Assessing the impact of diving in coralligenous communities : The usefulness of demographic studies of red gorgonian populations

Abstract

[EN] Coralligenous communities are an important “hot spot” of Mediterranean biodiversity. These communities are presently threatened by a combination of anthropogenic impacts partly related to global change. Managers of MPAs are increasingly concerned about the capacity of coralligenous communities to endure crowds of divers, and there is a call for descriptors and for quantitative methods for estimating this capacity. A good indicator to detect the impacts on coralligenous communities should gather the following essential traits: to be a long-lived, clonal, engineering species displaying an erect shape and a large size in order to facilitate the monitoring. Among the main taxonomic groups of the coralligenous, gorgonians display several advantages and, in this study, the red gorgonian Paramuricea clavata was selected as a model species to evaluate the effects of diving impact and climate change on coralligenous communities. We present three cases of study about the demographic monitoring of red gorgonian populations affected by the 1999 mass mortality event (Port-Cros) and subjected to high diving activity (Medes Islands and Cap de Creus). Finally, combining all these data we simulate the interacting effects of divers damage and mass mortality events on the persistence of gorgonian populations. The results of long-term monitoring of permanent plots allowed showing that although diving activity affects significantly the red gorgonian populations, the effect of climate change has more significant effects on the populations. Port-Cros populations affected by the 1999 mass mortality suffered a 60% biomass loss that has not been recovered in the following eight years. Simulations showed that the combination of both disturbances can have large and lasting effects on the future viability of red gorgonian populations. As gorgonian assemblages are only one facies of the coralligenous communities, other species should be selected for detecting the impact on these communities. Nevertheless, the development of similar demographic approaches for other species would be more difficult than for gorgonians