Immediate and delayed effects of a mass mortality event on gorgonian population dynamics and benthic community structure in the NW Mediterranean Sea

Abstract

In the boreal summer of 1999, many invertebrates of hard-bottom communities in the NW Mediterranean Sea suffered mass mortality. Our study assessed the population of the temperate octocoral Paramuricea clavata before the event and monitored the population over the following 4 yr. Spatial patterns showed decreasing mortality with increasing depth between 0 and 50 m, as well as high local variability. The temporal pattern was characterized by a sharp decrease in biomass (58%) shortly after the event caused by the combined effect of colony death (9% of the population) and an increase in the extent of colony injury (from 9% before the event to 52% shortly after it). After 4 yr of monitoring, our results indicated a large delayed effect of the event. Population density decreased continuously after November 1999, and by the completion of the study in November 2003 the accumulated density decrease was 48% of the initial population. This decrease was mainly due to the death of colonies subjected to extensive injury, and because recruitment did not offset mortality. After November 1999, biomass continued to decrease at a slow rate, becoming almost constant after November 2001. Overall, the delayed effect of the event accounted for a 70% loss in P. clavata biomass. The fact that a stabilization of the density and biomass of this species was observed during the last year of our study suggests that the delayed impact of the 1999 mass mortality event may be nearing its end. Nevertheless, given the low dynamics of P. clavata and its role as a habitat former, the delayed effect of the mass mortality event indicates the relevant role that disturbance can play on the population dynamics of this species and as a community structuring force on the coralligenous community. © Inter-Research 2005.