New Insights into the Functioning of Mediterranean Rocky Reef ecosystem When Algal Forests Are Lost

In the Mediterranean, overfishing of large predatory fish, particularly Diplodus spp., can cause severesea urchin outbreaks, leading to significant shifts in benthic communities. Variations in sea urchingrazing intensity may trigger transitions between complex algal-dominated states ("Forest") andsimpler, sea urchin-dominated states ("Barren"). Barren states, which are characterized by low diversityand productivity, can persist for long periods, even within marine protected areas (MPAs). The recoveryof key sea urchin predators in MPAs is slow due to initial overexploitation, resulting in persistent seaurchin population explosions.Forest states, with their rich biodiversity, transfer substantial quantities of carbon, nitrogen, andphosphorus to coastal food webs, whereas barren systems have lower diversity and productivity.However, recent studies suggest that barren systems can provide microhabitats for various cryptic andinvertebrate species, which may contribute to maintaining the barren state. The shift between forest andbarren states affects the entire food web, altering growth, feeding dynamics, and energy flow.This thesis aims to elucidate the functioning of forest and barren states in Mediterranean rocky reefsthrough a multidisciplinary approach, including descriptive, isotopic, mass balance, and molecularanalyses.1. Descriptive and Isotopic Approach: The study tested the hypothesis that corallinebarrens enhance benthic megafauna abundance and diversity. Isotopic analyses revealed a comparabletrophic structure between the two states, with higher isotopic uniqueness in barrens, primarilyinfluenced by sea urchins and carnivorous starfish. Results indicated that coralline barrens support adiverse benthic megafauna, challenging the notion of barrens as low-diversity habitats.2. Mass Balance Approach: Food-web models of algae forests and urchin barrens,representing pristine and collapsed states of rocky reefs, were developed. Both states showeddominance of low trophic level consumers and significant energy flow through detritus. Despitedifferences in primary production and energy utilization, both states exhibited similar complexity andstability. This highlights the importance of understanding ecosystem dynamics for effectivemanagement and conservation.3. Molecular Approach: Investigating the persistence of barren states despite predatorrecovery, this study explored the role of micropredation in controlling sea urchin populations. Specificprimers for detecting mtDNA of sea urchins Paracentrotus lividus and Arbacia lixula were designed.Testing invertebrates collected during urchin settling events identified potential micropredators,suggesting that micropredation may help maintain the forest state by controlling sea urchin populations.The findings provide insights into the structure and functioning of rocky reef ecosystems in differentstable states, underscoring the need for comprehensive management strategies to preserve these criticalhabitats.