Trawling-induced resuspension and dispersal of muddy sediments and dissolved elements in the Gulf of Lion (NW Mediterranean)

Abstract

International audienceA dedicated trawling experiment was performed at three sites on the Gulf of Lion continental shelf, with the aim of assessing the resuspension of particulate and dissolved matter triggered by different types of trawls on muddy sediments. The different configurations were: (i) bottom trawl, with bobbin for ground rope (Rockhopper): (ii) bottom trawl, without bobbin (Medits); and (iii) pelagic trawl, towed at 1 and 10m above the seabed.;The plumes of resuspended sediment were measured using the acoustic backscattered intensity, from a towed ADCP. Concomitant profiles of particle size-distribution, light transmission and water samples were collected, outside and inside the plumes. The analysis of the data enabled derivation of the major physical and chemical characteristics of the plumes generated by the trawls; likewise, and to quantify the resuspension fluxes of sediment, particulate (PN, POC) and dissolved (nutrients) elements. The residence time and dispersal of the plumes were monitored and modelled, considering the settling velocity of the particulate matter and the near-bottom turbulence.;The results indicate that the bottom trawls produce significant resuspension, whilst the near-bottom and mid-water pelagic trawls have no impact upon the sediment. The sediment clouds at several hundreds metres astern of the bottom trawls are 3-6m high and 70-200m, wide; they were generated both by the otter doors and the net. The average suspended sediment concentrations measured in the plumes reach 50 mg l(-1). Resuspension fluxes of sediment along the path of the trawls range from 190 g m(-2) s(-1), for the coarsest sediment (clayey silt) to 800 g m(-2) s(-1) for the finest sediment (silty clay). Whilst the resuspended loads of dissolved elements (nutrients) within the plume segment suggest a release of porewater, present at least in the first few centimetres of sediment, the particulate matter load only resulted from the resuspension of less than 1 mm thickness of the sediment bed. This discrepancy shows that a very small fraction of the sediment ploughed by the trawl is effectively injected into the water column.;The monitoring of the settling of the plumes indicates a rapid decay of the sediment load, during the first hour after its generation. Some of the sediment (about 10-15% of the initial load) remains in suspension; this is due, probably, to the near-bottom turbulence that prevents the redeposition of the fine particles and aggregates. Lateral spreading of the plume is strongly dependent upon the variability of horizontal currents. (c) 2005 Elsevier Ltd. All rights reserved

Rapid respiratory responses of the deep-water sponge Geodia barretti exposed to suspended sediments

“…The natural sediment (sieved to <1 mm) used in the experiment was collected using a box core at the site of sponge collection, and the top 5 cm of sediment was collected. The selected sediment concentrations are ecologically relevant according to measurements taken after bottom trawling (Schubel et al 1978, Schoellhamer 1996, Durrieu de Madron et al 2005, Bradshaw et al 2012.…”

“…1). Bottom trawlers are designed to penetrate several centimetres into the seafloor and re-suspend large quantities of bottom sediment into the water column (Schoellhamer 1996, Durrieu de Madron et al 2005, Bradshaw et al 2012. Suspen ded sediment concentrations after a single trawl pass have been measured to vary between 5 and 500 mg l −1 (Schubel et al 1978, Schoellhamer 1996, Durrieu de Madron et al 2005, Bradshaw et al 2012.…”

“…Suspen ded sediment concentrations after a single trawl pass have been measured to vary between 5 and 500 mg l −1 (Schubel et al 1978, Schoellhamer 1996, Durrieu de Madron et al 2005, Bradshaw et al 2012. The largest particles settle within minutes or hours due to gravity, but 10 to 20% of the suspended load remains in suspension for several days (Durrieu de Madron et al 2005, Bradshaw et al 2012. Bradshaw et al (2012) measured that 95% of suspended (1996−2000) on northern Norwegian continental shelf estimated for the smallest statistical subarea units reported for catch and effort in Norway, i.e.…”

Exposure of cod Gadus morhua to resuspended sediment: an experimental study of the impact of bottom trawling

“…Given the lack of a reference point for sediment concentration and extinction rates representative of commercial fishing, our experiment was designed to mimic the highest concentrations referenced behind trawls (Schubel et al 1979) in order to create a worst-case scenario. Concentrations may be expected to fall rapidly due to settling of large and flocculated particles (Durrieu de Madron et al 2005). However, approx.…”

“…Trawling can resuspend sediment up to a height of 10 m above the bottom (Churchill 1989) and may be the main agent of sediment transport in certain areas. While much of the suspended sediment settles within a few hours due to gravity and flocculation, ~10 to 30% of the initial load may remain in suspension for several days (Durrieu de Madron et al 2005). Near bottom sediment concentrations have been reported to be as high as several hundred mg l -1 (Schubel et al 1979, Schoellhamer 1996.…”