Influence of bottom trawling on sediment resuspension in the 'Grande-Vasière' area (Bay of Biscay, France)

Type Article
Date 2016-09
Language English
Author(s) Mengual Baptiste1, Cayocca Florence1, Le Hir Pierre1, Draye Robin2, Laffargue PascalORCID2, Vincent Benoit3, Garlan Thierry4
Affiliation(s) 1 : IFREMER DYNECO DHYSED, Ctr Bretagne, ZI Pointe Diable, CS 10070, F-29280 Plouzane, France.
2 : IFREMER EMH, Ctr Atlantique, Rue Ile Yeu,BP 21105, F-44311 Nantes 03, France.
3 : IFREMER STH LTBH, Stn Lorient, 8 Rue Francois Toullec, F-56100 Lorient, France.
4 : SHOM DOPS HOM Sedimentol, 13 Rue Chatellier,CS 92803, F-29228 Brest 02, France.
Source Ocean Dynamics (1616-7341) (Springer Heidelberg), 2016-09 , Vol. 66 , N. 9 , P. 1181-1207
DOI 10.1007/s10236-016-0974-7
WOS© Times Cited 12
Note This article is part of the Topical Collection on the 13th International Conference on Cohesive Sediment Transport in Leuven, Belgium 7–11 September 2015
Keyword(s) Bottom trawling, Sedimentresuspension, Seabed disturbance, Erosion flux, Bay of Biscay
Abstract Sea trials were performed on two zones with different fishing efforts on the continental shelf of the Bay of Biscay ('Grande-VasiSre' area of muddy sand) in order to assess particulate matter resuspension and seabed disturbances (i.e., penetration, reworking, grain size changes) induced by different types of trawls. Optical and acoustic measurements made in the water column indicate a significant trawling-induced resuspension mainly due to the scraping action of doors. It manifests as a highly dynamic turbid plume confined near the seabed, where suspended sediment concentrations can reach 200 mg l(-1). Concentration levels measured behind an "alternative" configuration (trawls with jumper doors instead of classical doors penetrating the sediment) are significantly lower (around 10-20 mg l(-1)), which indicates a potential limiting impact regarding the seabed. Grain size analyses of the surficial sediment led to highlight a potential reworking influence of bottom trawling. On the intensively trawled zone, this reworking manifests as an upward coarsening trend in the first 5 cm of the cores. A significant decrease in mud content (30 %) has been also witnessed on this zone between 1967 and 2014, which suggests an influence on the seabed evolution. The geometric analysis of bottom tracks (4-5-cm depth, 20-cm width) observed with a benthic video sledge was used to compute an experimental trawling-induced erosion rate of 0.13 kg m(-2). This erosion rate was combined with fishing effort data, in order to estimate trawling-induced erosion fluxes which were then compared to natural erosion fluxes over the Grande-VasiSre at monthly, seasonal and annual scales. Winter storms control the annual resuspended load and trawling contribution to annual resuspension is in the order of 1 %. However, results show that trawling resuspension can become dominant during the fishing high season (i.e., until several times the natural one in summer). In addition, the contribution of trawling-induced resuspension is shown to increase with water depth, because of the rapid decay of wave effects. Finally, the seasonal evolution of the respective contributions for erosion (mainly trawling and waves) could be mapped for the whole study area.
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