Mass mortality of coral reef ascidians following the 1997/1998 El Nino event

In April/May each year from 1995 to 2000, ascidians were sampled randomly with 35 1m(2) quadrats from three different reef habitats (intertidal reef tops, coastal reef walls and shallow-bank reefs) at four replicate localities (Praia do Forte, Itacimirim, Guarajuba and Abai) in northern Bahia (Brazil). As the sampling period included the 1997/1998 El Nino event, the most severe on record, for the first time these results allow a quantitative assessment of the impact of this major environmental stressor on the biodiversity of associated coral reef ascidians. Across all reef habitats, 22 ascidian species were recorded from three different orders (Aplousobranchia, Phlebobranchia and Stolidobranchia). After El Nino, all species showed significantly altered densities (ANOVA, F=602.90, p < 0.0001); many species were absent from the reefs within 2 years of the El Nino period, but densities of Lissoclinum perforatum (all reefs) and Echinoclinum verrilli (subtidal reefs) increased significantly from 1998 onwards. Univariate and multivariate analyses confirmed that significant changes in assemblage composition had occurred. BIOENV analysis identified turbidity, mean temperature and cloud cover as the main factors best explaining these assemblage changes. Our results suggest that although the 1997/1998 El Nino had a differential effect on the species contributing to the ascidian assemblage of Brazilian coral reefs, most species disappeared and those remaining are likely to enhance reef degradation through their bioeroding activities.