3D Scanning bathymetry applied for assessment and monitoring of protected marine habitats: El Cachucho case study (Spanish Cantabrian margin)

El Cachucho, also known as Le Danois bank, is the first and only marine zone declared as Marine Protected Area (MPA) in Spain since 2008. This bank consists on a 72 km-long E-W trending marginal platform located at Spanish Cantabrian margin (southern Bay of Biscay) and interpreted as horst block separated from the Spanish continental shelf by an interior basin. The bank seafloor has an almost flat-topped morphology with minimum water depth of 424 m, having only local structural and erosive features. During last decades researchers have highlighted the importance of the bathyal ecosystem developed in this geological formation. As a result, significant efforts are being carried out to asses and monitor the evolution of this MPA in order to ensure the conservation of its biodiversity, applying new techniques as 3D scanning bathymetry.

Since 2013 the Spanish Institute of Oceanography (IEO) is leading the ESMAREC project (founded by the Spanish Government) for the monitoring of El Cachucho in order to guarantee the continuity as MPA based European Union regulations. This monitoring mainly consists on repeated multibeam seafloor bathymetries to assess the geomorphological evolution and reflectivity mosaics in order to map and classify the seafloor that can be related to the different types of marine habitats. The survey plan of the ECOMARG-2019 oceanographic cruise included four different locations that were chosen along the El Cachucho for sampling stations with the remotely operated towed vehicle (ROTV) POLITOLANA in order to identify various species of gorgonians and sponges with video images. Furthermore, in those locations, a multi-parametric platform system (lander) was anchored to study the oceanographic dynamics of the Benthic Boundary Layer (BBL). Both ROTV operations and lander anchorages require a detailed knowledge of the seafloor morphology for instrumental safety and optimize efforts. Existing multibeam bathymetry along El Cachucho before the ECOMARG-2019 cruise was only 75 meter and then inadequate to carry out those seafloor operations.

With the aim to improve the existing bathymetry, during the ECOMARG-2019 cruise was used the Kongsberg EM710 multibeam echo-sounder using the 3D Scanning technique. In this technique the vessel navigates to 0.5 knots and 250 beams sweep the bottom with an 45º opening angle and 10º horizontal movement. Higher point density was achieved, so it was possible to increase the average resolution of bathymetry and reflectivity up to 5 meters. New high resolution data provided a precise image of the geomorphology and allowed a more detailed seafloor classification. In this way, potential risks were reduced during ROTV operations and anchorages. In addition, the locations for ROTV operations were optimized based on the reflectivity mosaics that allowed to identify hard seafloor zones, preferred typology of seabed for gorgonians and sponges. Using the 3D Scanning in El Cachucho has resulted in an essential tool for safety and to optimize the seafloor operations. This technique allows to achieve a detailed knowledge of the seafloor in order to better assess and monitor MPA.