Composition, structure and distribution of epibenthic communities within a mud volcano field of the northern Gulf of Cádiz in relation to environmental variables and trawling activity

Abstract

The Gulf of Cádiz represents an area of extensive seepage of the south-western Iberian Peninsula that is located between two continents and basins, where a high geomorphologic, sedimentary and environmental complexity occurs. In the present study, epibenthic communities were investigated in a mud volcano field containing four mud volcanoes (Gazul, Anastasya, Pipoca and Tarsis), one diapir/mud volcano complex (Chica) and several diapirs that are all located in the upper and middle slope of the Spanish margin of the Gulf of Cádiz (300–700 m depth). Faunistic samples were collected in 2010 and 2011 using a beam-trawl in different sectors of the mud volcano field, including fluid venting submarine structures and their adjacent seabeds. Environmental variables from the sediment and water column as well as the commercial bottom trawling activity were also measured in the same sampled areas for studying relationships with the fauna. A total of 325 taxa were found in the samples, including sponges, molluscs, cnidarians and decapods the most diverse groups. The epibenthic communities displayed differences between sectors that were related to depth, sediment features, characteristics of the water masses and the trawling activity. Six different assemblages were found including deep-sea sponge aggregations and cold-water coral banks in Gazul, echinoid and solitary coral aggregations in Gazul, sea-pen communities that were sometimes intermixed with sponge and gorgonian aggregations in Tarsis, Chica and Pipoca, and low density sea-pen communities with components of chemosynthesis-based communities in Anastasya. Species richness was higher in Gazul, Pipoca and Chica due to the presence of complex habitats (e.g. cold-water coral banks, sponge aggregations) and low in Anastasya due to the scarce epifauna, high trawling activity and low near-bottom current speed. Typical cold seep chemosymbiotic bivalves of the Gulf of Cádiz were not detected in beam-trawl samples due to their infaunal habit. The studied mud volcano field was declared a Site of Community Importance due to the presence of habitats and species with a conservation status, but several commercial species are exploited by bottom trawling. Therefore, a sustainable management of this mud volcano field is still needed according to current EU directives for combining fisheries and habitat conservation (Habitats Directive of the European Union, Directive 1992/43/EEC; Marine Strategy Framework, Directive 2008/56/EC).