Abstract
Sabellaria alveolata is a sedentary gregarious tube-building species widely distributed from southwest Scotland to Morocco. This species builds what are currently considered the largest European biogenic reefs in the bay of Mont-Saint-Michel (France). As an ecosystem engineer, S. alveolata generates small to large scale topographic complexity, creating numerous spatial and trophic niches for other species to colonize. Sabellaria reefs are also under anthropogenic pressures, leading locally to massive degradation. However, stakeholders lack spatially explicit measures of reef ecological status, at adapted spatial resolution to provide key management information for this protected habitat. Traditional field surveys are extremely time-consuming and rely on expertise for visual ecological status assessment. The present study aims at using an automatic processing approach based on optical airborne data to (i) assess the potential of hyperspectral imagery to discriminate Sabellaria bioconstructions and its main ecosystem associated habitats, including different types of substrate as well as biological components and (ii) to use the combination of the hyperspectral and LiDAR signals to estimate the spatial structure of the different bioconstruction types (veneers vs hummocks and platforms) and ecological phases (retrograding and prograding). A reef from Mont-Saint-Michel was used as a test site. We built a processing chain based on supervised classification using Mahalanobis distance to generate an accurate distribution map (overall accuracy of 88% and a Kappa of 0.85) of ten Sabellaria-related benthic features, including large reef developing on sand and smaller veneers encrusting rocky shore areas. Specific spectral indices were used to define the spatial distribution of the main primary producers, in particular the microphytobenthos. Joining the hyperspectral and LiDAR data led characterizing the distribution of S. alvealata’s ecological status (prograding and retrograding phases) with an overall classification accuracy and Kappa coefficient that can respectively amount to up to 93% and 0.86. In our study site, the Sabellaria reef area (between 5.52 ha and 6.76 ha) was dominated by retrograding phases (between 53% and 58%). Our results showed that this automatic processing chain could be relevant for the spatial characterization of other Sabellaria reef sites. Study perspectives tend towards a quantitative estimation of their ecological status index.
Highlights
Biogenic reefs are limited to coral reefs and include a wide variety of species capable of building biogenic structures, including the gregarious tube-building worms of the Sabellariidae family
To overcome limitations in large scale S. alveolata reef characterization, we propose an approach based on the combination of two complementary techniques: (i) LiDAR a widely used tool to depict the 3D habitat structural complexity (Wedding et al, 2008; Davies and Asner, 2014; D’Urban et al, 2020) and (ii) hyperspectral imagery that allows for the characterization of bio-optical properties variability in time and space (Fyfe, 2003; Schmidt and Skidmore, 2003; Chennu et al, 2013; Bajjouk et al, 2019b)
Field spectra were acquired to examine if S. alveolata bioconstructions have a sufficiently specific spectral signature leading to their identification and discrimination from the other benthic feature present in the study site
Summary
Biogenic reefs are limited to coral reefs and include a wide variety of species capable of building biogenic structures (or bioconstructions), including the gregarious tube-building worms of the Sabellariidae family. Large bioconstructions are found on the coasts of South America, built by the species Phragmatopoma lapidosa (Kinberg, 1866) and Sabellaria wilsoni (Lana and Gruet, 1989). On European coasts, the largest intertidal bioconstructions are built by the species Sabellaria alveolata (Linnaeus, 1767), which is presented from the north of the English coast to the south of the Moroccan coast. Sabellaria growths mainly on rocky area (Muir et al, 2016), but can be present on soft sediments. In France, the Sainte-Anne reef (Mont-Saint-Michel bay) is known as the largest European bioconstruction. By modifying local hydrodynamics, S. alveolata reef structures impact the habitats of benthic assemblages located in the immediately surrounding sediments (Dubois et al, 2002; Jones et al, 2018)
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