Effects of Local and Global Stressors on the Status and Future Persistence of Intertidal Canopy-Forming Algae

Canopy-forming seaweeds are worldwide disappearing due to the combined effects of human activities and climate instabilities. Identifying the type and strength of interactions between multiple anthropogenic and natural stressors can help setting achievable management targets for degraded ecosystems and support ecological resilience through local actions. This thesis aimed to understand how algal forests change from extensive to degraded, and what factors can enhance the ability of forests to withstand or recover from stressors. I contributed to a systematic review to infer potential important synergistic stressors interactions driving the loss of canopy-forming seaweeds at a global level. We found that management of excess nutrient levels would provide the greatest opportunity for preventing the shift from canopy to mat-forming algae, because of the higher prevalence of synergistic interactions between nutrient enrichment with other local and global stressors. Then, I focused my attention on fucoid algae of the genus Cystoseira that are the most typical canopy-forming seaweeds in the Mediterranean Sea. I explored which environmental and anthropogenic factors can explain the current status of the intertidal Cystoseira populations. I found that coastal urbanization and nutrient concentration were the factors most related to the status of Cystoseira. Finally, I carried out a series of manipulative field experiments to explore the effects of nutrient enrichment and heat-wave events on intertidal C. compressa. The results showed that C. compressa is sensitive to heat-wave events and that local biodiversity and thermal history of the alga seem to play a role reducing or increasing respectively the impact of such extreme events. I also characterised the epiphytic bacteria associated to the surface of C. compressa and showed their potential influence on the responses of C. compressa to environmental stressors.