The role of abiotic and biotic interactions in deep-sea benthic habitats of the northeastern Gulf of Cadiz
The Gulf of Cádiz is influenced by the collision of the Alborán Domain with the North African and South Iberian margins, which ultimately promotes hydrocarbon-rich fluid venting, among other geological processes. Regarding oceanography, the single and interactive effects of the Eastern North Atlantic Central Water and of the Mediterranean Outflow Water strongly influence seabed topography and substrate types. Both oceanographic and geological processes determine the distribution of a wide variety of geomorphic features, benthic habitats, and organisms. A high-resolution morpho-sedimentary analysis, based on geophysical data, sediment samples and submarine imagery, has been carried out; the onset and evolution of the main geoforms are discussed; and the influence of oceanographic, geological and biogenic processes conditioning substrate types and benthic community distribution are evaluated. The interplay of oceanographic (e.g., water mass circulation, secondary circulation, vertical eddies, internal waves), geological (e.g., salt and shale diapirism, mud volcanism), and biogenic (e.g., methane derived authigenic carbonates formation, coral mound aggradation) processes drive the formation of a large variety of geoforms including, among others, contourite drifts, channels, diapiric ridges, mud volcanoes, pockmarks, and coral mounds. At a smaller scale, the interaction between massive and fluid extrusive, biogenic, and bottom current-related processes (both depositional and erosive) determines the distribution of substrate types and benthic communities, which have been classified as belonging to different benthic habitats. In a second phase, based on the same high-resolution multibeam echosounder data (both bathymetric and backscatter) and ground-truthing data consisting in more than 100 videos and 50 samples, benthic habitats in a bathyal area of more than 1000 km2 in the shallow part of the Site of Community Importance (SCI) denominated Volcanes de fango del golfo de Cádiz(Mud volcanoes of the Gulf of Cádiz) have been mapped and characterised. A total of 11 types of benthic habitats were detected, some of them linked to Habitat 1180 and Habitat 1170 of the Habitats Directive but intermixed with other benthic habitats not contemplated in directives or conservation agreements. An interpretative benthic habitat map was created using expert judgement that represents a first approximation of the large scale distribution and composition of the different habitats for the northeastern Gulf of Cadiz. This map provides a valuable tool for management and conservation efforts in the region by identifying areas of high ecological value, as well as areas that are vulnerable to human impacts and others with a high resilience. Finally, the distribution and abundance of key species of sedimentary bathyal benthic habitats in the northeastern Gulf of Cádiz, such as sea pens, bamboo corals and the Norway lobster, were examined using Random Forest models. The effects of environmental variables and bottom trawling on the distribution and abundance of some of these species were assessed, and management strategies for habitat protection were proposed. Bottom trawling was found to decrease habitat suitability for large octocoral species while increasing the abundance of commercial species like the Norway lobster. The results suggested that bottom trawling causes significant reduction in the octocoral distribution, together with substrate homogenization, and habitat differentiation. A 200 km² area was recommended for a specific bottom trawling regulation to protect soft-bottom octocorals and support the recovery of commercial burrowing megafauna. These results have been essential to advise the competent authorities on management and zoning to develop the management plan for SCI-ESZZ12002 and its next declaration as Special Area of Conservation (SCA).
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