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Structure, fonctionnement, évolution des communautés benthiques ...

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tel-00009359, version 1 - 1 Jun 2005<br />

INTRODUCTION<br />

Chapitre 3 - Fonctionnement du réseau trophique benthique de la Grande Vasière<br />

The Bay of Biscay (NE Atlantic) extends from 48°50 to 43°50 N and 1° to 6° W, representing<br />

ca. 2.10 6 km 2 . The continental shelf of the Bay is classified as a meso- to macro-tidal (4-5 m tide<br />

range) and storm wave-dominated environment (Arbouille, 1987). On the northern part of the shelf,<br />

the main open-shelf field of fine-grained deposits is known as the ‘Grande Vasière’ (Allen and<br />

Castaing, 1977). This large sedimentary area represents nearly 75% of the total surface of fine deposits<br />

in the Bay of Biscay (Jouanneau et al., 1999). This mud Bank, situated between 80 and 130 m depth,<br />

extends on 150 nautical miles from south to north and 30 to 40 n.m. from east to west. It consists of a<br />

mosaic of various sediments mostly composed of mud, sandy mud and muddy sand, in which the<br />

entrapped fine-grained material may account for up to 25% of the total dry weight of the sediment<br />

(reviewed in Lesueur et al., 2001). The ‘Grande Vasière’ sustains important french demersal and<br />

benthic fisheries, mainly for sole (Solea solea), Atlantic hake (Merluccius merluccius), Norway<br />

lobster (Nephrops norvegicus) and anglerfish (Lophius sp.; Dardignac, 1988).<br />

The present study is part of the french National Program in Coastal Environment (NPCE),<br />

which main goals in the Bay of Biscay are: (i) to relate the distribution of the main pelagic and benthic<br />

fish species with environment variability, and (ii) to evaluate the impacts of fishing activities (mainly<br />

pelagic- and bottom-trawling) on the pelagic and benthic communities.<br />

The specific objectives of this study are to characterize hydrological parameters and vertical<br />

export of particulate matter within the muddy bank, the sedimented materials constituting potential<br />

food sources for benthic suspension and deposit feeders. Both spatial and temporal variability of<br />

sedimentation processes are considered; in particular, high frequency (daily) patterns linked to<br />

hydrodynamical conditions and seasonal (spring vs. late summer) variations were studied. Spring and<br />

late summer conditions were selected due to the occurrence of major phytoplankton blooms at these<br />

periods in the Bay of Biscay (Lampert, 2001; Loyer, 2001; Lampert et al., 2002). Re-suspension was<br />

also estimated, in order to determine the net particle fluxes originating from the water column and<br />

sedimenting on the muddy bank. The results obtained in this study will be used later on to determine if<br />

pelagic compartments may constitute significant food sources for the benthos living on the continental<br />

shelf of the Bay of Biscay (food web modelling; coll. O. Le Pape, IFREMER Nantes).<br />

At the european scale, major studies have been conducted in the North Sea, in the English<br />

Channel, in the Skagerrak and in the Baltic Sea to quantify the role of benthic-pelagic coupling in food<br />

web structure and long-term changes (Reise, 1982; Kroncke, 1995; Lindeboom and de Groot, 1998;<br />

Frid and Clark, 2000), as well as in carbon export (Van Haren et al., 1998; Van Raaphorst et al.,<br />

1998). This study is the first one addressing vertical export of particulate matter on the continental<br />

shelf of the Bay of Biscay in relation to benthic macrofauna biomass. Multiple sample programmable<br />

sediment traps and electromagnetic current meters were deployed at each site to measure the total<br />

downward sediment flux. Over the past three deca<strong>des</strong>, the use of sediment traps to measure coastal and<br />

oceanic particle fluxes and composition has proliferated. The sediment trap is currently the main<br />

existing tool for the direct measurement of settling particles (see Knauer and Asper, 1989). Various<br />

studies of the vertical transport of material using sediment traps have provided results which agree<br />

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