Large-scale surveys on the Florida Reef Tract indicate poor recovery of the long-spined sea urchin Diadema antillarum

The 1983-1984 Caribbean-wide mass mortality of the long-spined urchin (Diadema antillarum Philippi) had severe consequences for many coral reefs. Increases in macroalgal cover, declines in crustose coralline algae and reef corals, lower recruitment by corals, and greater sediment trapping by filamentous algae were well-documented changes resulting from the urchin mortality. The intensity of the effects has been especially great in those areas subjected to intense fishing. However, dramatic changes in coral reef community structure have also been documented in areas such as the Florida Keys where Scaridae and Acanthuridae are not intensively fished. Current patterns on many Florida Keys reefs include a dominance by algae, low coral cover, and low oral recruitment. Historical population densities of Diadema antillarum in the Florida Keys were similar to Caribbean reefs, but declined after the 1983-1984 mortality event, even suffering a second mass mortality in 1991. In contrast to recent observations of D. antillarum population recovery on the north coast of Jamaica, urchin densities remain very low in the Florida Keys. The consequences of the urchin mortality may be manifested in current widespread algal abundance patterns and illustrate that top-down controls on benthic community structure need to be considered. While there is consensus that Florida Keys reefs have changed in recent decades, there is substantial debate concerning the mechanisms. Water quality concerns in Florida are often used to explain increased coverage of algae on reefs, but other factors may be important. This short communication summarizes density and size structure estimates of D. antillarum quantified at multiple spatial scales on the Florida Reef Tract during 1999 (Fig. 1). The synoptic surveys focused on four offshore coral reef and hard-bottom types from 4-12 m depth within and adjacent to no-fishing zones established in the Florida Keys National Marine Sanctuary in 1997. The surveys were part of a broader assessment and monitoring program to evaluate large-scale ecological patterns in reef benthos and the responses of small reef areas to protection from fishing pressure in the Sanctuary.