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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 pages, 5 figures, 2 tables.-- This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY) ; Physical and biogeochemical processes in the Southern Ocean are fundamental for modulating global climate. In this context, a process-based understanding of how Antarctic diatoms control primary production and carbon export, and hence global-ocean carbon sequestration, has been identified as a scientific priority. Here we use novel sediment trap observations in combination with a data-assimilative ocean biogeochemistry model (ECCO-Darwin) to understand how environmental conditions trigger diatom ecology in the iron-fertilized southern Scotia Sea. We unravel the role of diatoms assemblage in controlling the biogeochemistry of sinking material escaping from the euphotic zone, and discuss the link between changes in upper-ocean environmental conditions and the composition of settling material exported from the surface to 1,000 m depth from March 2012 to January 2013. The combined analysis of in situ observations and model simulation suggests that an anomalous sea-ice episode in early summer 2012–2013 favored (via restratification due to sea-ice melt) an early massive bloom of Corethron pennatum that rapidly sank to depth. This event drove high biogenic silicon to organic carbon export ratios, while modulating the carbon and nitrogen isotopic signals of sinking organic matter reaching the deep ocean. Our findings highlight the role of diatom ecology in modulating silicon vs. carbon sequestration efficiency, a critical factor for determining the stoichiometric relationship of limiting nutrients in the Southern Ocean ; This work was funded by the Spanish Polar Program through the Spanish Research and Innovation (I+D+i) National Plan (grant numbers CTM2009-08287-E/ANT and CTM2011-14056- E/ANT), and supported by the Catalan Government Grups de Recerca Consolidats Grant (2017 SGR 315) and the Internal Research and Technology Development program of the Jet Propulsion Laboratory, California Institute of Technology. DZ was funded by a postdoctoral fellowship (Plan I2C) from Xunta de Galicia (Spain) and performed diatom analysis during her stay at Instituto Português do Mar e da Atmosfera (IPMA), Lisbon, Portugal. Diatom analysis was sponsored by Portuguese national funding through FCT-Fundação para a Ciência e a Tecnologia (UIDB/04326/2020 and DiatBIo PTDC/AAG-GLO/3737/2012 projects). GS was supported by the Deutsche Forschungsgemeinschaft (DFG) in the framework of the priority programme "Antarctic Research with comparative investigations in Arctic ice areas" SPP 1158 by Grant SITAnt (365778379) ; 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

The carbon cycle of the Arctic Ocean is tightly regulated by land-atmosphere-cryosphere-ocean interactions. Characterizing these environmental exchanges and feedbacks is critical to facilitate projections of the carbon cycle under changing climate conditions. The environmental drivers of sinking particles including organic carbon (OC) to the deep-sea floor are investigated with four moorings including sediment traps and currentmeters at the Arctic gateway in the eastern Fram Strait, which is the area where warm anomalies are transported northwards to the Arctic. Particles fluxes were collected over one year (July 2010-July 2011) and have been analysed to obtain the content of the lithogenic fraction, calcium carbonate, OC and its stable isotopes, opal, and the grain size. Records of near bottom current speed and temperature along with satellite observations of sea ice extent and chlorophyll-a concentration have been used for evaluation of the environmental conditions.We found increased lithogenic fluxes (up to 9872mgm-2d-1) and coarsening grain size of settling particles in late winter-early spring. At the same time, intensifications of the northward flowing west Spitsbergen current (WSC) were recorded. The WSC was able to resuspend and transport northwards sediments that were deposited at the outlet of Storfjordrenna and on the upper slope west of Spitsbergen. The signal of recurrent winnowing of fine particles was also detected in the top layer of surface sediments. In addition, an increased arrival of sea ice transported ice rafted detritus (>414 detrital carbonate mineral grains larger than 1mm per m2) from the southern Spitsbergen coast along with terrestrial organic matter was observed beyond 1000m of water depth during winter months. Finally, the downward particle fluxes showed typical temporal variability of high latitudes, with high percentages of the biogenic compounds (opal, organic carbon and calcium carbonate) linked to the phytoplankton bloom in spring-summer. However, on an annual basis local planktonic production was a secondary source for the downward OC, since most of the OC was advected laterally by the WSC. Overall, these observations demonstrated the sensitivity of the downward flux of particles to environmental conditions such as hydrodynamics, sea ice rafting, and pelagic primary production. Future alteration of the patterns of natural drivers due to climate change is thus expected to cause major shifts in the downward flux of particles, including carbon, to the deep sea ecosystems. © 2015 Elsevier Ltd. ; This research has been supported by the projects HERMIONE (FP7-ENV-2008-1-226354) and GRACCIE-CONSOLIDER (CSD2007-00067), and a Catalan Government Grups de Recerca Consolidats grant (2009 SGR 1305). LL was partly supported by the CNR-DTA project SNOW (Sensor Network for Oceanography in shallow Water - Kongsfjord experiment), and AS by a "Ramon y Cajal" contract from MICINN. BF is affiliated with the Centre of Excellence: Arctic Gas hydrate, Environment and Climate (CAGE) funded by the Norwegian Research Council (grant no. 223259). We are grateful to S. Buenz and the crew of RV Helmer Hansen (University of Tromsø) for their valuable support during the cruises, and R. Duran, S. Kunesch, J. Carbonne, A. Rumin, S. Aliani, and X. Rayo who assisted with the field and laboratory work. This is contribution N. XXXX of the CNR-ISMAR of Bologna. ; Peer reviewed