Sediment Accumulation Records in Blanes Canyon (NW Mediterranean): Anthropogenic Disturbance Caused by Bottom Trawling

Abstract

A comprehensive study of 15 sediment cores from the Blanes Canyon area was conducted to understand natural sedimentary processes and alterations caused by bottom trawling. Canyon axis and flanks, as well as adjacent open slope areas were sampled at depths ranging from 300 m to 2200 m. Grain size, dry bulk density and 210Pb concentration profiles were analyzed to obtain sediment properties and to study possible changes on sedimentation rates over the last century attributed to bottom trawling. The passage of trawling gear lead to eroded and disturbed surface sediments, altering the natural sedimentation rates within the canyon axis. Thus, shallower depths at the canyon axis (900-1200 m) presented considerably high sedimentation rates (0.9-2.1 cm·yr-1) accumulated after the 1970s. Further downcanyon, sedimentation rates increased two- to fourfold after this decade, from 0.1-0.2 cm·yr-1 to 0.2-0.8 cm·yr-1. This enhanced sedimentation is attributed to the resuspension and subsequent transport of sediments caused by the expansion of the operating trawling fleets at that time. These impacts are localized downslope from fishing grounds, and the anthropogenically-enhanced sedimentation rates decreased downcanyon at a greater distance from trawling grounds. On the lower continental slope (900-1500 m) and in the canyon axis (1500-2200 m), natural sedimentation rates (pre-1970s) were similar in both environments, indicating that sediment deposition at those depths is governed by the offshore distance from the continental sediment inputs. These results are a manifest of the capacity of trawling activities to erode flanks and rims of the Blanes Canyon and remobilize large amounts of sediments towards its interior, ultimately enhancing sedimentation rates along the canyon axis. While submarine canyons have been regarded as preferential conduits of particulate matter, they should also be considered depocenters of trawling-induced resuspended sediments