Biogeographical patterns in intertidal macroalgal communities along the coasts of Normandy (Channel Sea)Reef-associated Biodiversity

Mathieu Helias^1,2, Julie Camus¹, Pascal Bailly du Bois^1,2, Antoine Collin³, Emmanuel Poizot^1,2, Régis Gallon-Garcia^1,2

¹Conservatoire National des Arts et Métiers, INTECHMER, France - ²Université de Caen, France - ³Ecole Pratique des Hautes Etudes, France

Habitats formed by intertidal macroalgae are highly productive zones that support significant biodiversity. These habitats are monitored along the French coasts as part of the Water Framework Directive and the Marine Strategy Framework Directive. Despite a long historical background in phycology, both long- and short-term variations in intertidal macroalgal communities remain poorly understood along the Normandy coasts. The Normandy coastline is notable for its diverse geological features and pronounced tidal hydrodynamics, combined with substantial anthropogenic pressures. This study aims to describe biogeographical patterns in intertidal macroalgal communities along the Normandy coasts to refine specific habitat typologies.

Twelve sites were monitored over four seasons along the Normandy coastline. The structure and composition of macroalgal communities were characterized through in situ sampling, and zonation patterns were investigated using the Tidal Height Normalization (THN) protocol. Over 250 species were identified during the survey. A biodiversity gradient was observed, with a clear distinction between North Cotentin, where cartilaginous rhodophytes dominate sub-canopies, and the rest of Normandy, which is characterized by filamentous and turf-forming species. However, fewer differences were observed in community structure, as estimated by the Index of Community Structure. The causes of local and regional variations are currently under investigation, with hydrodynamics and anthropogenic pressures expected to play significant roles. The intertidal zones of North Cotentin appear to be the most vertically structured, supporting up to six distinct communities on the shore. THN values highlight local colonization patterns and emphasize the importance of geo-morphological factors, such as local geology (e.g., granite versus chalk shores).

In the context of the EQAMAT project, these results will enrich an in situ database that, combined with in silico imaging (drone/aircraft), will enhance mapping products and ecological status assessments of intertidal macroalgal communities.