3D architecture evolution of the largest honeycomb reef (Sabellaria alveolata) in the world over two decades

Antoine Collin¹, Laurine Nogue^1,2, Nancy Lamontagne¹, Dorothée James¹, Manuel Lesacher¹, Arnaud Thulie³, Stanislas Dubois⁴

¹EPHE-PSL University, France - ²Institut Agro Rennes-Angers, France - ³Réseau d’Observation Littoral Normandie Hauts de France (ROL), France - ⁴Ifremer, France

Honeycomb (Sabellaria alveolata) reefs provide a cornucopia of ecosystem services such as provisioning ecological niches, dampening waves or stimulating peripheral biodiversity. The largest honeycomb reef (S. alveolata) in the world is located within the macro- (even mega-) tidal bay of Mont-Saint-Michel, 4 km offshore of Sainte-Anne’s chapel, thus its name: Sainte-Anne’s reef. The surrounding envelope of the reef ranges from 2 to 4 m above the sea level corresponding to the lowest astronomical tide, which constrains its 2-hour emerged monitoring to the lowest tide during the spring/fall tides. The reef, actually composed of five distinct sub-reefs, extends over 3 km long, 1 km wide, and vertically ranges from a few centimeters to several decimeters height. Even if the reef’s envelope (in 2D) has been surveyed in its entirety through the aerial imagery for more than 70 years, the reef’s topography or 3D architecture was not collected over that time-series, owing to the absence of ad hoc technology. Nevertheless, Sainte-Anne’s reef has benefited from a first dataset of airborne lidar-derived information acquired in 2002, followed by a second dataset collected in 2018, and finally a third one carried out in 2020. We propose for the first time to analyze the 18-year-old evolution of the 3D architecture of that emblematic reef by (1) computing the height above ground, (2) deriving the volume, and (3) comparing the “initial” and “final” states. The resulting spatially-explicit map of the architectural dynamics highlights erosion and accretion hotpoints. Those original eco-morphometric patterns are discussed in the light of oceanographic parameters driven by global changes and local influences.