [Tonicella rubra (Linnaeus)] limited macroalgal growth.