Very high-resolution mapping of emerging biogenic reefs using airborne optical imagery and neural network: the honeycomb worm (Sabellaria alveolata) case study

ABSTRACT

Biogenic reefs provide a wide spectrum of ecosystem functions and services, such as biodiversity hotspot, coastal protection, and fishing practices. Honeycomb worm (Sabellaria alveolata) reefs, in the Bay of Mont-Saint-Michel (France), constitute the largest intertidal bioconstruction in Europe but undergo anthropogenic pressures (aquaculture-stemmed food/space competition and siltation, fishing-driven trampling). Very high-resolution (VHR) airborne optical data enable cost-efficient biophysical measurements of reef colonies, strongly expected for conservation approaches. A synergy of remotely sensed airborne optical imagery, calibration/validation photoquadrat ground-truth (202/101, respectively), and artificial neural network (ANN) modelling is first used to map S. alveolata relative abundance, over the largest bioconstruction in Europe. The best prediction of S. alveolata abundance was reached with the infrared–red–green (IRRG) spectral combination and ANN model structured with six neurons (R² = 0.72, RMSE = 0.08, and r = 0.85). The six hyperbolic tangent formulas were applied to the three input spectral bands (IRRG) in order to build six hidden neuronal images, resulting in VHR digital S. alveolata abundance model (6547 × 6566 pixels with 0.5 m pixel size). The innovative model revealed undescribed spatial patterns, namely a reef polarization (perpendicular to the shoreline) of S. alveolata abundance: high abundance on forereef and low abundance on backreef.

Acknowledgements

Authors gratefully thank French Minister for Ecology, Sustainable Development and Energy for the airborne multispectral acquisition and dissemination related to Ortholittorale V2 product (http://cartelie.application.developpement-durable.gouv.fr/cartelie/voir.do?carte=telecharg_ol_v2_l93&service=CEREMA). Hélène Gloria and Dorothée James are also greatly acknowledged for their fieldwork involvement.

Disclosure statement

No potential conflict of interest was reported by the authors.