The Spatial Network of the Largest Honeycomb Reef in the World as Revealed by an Ultra-High Resolution Lidar Drone

Collin, A.; James, D.; Gairin, E.; Aubert, M.; Daniel, Y.; Feunteun, E., and Dubois, S., 2024. The spatial network of the largest honeycomb reef in the world as revealed by an ultra-high resolution lidar drone. In: Phillips, M.R.; Al-Naemi, S., and Duarte, C.M. (eds.), Coastlines under Global Change: Proceedings from the International Coastal Symposium (ICS) 2024 (Doha, Qatar). Journal of Coastal Research, Special Issue No. 113, pp. 961-965. Charlotte (North Carolina), ISSN 0749-0208. On temperate coastal areas, Sabellaria alveolata worms are honeycomb reef-builders, providing multiple ecosystem services. The largest honeycomb reef in the world is located in the Bay of Mont-Saint-Michel (France) and has never been surveyed in its entirety at ultra-high spatial resolution. This original research addresses this gap by using a medium-cost drone topographic lidar system over the time window of a low spring tide. A denoised lidar point cloud, composed of 702 million illuminations (on average, 118 points/m2), enabled to compute point-scaled height above the ground at an accuracy of 0.06 m. The 3033 reef patches of height ≥ 0.5 m allowed to draw a graph-based network, woven with 3032 edges. The contribution of each node and edge was estimated and mapped using Betweenness Centrality, a local weighted connectivity metric. The nodes of the central and seaward front sub-reefs primarily contributed to the network's anchoring, while the edges linking the easternmost and westernmost sub-reefs were crucial to ensure full connectivity. The use of ultra-high spatial resolution, at very high temporal resolution, paves the way to seasonally monitor the world's largest honeycomb reef in order to outline areas subjected to erosion, and rapidly manage threats upon that ecosystem.