Habitat fragmentation in marine macroalgal forest conservation: implementing remote sensing techniques for high-resolution subtidal landscape characterisationNovel Approaches

Anaïs Barrera^1,2, Sònia de Caralt^1,2, Emma Cebrian^1,2, Jordi Boada², Cèlia Sitjà², Jorge Santamaria²

¹Institute of Aquatic Ecology, Spain - ²Center of Advanced Studies of Blanes, Spain

Coastal areas are the most populated regions in the world suffering rapid environmental changes derived from human action. Multiple impacts threaten the conservation of coastal habitats such as the macroalgal forests on shallow rocky reefs. As a result, these valuable and productive ecosystems experience a loss of continuity and are replaced by diverse configurations ranging from small loss of connectivity to areas with severe fragmentation, associated with impacts on the ecosystem functioning. Given the widespread nature of habitat loss and the high ecological value of these habitats, it is crucial to develop and implement efficient techniques that allow us to accurately assess the degree of fragmentation. This, in turn, will enable us to determine the consequences of fragmentation on the conservation status and vulnerability of these ecosystems. In this sense, the use of drones has been successfully applied as a cost-effective method for mid-to-large spatial scale intertidal and shallow subtidal coastal mapping of high resolution. In this study, we use shallow macroalgal forests as a case study to assess the suitability of drones for measuring habitat fragmentation. To conduct this assessment, we identified sites with the presence of shallow macroalgal forests with three levels of fragmentation (high, medium and low) in two different regions in the Mediterranean Sea: the Catalan Coast and the Balearic Islands. In each site, we measured the fragmentation of macroalgal forests dominated by the canopy-forming species (Ericaria sp.) both from high-resolution orthomosaics using UAV (i.e. drones) and the characterized forest patterns by in situ 25m SCUBA transects using the contact point method (N=3 per site and fragmentation level). From these data we developed a fragmentation index based on the size and frequency of forest patches and the ratio of corridors between patches where the focal species was absent. We compared the fragmentation patterns obtained from the drone surveys with the in situ data and quantified the robustness of the method. Our results show that high-resolution mapping from remote-sensing techniques are a powerful tool to detect the spatial distribution of structural macroalgae in shallow subtidal systems, which can be crucial to efficiently determine the fragmentation of these habitats and apply it to the monitoring and conservation of these coastal ecosystems. This is, to our knowledge, one of the first instances of the application of remote-sensing techniques to ecological processes beyond habitat identification.