| Summary: | The Mediterranean Sea is characterized by homeothermia (~14ªC) and a gradient of increasing oligotrophy from west to east which makes it of particular interest to study distribution patterns of deep-sea fauna. Particular oceanographic processes and topographic characteristics vary in different regions. The northwestern Mediterranean, where the shelf is deeply incised by numerous submarine canyons, is typically more productive and it is also subjected to an intense anthropogenic pressure mainly by deep-sea bottomtrawling fisheries. The suprabenthos, loosely defined as the macrofauna living in the sediment/water column interface, is an important component of the benthic fauna, with a relevant role in deep-sea food webs, albeit poorly investigated. In this context, suprabenthic assemblages were studied along an oligotrophic gradient (600-3000 m water depths; western region, Balearic Sea; central region, Ionian Sea; eastern region, South of Crete) and in a submarine canyon and adjacent slope (400-2250 m; northwestern Mediterranean Sea, Catalan Sea) aiming to examine their biodiversity, abundance and community structure in relation to varying environmental conditions. In each sampling site, samples were collected at three water layers above the sediment (10-50 cm; 55-95 cm; 100-140 cm), allowing to characterize the vertical distribution in the close vicinity of the seafloor. The specimens collected were ascribed to 232 taxa, from which amphipods and cumaceans were the most species-rich groups. Amphipods, mostly predators on zooplankton, followed by mysids and isopods, mostly omnivores, were the most abundant groups. The analysis of the near-bottom vertical distribution of the suprabenthic fauna showed a marked decreased in densities from the layer closer to the sediment (10-40 cm water layer) to the upper layers. Community structure varied in relation to environmental variables such as food input, hydrodynamic regime, topographic features (e.g. canyon-associated conditions) and properties of the water masses. The general results showed high variability in densities (3.5-538.9 ind.100 m-2) with maximum values registered in the Blanes Canyon and adjacent slope at 900 m depth. The number of species and the Shannon biodiversity index varied from 21 to 84 and from 1.28 to 3.35, respectively, with the lowest biodiversity observed in the canyon. Along the longitudinal gradient, densities and number of species decreased, the relative abundance of animals relying on food sources from the sediment decreased in parallel with an increase in the relative abundance of animals feeding on the water column. These results likely reflect the low organic matter input to the sediments in the more oligotrophic region. The nearbottom vertical distribution of the fauna changed along the longitudinal gradient, which may be associated to the functional traits of the species (e.g. motility, dispersion capability, feeding mode), to the different responses of individual species to changing abiotic conditions, the occurrence of topographic barriers and to the oligotrophy. These changes in the composition of the suprabenthic assemblages maintained similar values of α- diversity across the longitudinal/oligotrophy gradient, but resulted in high turnover (β-diversity). In the northwestern region the community structure appeared to be driven by the quantity and quality of food sources, revealed by the presence of surface predators in the Blanes Canyon and adjacent slope and also detritivores in the latter environment. In the canyon head and upper reaches, the community structure and biodiversity appeared to be driven by the temporal variability in hydrodynamic conditions with increased intensity of currents and particle fluxes in autumn and winter (river discharges, storms). Under disturbance conditions, densities increased and biodiversity decreased due to the dominance of omnivores with high motility (e.g. mysids). In the slope, the assemblages appeared to respond to the lower particle fluxes but higher quality of the predominantly pelagic organic input, by showing an increased biodiversity, particularly in spring. At deepest sites, biodiversity was similar between canyon and open slope, probably owing to the lower intensity of natural disturbance and lessening of a putative canyon effect. Nevertheless, after the occurrence of high energetic processes, such as a dense shelf cascading event (e.g. in 2012), an important increase in the number of species and densities was observed both in the canyon and slope, probably reflecting the increment of fresh organic matter in the lower slopes and basin. Despite the overall high resilience of suprabenthic assemblages, they were affected by high and continued trawling disturbance. In conclusion, this Thesis showed evidence of highly variable patterns in the composition, biodiversity and structure of the suprabenthic assemblages typified by the occurrence of a variety of trophic groups and life styles. High levels of spatial and temporal turnover in species composition was attributed to food availability, habitat heterogeneity and natural disturbance. In order to improve the knowledge on deep-sea suprabenthos, more studies on its auto-ecology and biotic interactions are needed. Also important to enable biogeographical and even regional comparisons, is to reach a consensus on a standardized terminology and conceptual definition concerning this faunal compartment, as well as to improve the spatial and temporal replication of sampling. Finally, given the important functional role of suprabenthos in marine food webs, it is strongly recommended to include this benthic compartment in future studies focusing on deep-sea ecosystem functioning.
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