Algal-urchin biomass responses following mass mortality of Diadema antillarum Philippi at Saint John, U.S. Virgin Islands

Abstract

A mass mortality occurred in 1983, killing 95–99% of the Diadema antillarum Philippi in the Caribbean. This study, conducted in Lameshur Bay, Saint John, U.S. Virgin Islands, reports the impact of the mass mortality on Diadema population density, algal biomass, and Diadema body size. Census dates were 6 months before and 6, 12, 18, 26, 30, and 42 months after the mass mortality. There was an initial 100-fold decrease in population density after the mortality. During the first 12 months after the mortality, there was a slight increase in density. During the next 30 months, however, population density remained constant. There was a 30-fold increase in algal biomass 6 months after the mortality. During the next 36 months, however, algal biomass decreased to only 5-fold premortality levels. After the mass mortality, mean and maximum urchin test diameter increased 2- and 1.5-fold, respectively. Median Diadema weight increased 10-fold from 1984 to 1987. During the same time period, Diadema biomass per unit area increased 42-fold. Although postmortality densities may not return to premortality levels for several decades, the increase in body size of Diadema may return urchin and algal biomass to premortality levels much sooner.