Impact of the 1997-98 El Nino event on the coral reef-associated echinoderm assemblage from northern Bahia, northeastern Brazil

The 1997-98 El Nino event was the most severe on record, resulting in record elevated water temperatures across much of the world's tropical ocean regions. This event triggered extensive bleaching of coral reefs world-wide, but little information is available on the impact of this major global event on the non-coral invertebrates associated with reef systems. Here, we report the results of a 6 yr (1995-2000) survey of the echinoderm assemblage from 4 coral reef systems in Bahia, Brazil; the 1997-98 El Nino event occurred in the middle of the survey, allowing impact and recovery to be assessed. At each location, 3 contrasting reef habitats were sampled (intertidal emergent reef tops, coastal reef walls, offshore shallow-bank reefs), with all echinoderm species being identified and enumerated. The El Nino event had a dramatic and consistent impact on the echinoderm assemblage, with a sharp post-El Nino decrease in the number of species from all habitats. However, declines in diversity continued into subsequent years, with the local extinction of the majority of echinoderm species by 2000. In contrast, echinoderm density peaked in 1998, due to opportunistic increases in urchin populations (Diadema antillarum and Echinometra lacunter) across the studied reef systems. Multivariate analysis confirmed a marked change in echinoderm assemblage composition between 1997 and 1998 for all reef habitats, with no evidence of recovery to a pre-El Nino assemblage in the 2 subsequent years. The stresses associated with the El Nino event, in the case of Bahia increases in water temperature and ultraviolet light reaching the reef (reduced cloud cover and turbidity), appear to have had a lethal impact on the majority of reef echinoderm species with no evidence of recovery over 2 yr following El Nino. Increases in urchins are most likely due to migration of individuals onto the reef from deeper areas to exploit the reduced competition and potential increasing algal food resource. The study highlights that El Nino can have severe impacts on reef systems additional to the effect on corals.