Influence of red gorgonian (Paramuricea clavata) forests on coralligenous community structure across different regions, seasons, thermal environment, and water trophic state

Habitat-forming species are essential for maintaining biodiversity and ecosystem functioning in marine benthic ecosystems, but their role in modulating community responses to environmental stressors across spatial and temporal scales remains underexplored. In the Mediterranean Sea, the red gorgonian Paramuricea clavata forms structurally complex animal forests within coralligenous assemblages, able to enhance habitat heterogeneity. This study assessed the influence of P. clavata forests on the structure of coralligenous communities across five Mediterranean regions and two periods (summer and autumn), incorporating thermal (median temperature, temperature range, frequency and duration of heating events) and trophic (chlorophyll, pheophytins, carbohydrates, proteins, organic and inorganic suspended matters) environmental descriptors. Using Generalized Linear Latent Variable Models, we found that environmental variables—particularly temperature, organic matter, and trophic resource availability—strongly shaped coralligenous community structure. Forested sites exhibited a reduced abundance of opportunistic groups (e.g., turf algae, hydroids, perforating sponges) under conditions of elevated temperature and organic matter concentrations, highlighting the buffering role of P. clavata forests. However, under intense and prolonged thermal stress combined with high concentrations of carbohydrates and proteins, P. clavata itself showed increased necrosis, indicating limits to its resilience. Considering the regional and temporal scales, region emerged as the strongest predictor of community structure (explaining 45% of the variation in group abundance), followed by environmental variables (19%) and period (12%). Further, the presence of P. clavata forests significantly increased species richness, Shannon diversity, and evenness. These findings emphasize the importance of P. clavata forests in promoting ecological stability and highlight the vulnerability of this foundation species to climate-driven stressors. Understanding these dynamics is pivotal for predicting future changes in biodiversity and for informing conservation strategies targeting Mediterranean coralligenous reefs.