The mass mortality of the echinoid Diadema antillarum Philippi in 1983/1984 resulted in dramatic changes in the benthic algal community. This study reports data on the population densities of sea urchins prior to and following the mass mortality and data on the biomass, species composition, and rates of primary productivity of algal communities at several study sites in St. Croix, U.S. Virgin Islands. The mass mortality reduced D. antillarum population densities by 95 to 99%. Population densities were reduced further by a second, less severe mortality event in October 1985. Over a period of 2 yr, recruitment of D. antillarum larvae to the study sites was low and population densities remained at 2 to 30% of their premortality levels. The effects of the mass mortality on the algal community were significant. Algal biomass increased by 22 to 439% across reef zones 16 mo after the die-off. Rates of primary productivity per unit biomass (chlorophyll a) remained at approximately 60% of premortality levels in shallow-reef habitats for 25 mo following the die-off. The magnitude of the response of the algal community was positively correlated with the previous population densities of D. antillarum. The species composition of the algal community also shifted dramatically. Prior to the mass mortality, algal communities were dominated by algal turfs and crustose algae, and macroalgae were either rare or absent. Twenty-five months after the mass mortality, algal turfs covered 40% of the area and macroalgae covered 47%. Many of the macroalgal species are not consumed by herbivores and much of the algae is dislodged by storm waves and is exported from the reef. This represents a transition from a community dominated by a grazing-based food web to one where the majority of primary production may be exported to adjacent communities. © 1990 Springer-Verlag.
