Thomas Burel1, Wendy Diruit1, Rémi Brosse1, Jacques Grall2, Gauthier Schaal1, Michel Le Duff2, Georges Chapalain3, Nicolas Guillou3, Erwan Ar Gall1
1 Univ Brest, CNRS, IRD, Ifremer, LEMAR, 29280, Plouzané, Brittany, France
2 UAR 3113, Observatoire Marin, Université de Brest, 29280, Plouzané, Brittany, France
3 Laboratoire de Génie Côtier et Environnement (LGCE), Cerema, DtecEMF, ER, 155 rue Pierre Bouguer, Technopôle Brest-Iroise BP 5, 29280, Plouzané, Brittany, France
Wave exposure is commonly recognized in ecology as one of the most influential environmental factors conditioning the distribution and development of coastal seaweeds. Studies relying on a single proxy, either field-measured or derived from coastline topography, to account for exposure, face the risk to produce biased, qualitative and overall poor-quality estimates. We assessed the performance of 8 hydrodynamics proxies (based on biological assemblages, topography, in situ measurements, remote sensing, GIS and physical models) along 12 coastal sites of Brittany (NW-France) at four seasons. Mid-shore benthic community structure was described at low tide, revealing the presence of 90 seaweed species and 26 faunal taxa. Two types of mid-shore seaweed communities were identified, either dominated by Ascophyllum nodosum or by Fucus vesiculosus, clearly distinguished by their canopy-forming and understorey algal and faunal species. Positive correlations were observed between model and GIS-based proxies, while a negative correlation was observed between the biological scale and remote sensing-based proxy. No clear relationship could be highlighted between biological communities and any single hydrodynamics proxy. Hydrodynamics conditions macroalgal communities in two different ways: 1- through local exposure, directly limiting the development of canopy-forming species, and 2- through offshore exposure, which can regulate water bodies characteristics (e.g. temperature, nutrients) and consequently the species that inhabit it. Our results suggest that the use of multiple proxies, integrating multiple operating scales, is the most promising approach when trying to account for the complexity of this major structuring factor of seaweed communities.