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European Geosciences Union (EGU) General Assembly, 19–30 Apr 2021.-- 2 pages.-- The authors wish to thank the Secretaría General de Pesca and Tragsa for the 2004 Espace Project dataset ; The north-western Mediterranean continental margin is one of the few regions in the world where bottom trawling has been continuously practised since several decades. Among the existing trawling techniques, the one practised on this region is the "otter trawling", which has a strong impact on the seafloor morphology via scraping and ploughing, especially on muddy substrates. High-resolution multibeam bathymetry and backscatter data, side scan sonar images, sediment cores and satellite based Vessel Monitoring System (VMS) data have been integrated to investigate the impact of bottom trawling on the seafloor morphology of the northern Catalan continental shelf (NW Mediterranean). Satellite-based navigation tracks from bottom trawlers operating in the study area during 6 years (2006-2011) reveal the spatial distribution of fishing grounds and the occurrence of an intense trawling effort around the 50-60 m isobaths, since trawling is banned at shallow depths. Backscatter imagery shows a narrow (120-250 m wide) and discontinuous high backscatter facies along this depth range, extending parallel to the coastline for more than 40 km from Portbou to l'Estartit. In the bathymetric data, this high backscatter region also coincides with an abrupt change in the mean seafloor gradient (from 0.8° in the inner shelf to 0.4° in the middle shelf), or locally with a narrow (50-150 m wide) slightly depressed (0.2-0.6 m deep) channeled morphology. Side-scan sonar images display high density of trawl marks generated by fishing gears in this area. Further offshore, scattered narrower trawl hauls are also observed on the middle shelf (60-90 m deep), where they can be traced across several thousands of meters. Sediment cores retrieved from the area of high backscatter and largest trawling intensity display sediment coarsening in the upper layers (0-4 cm) caused by winnowing of finer fractions. These findings demonstrate that chronic stirring, mixing and erosion of surface sediments induced by recurrent trawling persisting over the same fishing grounds can cause long-term morphological and sedimentary changes on the continental shelf seafloor ; This study has received funding from the ABIDES (Assessment of Bottom-trawling Impacts in the Deep-sea Sediments) Spanish Research Project (CTM2015-65142-R) and the European Union's Horizon 2020 research and innovation programme under Marie Sklodowska-Curie grant agreement No. 867471. Additional funds were provided by the Generalitat de Catalunya Generalitat de Catalunya (2017 SGR-663 and -1588) and by the Spanish Research Project ABRIC (RTI2018-096434-B-I00). This work is contributing to the ICM's 'Center of Excellence' Severo Ochoa (CEX2019-000928-S) ; Peer reviewed

European Geosciences Union (EGU) General Assembly, 19–30 Apr 2021.-- 2 pages.-- The authors wish to thank the Secretaría General de Pesca and Tragsa for the 2004 Espace Project dataset ; Submarine canyons are morphological features found along continental margins that play a key role channeling and connecting sediment from continental shelves to the abyssal plains. The current morphological characterization of the Blanes and Cap de Creus canyon heads, located on the Catalan continental margin (NW Mediterranean Sea), has been recently conducted during the CRIMA cruise in September 2020 using high-resolution (4 m grid size) multibeam bathymetry data. These data have been compared with a previous dataset collected in 2004 during the ESPACE project to evaluate the morphological changes during this 16-year interval. Since these canyon heads are located at shallow water depths and at short distances from the shoreline, their short-term evolution is related to the sediment dynamics on the continental shelf. A large-scale change in the seafloor morphology was observed in the Blanes canyon head, indicating the prevalence of erosion in the western canyon rim and non-deposition in the eastern rim. In the Cap de Creus canyon head, the excavation of pre-existing erosive structures was also evidenced in the southwestern canyon rim. These changes mainly happen in the area where the shelf is narrower, which coincide with the main zone of dense water advection along the shelf and toward the canyon interior. The different small-scale morphological evolution between both canyon heads seems to be related to the local geological characteristic of the subsurface deposits of the continental shelf. The Blanes canyon head incises a succession of relict (Holocene) sediment bodies that can act as a source of erodible sediments to the canyon, mainly during strong storms. The continental shelf in the vicinity of the Cap de Creus canyon head, however, is characterized by a rocky substratum (Paleozoic) with a limited sediment coverage and numerous erosive features that evidence relative sand starvation. This creates a greater erosive resistance, although the erosive character of strong storms and major dense-shelf water cascading events occurring during the studied time interval is evident. Additionally, small changes in the shelf bedforms indicate that such high-energetic oceanographic processes also modify the fine-scale seafloor morphology. These results reveal that both submarine canyon heads are dynamic and sensitive to oceanographic processes that enhance the erosion and transport of sediment from the shelf into the canyon, particularly during energetic storms and dense shelf water cascading events. Nevertheless, their small-scale evolution seems to be closely related to the type of geological substrate of the shelf on which they are developing ; This study has received funding from the ABRIC (RTI2018-096434-B-I00) and CRIMA (RTI2018-095770-B-I00) Spanish Research Projects, the European Union's Horizon 2020 research and innovation programme under Marie Sklodowska-Curie grant agreement No. 867471 and the Generalitat de Catalunya (2017 SGR-663 and -1588). This work is contributing to the ICM's 'Center of Excellence' Severo Ochoa (CEX2019-000928-S) ; Peer reviewed

22 pages, 12 figures, 2 tables.-- Dataset https://doi.org/10.20350/digitalCSIC/13748 ; Increases of water turbidity and suspended sediment transport in submarine canyons have been associated with high-energy events such as storms, river floods and dense shelf water cascading (DSWC), and occasionally with bottom trawling along canyon flanks and rims. To assess the variations on the water column turbidity and sediment transport in the Palamós Canyon linked to both natural and trawling-induced processes, an autonomous hydrographic profiler, as well as a near-bottom current meter and a turbidimeter were deployed in the canyon axis (929 m depth) from February to June 2017, covering a trawling closure (February) and trawling activities (March-June). Periods of enhanced water turbidity during the trawling closure were mostly associated with storms and DSWC events, transporting turbid dense waters into the canyon. In absence of such events, the water column displayed low suspended sediment concentrations (~ 0.3 mg L− 1) until the trawling season began, when particulate matter detachments, ranging between > 1 mg L− 1 and 3.8 mg L− 1, were observed at the water depths where the trawling grounds are found. During the trawling closure, high near-bottom suspended sediment fluxes (35–44 g m− 2 s− 1) were sporadically registered at ~ 920 m depth associated with a major storm and DSWC event. Smaller but more frequent increases of near-bottom suspended sediment fluxes (0.1–1.4 g m− 2 s− 1) were recorded during trawling activities. Despite these smaller trawling-induced suspended sediment fluxes, 30 days of continuous bottom trawling activity transported a total amount of 40 kg m− 2, of similar magnitude to that generated by a major DSWC event (50 kg m− 2). Since bottom trawling in Palamós Canyon is practiced on a daily basis throughout the year, a much larger contribution of anthropogenically derived water turbidity and suspended sediment transport can be expected ; This work has been supported by the ABIDES (CTM2015-65142) Spanish Project and by the autonomous government of Catalonia grants 2017 SGR 863 and 2017 SGR 1241, as well as by the TrawledSeas Project receiving funding from the European Union's Horizon 2020 Research and Innovation programme under a MSCA grant agreement (No. 867471). M. Arjona-Camas is supported by a predoctoral FPI grant from the Spanish Ministry of Economy, Industry and Competitiveness ; With the institutional support of the 'Severo OchoaCentre of Excellence' accreditation (CEX2019-000928-S) ; Peer reviewed

11 pages, 5 figures, 2 tables ; Bottom trawling in submarine canyons can affect their natural sedimentation rates, but studies addressing this issue are still scarce. In the Gulf of Palermo (SW Mediterranean), bottom trawling occurs on the slope around Oreto, Arenella and Eleuterio canyons. Analyses of excess 210Pb concentrations and grain size fractions in sediment cores from their canyon axes revealed that sedimentation rates and silt contents increased in all canyons in the 1980s, due to the expansion of more powerful trawlers (>500 HP) to deeper fishing grounds. In Eleuterio and Arenella canyons, sedimentation rates increased by an order of magnitude (0.1-1.4 cm·yr-1), whereas they increased less (0.1-0.7 cm·yr-1) in Oreto Canyon, since the enhanced trawling-derived sediment fluxes into this canyon are affected by sediment resuspension from trawling along its axis. Considering the global expansion of bottom trawling, we anticipate similar alterations in other trawled canyons, with ecological consequences that should be addressed by management strategies ; The results presented in this study were obtained within the Exploring SiciLian CAnyoN Dynamics (ISLAND) project, supported by the European Commission, Seventh Framework Programme (EUROFLEETS2 (grant no. 312762)). The analyses were funded by the "Assessment of Bottom-trawling Impacts in Deep-sea Sediments" (ABIDES) Spanish Research Project (CTM2015-65142-R). Additional funds were provided by the Generalitat de Catalunya (2017 SGR-863 and SGR-1588), by the Australian Research Council LIEF Project (LE170100219) and by the Spanish Research Project ABRIC (RTI2018-096434-B-I00). This work is contributing to the ICTA's "Unit of Excellence" Maria de Maetzu (CEX2019-000940-M), and ICM's "Center of Excellence" Severo Ochoa (CEX2019-000928-S). The IAEA is grateful for the support provided to its Environment Laboratories by the Government of the Principality of Monaco. CLI was supported by the H2020 MSC Action HABISS (GA 890815) ; Peer reviewed

5 pages, 4 figures ; Bottom trawling is a non-selective commercial fishing technique whereby heavy nets and gear are pulled along the sea floor. The direct impact of this technique on fish populations1, 2 and benthic communities3, 4 has received much attention, but trawling can also modify the physical properties of seafloor sediments, water–sediment chemical exchanges and sediment fluxes5, 6. Most of the studies addressing the physical disturbances of trawl gear on the seabed have been undertaken in coastal and shelf environments7, 8, however, where the capacity of trawling to modify the seafloor morphology coexists with high-energy natural processes driving sediment erosion, transport and deposition9. Here we show that on upper continental slopes, the reworking of the deep sea floor by trawling gradually modifies the shape of the submarine landscape over large spatial scales. We found that trawling-induced sediment displacement and removal from fishing grounds causes the morphology of the deep sea floor to become smoother over time, reducing its original complexity as shown by high-resolution seafloor relief maps. Our results suggest that in recent decades, following the industrialization of fishing fleets, bottom trawling has become an important driver of deep seascape evolution. Given the global dimension of this type of fishery, we anticipate that the morphology of the upper continental slope in many parts of the world's oceans could be altered by intensive bottom trawling, producing comparable effects on the deep sea floor to those generated by agricultural ploughing on land ; This work was supported by various research projects: HERMIONE (226354 and CTM2010-11084-E), DOS MARES (CTM2010-21810-C03),OASIS DEL MAR—Obra Social ''la Caixa'', GRACCIE-CONSOLIDER (CSD2007-00067) and REDECO (CTM2008-04973-E). We also received support from Catalan Government Grups de Recerca Consolidats grants (2009 SGR 899 and 1305). VMS data and support were provided by the Spanish General Secretariat of Maritime Fishing (SEGEMAR). Assistance at sea by the crews of RV Hespérides, RV Sarmiento de Gamboa and RV García del Cid is also acknowledged. J.M. was funded through a JAE-DOC contract granted by Consejo Superior de Investigaciones Científicas and co-financed by the European Social Fund. F. Sarda and T. Milligan provided comments on the manuscript. The final document benefited from a review by P. Talling ; Peer reviewed

15 páginas, 8 figuras, 3 tablas. ; Settling particles were collected using sediment traps deployed along three transects in the Lacaze-Duthiers and Cap de Creus canyons and the adjacent southern open slope from October 2005 to October 2006. The settling material was analyzed to obtain total mass fluxes and main constituent contents (organic matter, opal, calcium carbonate, and siliciclastics). Cascades of dense shelf water from the continental shelf edge to the lower continental slope occurred from January to March 2006. They were traced through strong negative near-bottom temperature anomalies and increased current speeds, and generated two intense pulses of mass fluxes in January and March 2006. This oceanographic phenomenon appeared as the major physical forcing of settling particles at almost all stations, and caused both high seasonal variability in mass fluxes and important qualitative changes in settling material. Fluxes during the dense shelf water cascading (DSWC) event ranged from 90.1 g m−2 d−1 at the middle Cap de Creus canyon (1000 m) to 3.2 g m−2 d−1 at the canyon mouth (1900 m). Fractions of organic matter, opal and calcium carbonate components increased seaward, thus diminishing the siliciclastic fraction. Temporal variability of the major components was larger in the canyon mouth and open slope sites, due to the mixed impact of dense shelf water cascading processes and the pelagic biological production. Results indicate that the cascading event remobilized and homogenized large amounts of material down canyon and southwardly along the continental slope contributing to a better understanding of the off-shelf particle transport and the internal dynamics of DSWC events. ; This research has been supported by HERMES (GOCE-CT-2005-511234-1), SESAME (GOCE-036949), HERMIONE (FP7-ENV-2008-1-226354), and GRACCIE (CSD2007-00067, Consolider-Ingenio Program) research projects, a Generalitat de Catalunya "Grups de Recerca Consolidats" grant (2009 SGR-1305) and a FP7 Marie Curie Reintegration Grant (PERG04-GA-2008-239175). C. Pasqual is supported by an F.P.U. grant from the Spanish government. ; Peer reviewed