A rapid assessment method to monitor the health status of habitat-forming species in coastal benthic ecosystems

1. Marine habitat-forming (MHF) species in the Mediterranean are among the most threatened coastal species by human activities. In recent decades, different stressors (e.g., warming-induced marine heatwaves and algal blooms) have caused mass mortality events in these key species. Overall, a common method to assess their health status at the Mediterranean Sea scale is lacking. 2. To fill this gap, the aim of this work is to present and validate a cost-effective method, the Mortality Rapid Assessment Method, that is able to assess the health status of key MHF species, even through Citizen Science. 3. The Mortality Rapid Assessment Method is based on determining the impact of mortality on MHF species derived from the metric percentage of affected colonies or individuals. To validate the ability of the proposed method to assess the health status, it was compared to a more commonly used but time-consuming and expert-required metric based on the injured surface percentage of the colonies or individuals. For the validation, one of the most extensive (>47,500 colonies) demographic datasets of the octocoral Paramuricea clavata was used to conduct a comprehensive metric comparison. 4. The results showed a highly significant correlation between metrics from both methods (rho = 0.86), confirming that the percentage of affected colonies provides a reliable assessment of the health status of gorgonian populations over broad spatial and temporal scales. 5. Bearing in mind that this metric can be applied to different MHF species, such as sponges, bryozoans and calcareous algae, and by non-scientific personnel (managers and trained volunteers), its implementation can contribute to inform and enhance the effectiveness of the conservation and management plans for key MHF species at the scale of the Mediterranean Sea.