Bottom trawling has many impacts on marine ecosystems, including seafood stock impoverishment, benthos mortality, and sediment resuspension. Historical records of this fishing practice date back to the mid-1300s. Trawling became a widespread practice in the late 19th century, and it is now progressively expanding to greater depths, with the concerns about its sustainability that emerged during the first half of the 20th century now increasing. We show here that compared with untrawled areas, chronically trawled sediments along the continental slope of the north-western Mediterranean Sea are characterized by significant decreases in organic matter content (up to 52%), slower organic carbon turnover (ca. 37%), and reduced meiofauna abundance (80%), biodiversity (50%), and nematode species richness (25%). We estimate that the organic carbon removed daily by trawling in the region under scrutiny represents as much as 60-100% of the input flux. We anticipate that such an impact is causing the degradation of deepsea sedimentary habitats and an infaunal depauperation. With deep-sea trawling currently conducted along most continental margins, we conclude that trawling represents a major threat to the deep seafloor ecosystem at the global scale.

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Meiofaunal biodiversity, with a special focus on nematodes, was investigated in 6 submarine canyons and 5 adjacent open slopes along bathymetric gradients (from ca. 200 to 5000 m depth) from 3 deep-sea regions (northeastern Atlantic, western and central Mediterranean) spanning > 2500 km and across a wide gradient of trophic and physicochemical conditions. The analysis of local (α) diversity at equal depths showed the presence of similar values in the NE Atlantic and Mediterranean deep-sea sediments. The comparison of the α diversity between different deep-sea habitats (canyons versus adjacent open slopes) revealed the lack of significant differences in species richness in most of the investigated systems. However, the analysis of nematode species composition showed the presence of major differences among different sampling depths (i.e. 500 versus 1000 versus 2000 m depth) and habitats. Turnover (β) diversity was high in all of the investigated deep-sea systems, but was higher in the NE Atlantic (87%) than in the Mediterranean margins (range 51 to 60%), resulting in higher values of regional (γ) diversity in the Atlantic margin. Turnover diversity among regions (δ diversity) was highest (~91%) between the NE Atlantic and western Mediterranean, but still extremely high between the western and central Mediterranean margins (~80%), thus leading to similar values of biogeographical diversity (ε) in the NE Atlantic and Mediterranean deep biogeographical provinces. The results suggest that biogeographic differences in deep-sea species composition are related to differences in β and δ diversity and not to differences in α diversity, and that the analysis of the factors driving β diversity are crucial to understand the spatial patterns of biodiversity in the deep sea.

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