Temporal trends of two iconic Mediterranean gorgonians (Paramuricea clavata and Eunicella cavolini) in the climate change context

Gorgonians are iconic species of Mediterranean benthic communities. They are ecosystem engineers and their conservation is essential for the biodiversity of marine communities. Yet these long-lived species are particularly vulnerable to natural or anthropogenic disturbances. The objective of this study is to define the demographic characteristics and health status of the gorgonian forests for two species of gorgonians Paramuricea clavata (Risso, 1826) and Eunicella cavolini (Koch, 1887) over fifteen years. The potential impact of changing environmental conditions is assessed by studying, at different depths, trends in temperature and planktonic changes (phytoplankton biomass and zooplankton abundance) over the study period. Our results indicate that there is a change in population structure of the two gorgonian species with a significant decrease in recruitment in recent years. For E. cavolini, the necrosis significantly increased between 2004 and 2019 (from 9.66% to 25.63% of injured colonies and from 0.24% to 4.75% of dead colonies, respectively). For P. clavata, the population was particularly damaged in 2004 with 14.81% of dead colonies. While necrosis significantly decreased between 2004 and 2014, a significant increase is observed between 2014 and 2019 (from 1.92% to 4.44% of dead colonies). In addition, it appears that large size colonies are more affected by necrosis, and reciprocally. Our main hypothesis is that these changes could be related to the consequences of climate change. Seawater temperature recorded on the same site, over a period of 32 years, shows a significant increase of the number of marine heat waves (MHWs) per year, especially since 2008. In addition, the study of the temperature changes along depth showed fewer and shorter MHWs in deep waters. Disturbance levels observed in these two gorgonian species (shift in population structure, temporal development of necrosis) are discussed in relation to past and present human-induced threats. The quantitative information obtained in this study provides a data baseline precious for future long-term monitoring, appearing as particularly relevant in a context of climate change.