ABSTRACT
An outbreak of disease in the fall of 1983 resulted in the elimination of Strongylocentrotus droebachiensis along 108 km (straight-line distance) of the Atlantic coast of Nova Scotia. This extends the range of complete mortality of echinoids (above 15m depth) to 280 km of coastline between 1980 and 1983. Partial mortality (estimated as 50%) has occurred along another 220 km of coastline during this period, Mortality is estimated as 246 thousand metric tons (wet weight) or 25 billion individuals in the area of complete die-off and 96 thousand metric tons or 10 billion individuals in the area of partial die-off. The elimination of echinoids and concomitant emergence of macroalgae have occurred along different parts of the coast in different years since 1980. Macroalgal communities in areas where echinoids died off one, three and four years previously were compared to existing echinoid-dominated barren grounds and a mature kelp bed where echinoids have been rare or absent for decades. Changes in species’ composition, and increases in macroalgal cover, biomass, and density and size of kelp species, characterize the succession from barren grounds to three and four year old kelp (Laminaria) beds. The greatest change occurred between one and three years following echinoid mass mortality. Macroalgal biomass increased from 0.8 kg m−2 at one year sites to 7.5 kg m−2 at three and four year sites due to the development of a dense canopy of L. longicruris and L. digitata. Macroalgal species diversity (H’) was highest at one year sites; diversity was lowest at three and four year sites due to dominance by Laminaria. The estimated increase in macroalgal biomass along 280 km of coastline affected by the echinoid die-off is 2.2 million metric tons. The emergence of kelp beds in these areas reverses another major transition which occurred 1-2 decades earlier, i.e., the destructive grazing of kelp by expanding echinoid populations. Large-scale reciprocal fluctuations in echinoid and macroalgal abundance may characterize the trajectory of a dynamic system which cycles between two alternative community states: kelp beds and echinoid-dominated barren grounds. Disease, by periodically regulating echinoid abundance, may be an important mechanism underlying this cyclicity.
