Population collapse of habitat-forming octocorals in Scandola Marine Protected Area: the long-term study of gorgonian populations affected by the 2003 extreme warm summer

Abstract

Mediterranean gorgonians are foundation species that support the high diversity associated with Mediterranean coralligenous assemblages. Through the formation of 3D structures, they provide structural complexity and thereby habitat and refugia to numerous associated species, among other ecological benefits. However, these organisms are especially vulnerable to the increasingly frequent and intense marine heatwaves (MHWs). In recent decades, MHWs deeply impacted octocoral species such as the red coral Corallium rubrum and the red gorgonian Paramuricea clavata across large geographical scales, leading to mass mortality events (MMEs). Although the MME immediate and mid-term (3-4 years) effects on the population dynamics of these species are well documented, a long-term evaluation of the impacted populations trajectories is still lacking. This hinders our understanding of the recovery capacity of these key species, and thus ofthe potential delayed effects that MMEs could have on their associated communities. In this study, we performed a long-term status assessment of different populations of P. clavata and C. rubrum that were affected by the MME occurred in the summer of 2003in Scandola Marine Protected Area (MPA), Corsica. Our results show that since 2003, impacted populations have followed different trajectories in terms of density, size structure or tissue necrosis, but a general progressive reduction in biomass. This indicates that the affected populations have no recovered over almost two decades, despite the high protection regime existing in the MPA. Furthermore, many populations followed collapse trajectories that have brought them to the brink of ecological extinction, probably due to the MHW recurrence observed since 2003. Since Mediterranean octocorals play a unique role on the functioning of coralligenous assemblages, the long-term delayed effects of the MME observed in our study underpins the relevant role that stochastic climatic events can play not only on their population dynamics, but also for the future functioning of coralligenous assemblages