Deep-sea habitat characterization using acoustic data and underwater imagery in Gazul mud volcano (Gulf of C´adiz, NE Atlantic)

Gazul is the shallowest mud volcano (MV) within the Shallow Field of Fluid Expulsion (SFFE) of the northeastern
Gulf of C´adiz (NE Atlantic; 300–1200 m depth). The SFFE represents an important geo- and biodiversity area that
was designated as a Site of Community Importance under the European Habitats Directive in 2014. In this study,
geological features, habitats and associated biodiversity, as well as anthropogenic impacts, were characterized at
Gazul MV from underwater imagery and multibeam bathymetry. Multivariate methods using the Bray-Curtis
similarity index identified six main habitats, each of which harbored a characteristic faunal assemblage that
included: (1) sandy ripple bottoms typified by the actiniarian Actinauge richardi; (2) sandy, muddy, coarse sand
and bioclastic bottoms dominated by the solitary coral Flabellum chunii; (3) coarse sand and bioclastic bottoms,
together with soft sediments covered by scattered methane-derived authigenic carbonates (MDACs) (mixed
bottoms), characterized by the echinoid Cidaris cidaris; (4) hard bottoms comprising MDACs dominated by a wide
variety of sponges and gorgonians; (5) coral-rubble bottoms typified by the presence of colonial scleractinian
communities dominated by Madrepora oculata; and (6) mixed bottoms characterized by the presence of a styelid
ascidian. Slope and water depth were the main factors explaining assemblages’ distribution, which was also
supported by the presence of MDACs such as slabs, crusts and chimneys on the seafloor, as well as by the
geomorphologic diversity of Gazul MV. The results highlight Gazul MV as an eco-biologically important area
harboring different vulnerable marine ecosystem (VME) elements with indicator taxa such as scleractinians,
sponges, gorgonians and black corals. ROV images revealed abandoned or lost fishing gears and marine debris on
the seafloor, indicating anthropogenic impacts in Gazul MV and adjacent areas. Indeed trawling fisheries activities
have also been detected in Vessel Monitoring System datasets. A fishery restricted area is recommended in
Gazul MV due to the occurrence of diverse VMEs and species included in different conservation directives and
conventions.