Response of a gorgonian (Paramuricea clavata) population to mortality events: recovery or loss?

Abstract

Abstract During late summer 1999 and 2003 two mass mortality events affected the population of the slow growing, long‐lived Mediterranean gorgonian Paramuricea clavata living in the Gulf of La Spezia (Italy). The population was monitored for three years after the mortality events. Availability of pre‐event data (1998) allowed comparison of population density and population size structure of the healthy population with those recorded in the three years following the mortality events. In 1998, before the two mass mortality events, mean colony density was 33.3 ± 3.7 colonies m−2 and had fallen to 6.7 ± 1.9 colonies m−2 in 2004. In the post‐event period the population size structure changed and the modal class of colonies shifted from 16–21 cm to 6–15 cm height. In 2004 mortality affected 75 ± 6.4% of colonies. A significant, positive correlation between the extent of damage and colony size was found throughout the monitoring period. In the three years following the two mortality events, a small increase in density of recruits and of older undamaged colonies was recorded suggesting that the population was slowly recovering. The bathymetric distribution of P. clavata straddles the summer thermocline making this population particularly sensitive to temperature increases. The lack of deeper colonies (less exposed to warming) and the geographical isolation of this population is likely to prevent any substantial external larval supply. An increased frequency of mass mortality events associated with ever increasingly high temperature events represent a considerable threat to the persistence of a P. clavata population in the Gulf of La Spezia. Copyright © 2007 John Wiley & Sons, Ltd.