Changes in sediment accumulation rates within NW Mediterranean submarine canyons caused by bottom trawling activities

Submarine Canyon Dynamics in the Mediterranean and Tributary Seas - An integrated geological, oceanographic and biological perspective, 15-18 April 2015, Sorrento, Italy.-- 8 pages, 5 figuresThe disturbance of the marine sedimentary environments by commercial bottom trawling is a matter of concern. The direct physical effects of this fishing technique include scraping and ploughing of the seabed and increases of the near-bottom water turbidity by sediment resuspension. However, the quantification of the sediment that has been resuspended by this anthropogenic activity over the years and has been ultimately transferred along and across the margin remains largely unaddressed. The analysis of sediment accumulation rates from sediment cores collected along the axes of several submarine canyons in the Catalan margin (northwestern Mediterranean) has allowed to estimate the contribution of bottom trawling to the present-day sediment dynamics. 210Pb chronologies, occasionally supported by 137Cs dating, indicate a rapid increase of sediment accumulation rates since the 1960-70s, along with a strong impulse in the industrialization of the trawling fleets of this region. Such increase has been associated to the enhanced delivery of sediment resuspended by trawlers from shelf and upper slope trawling grounds towards submarine canyons, as a consequence of the rapid technical development at that time, in terms of engine power and gear size. This change has been observed in La Fonera (or Palamós) Canyon (one of the most prominent canyons of the region) at depths greater than 1700 m, while in other canyons not so deeply incised it is restricted to shallower regions (-1000 m in depth) closer to fishing grounds. Two sampling sites from La Fonera and Foix submarine canyons that exhibited high sediment accumulation rates (0.6-0.7 cm y-1) were revisited several years after the first chronological analyses. These two new cores revealed a second and even more significant increase of sediment accumulation rates in both canyons which occurred circa 2000 and reached values higher than 2 cm y-1. This second change at the beginning of the 21st century has been attributed to a preferential displacement of the trawling fleet towards slope fishing grounds surrounding submarine canyons, and to new technical improvements in trawling vessels, presumably related to subsidies and aids provided by the European Commission to the fishing industryThis work was funded by the HERMIONE and PERSEUS projects (7FP Grant agreements: 226354 and 287600) and the Generalitat de Catalunya grants (2014 SGR 1356 and 1642)Peer Reviewe