Consequences of a mass mortality in populations of Eunicella singularis (Cnidaria: Octorallia) in Menorca (NW Mediterranean)

Abstract

At the end of the boreal summer of 1999, many invertebrates in hard-bottom communities in the NW Mediterranean Sea suffered an episode of mass mortality. Our study examined the effects of this event on populations of the temperate octocoral Eunicella singularis at Menorca (Balearic Islands). The event affected colonies over the entire depth range where the species is present (15 to 40 m). Four years after the occurrence of the event, 59% of the colonies still exhibited some damage. The extent of injury of the colonies varied among locations, ranging between 37 and 67% with a mean of 50% of the colony surface. The proportion of dead colonies also varied among locations from 26 to 62%, with an overall mean of 46% of the population. The proportion of dead colonies was inversely correlated with density. Current size distribution of the colonies indicated the occurrence of low recruitment during the years subsequent to the mass mortality event. Our study demonstrates that the 1999 mass mortality event was geographically more extensive than previously thought (Menorca is located about 400 to 700 km from areas where mass mortality was previously reported, along the coast of Provence and Ligury). The mortality rate of E. singularis estimated for the Menorca populations was the highest value recorded to date for this species, showing that the delayed effects of the mortality event exerted a much greater impact on these populations than the immediate effects. Additional impacts result from a reduction in recruitment. These delayed consequences may lead to important changes in community composition, structure, processes and function, as gorgonians are key structural and functional organisms within these communities