Three-quarter century morphological dynamics of the largest honeycomb reef (Sabellaria alveolata) in the world

Laurine Nogue^1,2, Antoine Collin¹, Manuel Lesacher¹, Nancy Lamontagne¹, Dorothée James¹, Stanislas Dubois³

¹EPHE-PSL University, France - ²L’institut Agro Rennes-Angers, France - ³IFREMER, France

Biogenic reefs engineered by the gregarious marine worms of Sabellaria alveolata can either take place on hard substrates in the form of veneers, or on soft (sandy to muddy) sediments in the form of hummocks until platforms. The megatidal (a 14-m amplitude during the spring/fall tide) bay of Mont-Saint-Michel hosts the largest clustering of honeycomb reefs growing on tidal flats at the world-scale. The clustering is called Sainte-Anne’s reef since it is located 4 km offshore of Sainte-Anne’s chapel, which lies on the Brittany shoreline. Surrounded by sandy and mud flats, that reef is characterized by horizontal dimensions of 3 km long and 1 km wide, as well as a vertical variability spanning 1 m height. It is actually composed of five sub-entities : an easternmost small but high sub-reef; two extended central sub-reefs in the shape of a butterfly; a southernmost retro-sub-reef; and a westernmost sub-reef. Evolving between 2 and 4 m above the chart datum (that is to say the lowest astronomical tide level in France), Sainte-Anne’s reef can only be monitored during two efficient hours during the equinoctial lowest tide levels, when emerged. Several airborne mapping campaigns were carried out since World War II, offering an insighful (ecologically-) landscape approach to survey the reef. Our innovative research aims to better understand the morphological dynamics of that world-class honeycomb reef by harnessing airborne imagery since 1947. First, the compilation of aerial (either panchromatic or colored) imagery was achieved. Second, the mosaicking of overlapped imageries were undertaken using photogrammetry. Third, a rigorous georeferencing was applied. Fourth, the envelope of contiguous honeycomb patches was delineated for 1947, 1966, 1973, 1995, 2000, 2014, and 2020. Spatial patterns have emerged across the last eight decades: the sustainability of the easternmost sub-reef; the erosion and sharpening of both central largest sub-reefs; the establishment and densification of the westernmost and southernmost sub-reefs. Those spatial trends are contextualized for the stakeholders’ tasked with the protection of that unique reef.