Trawling-induced daily sediment resuspension in the flanks of La Fonera Canyon

VIII Congreso Geológico de España (Oviedo, 2012). -- 4 pages, 3 figures[ES] El impacto de la pesca de arrastre en los fondos marinos y sus recursos naturales ha adquirido creciente relevancia durante las últimas décadas. Sin embargo, su posible contribución a la dinámica sedimentaria contemporánea permanece poco estudiada. Una línea equipada con sensores de turbidez fue fondeada a 1000 metros de profundidad en un tributario del flanco norte del Cañón de la Fonera (=Palamós), el cual es explotado por una flota de arrastre a profundidades entre 400 y 800 m. Durante días y horas de trabajo únicamente, se detectaron plumas de turbidez de un espesor mínimo de 100 metros, con concentraciones de sólidos en suspensión hasta 200 mg l-1 cerca del fondo, 1-2 órdenes de magnitud superiores a los valores medidos en días sin actividad pesquera. Estos incrementos de turbidez observados a distancia de los caladeros están asociados a flujos de gravedad generados por las pesadas artes de arrastre. Los resultados muestran que la pesca industrial de arrastre puede modificar dramáticamente los patrones naturales de resuspension y transporte de sedimento en márgenes continentales, principalmente en fondos de elevada pendiente como los flancos de cañones submarinos[EN] The offshore displacement of commercial bottom trawling has raised concerns on the impacts of this activity over the deep-sea physical environment, but the issue still remains largely unaddressed. A focussed study to address the alteration of natural sediment fluxes by trawling activities was conducted at the northern flank of the La Fonera Canyon (northwestern Mediterranean), where a bottom trawling fishery is active at depths from 400 to 800 m. A moored line equipped with turbidimeters at several heights over the seafloor was deployed at 1000 m depth to document the downslope effects of trawling on water turbidity. During working days and working hours of the trawling fleet, plumes of resuspended sediment extending to at least 100 m above the bottom were observed, with near-bottom suspended sediment concentrations up to 200 mg l-1. In the absence of trawling, turbidity dropped back to background levels ~2 mg l-1. These results highlight that, in steep environments such as the flanks of submarine canyons, the effects of bottom trawling on sediment resuspension and water turbidity can propagate to larger and deeper areas than the fishing grounds and dramatically change the natural patterns and rates of resuspension and advection of sedimentsEl proyecto HERMIONE fue financiado por la Comisión Europea (FP7 Grant agreement no: 226354). J. Martín se benefició de una beca FPI (MEC) y un contrato del programa JAE-DOC (CSIC)Peer reviewe

Role of internal waves in the generation of nepheloid layers on the northwestern Alboran slope: Implications for continental margin shaping

11 pages, 10 figuresThe role of internal waves in the sediment dynamics of the northwestern Alboran continental slope was investigated in a selected area around the Guadiaro submarine canyon. Nepheloid layer distribution was identified using closely spaced CTD/transmissometer profiles collected during two hydrographic surveys. A well-defined pattern of suspended sediment distribution consisting of surface, intermediate, and near-bottom nepheloid layers was found. Intermediate and bottom nepheloid layers were always observed within the canyon and on the adjacent continental slope, spanning mainly from 200 to 500 m depth. In addition, a current meter with a turbidimeter was deployed in the lower section of the Guadiaro Canyon at 600 m depth, 25 meters above the seafloor. Time series analysis indicated that the currents, temperature, and turbidity within the canyon fluctuate mainly at semidiurnal tidal frequencies, suggesting the presence of semidiurnal internal tides affecting the near-bottom suspended sediment concentration along the canyon axis. High-resolution bathymetry from the study area was used to evaluate the internal wave reflection conditions at semidiurnal tidal frequency for the entire continental slope region. Critical slope conditions were reached on the upper continental slope and along the canyon axis, coinciding with the region in which nepheloid layers were observed. This region also coincides with a zone of erosion on the upper continental slope of the study area previously identified by Hernández-Molina [1993]. These results indicate that the generation of intermediate and bottom nepheloid layers, as well as the erosion and shaping of the northwestern Alborán continental slope, may result from the interaction of internal waves and the seafloor morphology. Copyright 2004 by the American Geophysical UnionThis work was funded by the ‘‘Comisión Interministerial de Ciencia y Tecnología’’ (project MAR96-1781-CO2-01) and by the ‘‘Dirección General de Enseñanza Superior e Investigación Científica’’ (project MAR99-1060-CO3-01)Peer Reviewe

Seasonal sediment dynamics on the Barcelona inner shelf (NW Mediterranean): a small Mediterranean river- and wave-dominated system

The seasonal pattern of sediment dynamics on an inner shelf characterized by the presence of sediment delivered by a small, mountainous river (with a “flash-flood” regime) was investigated. Near-bottom suspended sediment fluxes across the shelf (i.e. 20, 30 and 40 m water depth) were estimated using observations from three benthic tripods deployed from September 2007 to June 2008. Near-bottom sediment resuspension was controlled by wave-induced currents and river-born sediment availability, whereas the shelf currents played a secondary role. Fourteen sediment transport events were identified (eight in autumn, two in winter and four in spring), with transport rates according to storm intensity and sediment availability. These few energetic events induced a large percentage of the cumulative sediment transport near the bottom. However, the lack of proportionality between suspended sediment transport rates and the combined wave-current bottom shear stress in some events highlights the importance of the sequence of events in sediment dynamics. Since wave activity, hydrography and river discharges display a strong seasonal pattern in the NW Mediterranean, the resulting sediment dynamics across the shelf also correspond to a seasonal cycle. This seasonal variability leads to a temporal evolution of the bottom grain size (coarser in winter) and the near-bottom sediment transport rates (higher in spring and autumn) which is consistent with the seasonal pattern of the hydrodynamic events and the river discharge load.Peer ReviewedPostprint (author's final draft

Trace metal variability controlled by hydrodynamic processes in a polluted inner shelf environment (Besòs prodelta, NW Mediterranean)

14 pages, 11 figures, 3 tables, supplementary material https://doi.org/10.1016/j.scitotenv.2020.139482Trace metal pollution of coastal sediment is monitored in many countries to control its evolution and the effectiveness of preventive and corrective measures. However, temporal variability of trace metal pollution is not always due to changes in pollution management, as natural processes can induce a significant variability in the trace metal content of sediment and particulate matter, especially in strongly polluted coastal areas. To study this variability, time series of trace metals in particulate matter and bottom sediments were recorded along with hydrographic and hydrodynamic parameters in the most highly polluted zone of the Besòs River prodelta. Two benthic tripods equipped with current meters, turbidimeters and sediment traps were deployed at 20 and 30mwater depth from late-September to mid-June and sediment cores were taken four times at each site during the deployment period. Trace metal content in the trapped particulate matter and the surface sediment increased during storm events, which can resuspend and erode several cm of subsurface sediments with higher pollution levels from earlier industrial times. After the storms, significant accumulation of less polluted sediment began, and near bottom currents redistributed it, decreasing trace metal contents in surface sediments and trapped particulate matter. Therefore, energy conditions previous to monitoring sampling must be considered in order to evaluate the evolution of trace metals in inner shelf polluted sedimentsA.P., J.G. and P.P. belong to CRG on Littoral and Oceanic Processes, supported by Grant 2017 SGR 863 of the Generalitat de CatalunyaWith the funding support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S), of the Spanish Research Agency (AEI

Interaction of dense dense shelf water cascading and open-sea convection in the Northwestern Mediterranean during winter 2012

Durrieu de Madron, Xavier ... et al.-- European Geosciences Union General Assembly 2013, 7-12 April, Vienna, AustriaThe winter of 2012 experienced peculiar atmospheric conditions that triggered a massive formation of dense water on the continental shelf and in the deep basin of the Gulf of Lions. Multi-platforms observations enabled, with an unprecedented resolution, a synoptic view of dense water formation and spreading at basin scale. Five months after its formation, the dense water of coastal origin created a distinct bottom layer up to few hundreds of meters thick over the central part of the NW Mediterranean basin, which was overlaid by a layer of newly formed deep water produced by open-sea convection. These observations highlight the role of intense episodes of both dense shelf water cascading and open-sea convection to the alteration of the characteristics of the NW Mediterranean deep watersPeer Reviewe

Bottom-trawling along submarine canyons impacts deep sedimentary regimes

Many studies highlight that fish trawling activities cause seafloor erosion, but the assessment of the remobilization of surface sediments and its relocation is still not well documented. These impacts were examined along the flanks and axes of three headless submarine canyons incised on the Barcelona continental margin, where trawling fleets have been operating for decades. Trawled grounds along canyon flanks presented eroded and highly reworked surface sediments resulting from the passage of heavy trawling gear. Sedimentation rates on the upper canyon axes tripled and quadrupled its natural (i.e. pre-industrialization) values after a substantial increase in total horsepower of the operating trawling fleets between 1960 s and 1970 s. These impacts affected the upper canyon reaches next to fishing grounds, where sediment resuspended by trawling can be transported towards the canyon axes. This study highlights that bottom trawling has the capacity to alter natural sedimentary environments by promoting sediment-starved canyon flanks, and by enhancing sedimentation rates along the contiguous axes, independently of canyons’ morphology. Considering the global mechanisation and offshore expansion of bottom trawling fisheries since the mid-20th century, these sedimentary alterations may occur in many trawled canyons worldwide, with further ecological impacts on the trophic status of these non-resilient benthic communities

Contrasting particle fluxes and composition in a submarine canyon affected by natural sediment transport events and bottom trawling

Unidad de excelencia María de Maeztu CEX2019-000940-MSubmarine canyons are important conduits of sediment and organic matter to deep-sea environments, mainly during high-energy natural events such as storms, river floods, or dense shelf water cascading, but also due to human activities such as bottom trawling. The contributions of natural and trawling-induced sediment and organic matter inputs into Palamós Canyon (NW Mediterranean) were assessed from three instrumented moorings deployed in the axis and northern flank of the canyon covering the trawling closure (February) and the trawling season (March-December) of 2017. During the trawling closure, large sediment fluxes with high contents of labile marine organic matter content were registered in the canyon axis, associated to storm resuspension on the shelf that coincided with dense shelf water cascading and high surface water productivity. Although no major natural sediment transport events occurred during the following spring and summer months, near-daily trawling-induced sediment gravity flows were recordedin the northern flank mooring, placed directly below a fishing ground, which sometimes reached the canyon axis. Compositionally, the organic matter transferred by trawling resuspension was impoverished in the most labile biomarkers (fatty acids, amino acids, and dicarboxylic acids) and had a high degree of degradation, which was similar to surficial sediment from the adjacent fishing ground. Trawling resuspended particles masked the transfer of organic matter enriched in labile biomarkers that naturally occur during the quiescent summer months. Overall, bottom trawling enhances the magnitude of particle fluxes while modifying its organic carbon composition, increasing the re-exposure and transfer of degraded organic carbon and potentially affecting benthic communities that rely on the arrival of fresh organic matter