How resilient are Mediterranean algal forests? New insights based on the brown alga Gongolaria barbata (Stackhouse) Kuntze

Although representing only 0.82% of the world ocean surface, the Mediterranean Sea is a major biodiversity hotspot, hosting about 17,000 marine species among which more than 1,200 macroalgae. Forests formed by brown seaweeds of the genera Cystoseira, Ericaria and Gongolaria (Cystoseira s.l. complex) are a key repository of marine biodiversity in this basin. These communities, widely distributed in the shallow subtidal zone, play many important ecosystem functions. They are, however, impacted by numerous local anthropogenic stressors that have caused their large-scale regression. Additionally, the enclosed nature of the Mediterranean makes them highly vulnerable to climate-related stressors, particularly heat waves. Gongolaria barbata is a canopyforming species of this complex with widespread distribution, but typically occurring in fragmented and geographically isolated populations; as such, it is at risk of local extinction. We are investigating in depth its biology, in order to predict its future responses and plan measures of conservation and restoration. Studies combining microscopic investigations and DNA barcoding and metabarcoding confirm that G. barbata canopies support great biodiversity. Population genetic studies confirm a strong spatial structure and limited connectivity between populations of different regions. Experiments of thermal tolerance showed that recruits of this species are relatively resistant to thermal stress, being able to survive temperatures up to 30°C; however, prolonged periods with temperature above this limit are likely to be lethal. Field-based experiments suggest that recruitment is a critical phase in the development of this species, and is severely affected by high sediment loads and competition with ephemeral algae. Overall, our results suggest that the species is likely to face further future decline and local extinctions, with major losses of associated diversity.