Structure, fonctionnement, évolution des communautés benthiques ...

tel-00009359, version 1 - 1 Jun 2005
INTRODUCTION
Chapitre 3 - Fonctionnement du réseau trophique benthique de la Grande Vasière
The Bay of Biscay (NE Atlantic) extends from 48°50 to 43°50 N and 1° to 6° W, representing
ca. 2.10 6 km 2 . The continental shelf of the Bay is classified as a meso- to macro-tidal (4-5 m tide
range) and storm wave-dominated environment (Arbouille, 1987). On the northern part of the shelf,
the main open-shelf field of fine-grained deposits is known as the ‘Grande Vasière’ (Allen and
Castaing, 1977). This large sedimentary area represents nearly 75% of the total surface of fine deposits
in the Bay of Biscay (Jouanneau et al., 1999). This mud Bank, situated between 80 and 130 m depth,
extends on 150 nautical miles from south to north and 30 to 40 n.m. from east to west. It consists of a
mosaic of various sediments mostly composed of mud, sandy mud and muddy sand, in which the
entrapped fine-grained material may account for up to 25% of the total dry weight of the sediment
(reviewed in Lesueur et al., 2001). The ‘Grande Vasière’ sustains important french demersal and
benthic fisheries, mainly for sole (Solea solea), Atlantic hake (Merluccius merluccius), Norway
lobster (Nephrops norvegicus) and anglerfish (Lophius sp.; Dardignac, 1988).
The present study is part of the french National Program in Coastal Environment (NPCE),
which main goals in the Bay of Biscay are: (i) to relate the distribution of the main pelagic and benthic
fish species with environment variability, and (ii) to evaluate the impacts of fishing activities (mainly
pelagic- and bottom-trawling) on the pelagic and benthic communities.
The specific objectives of this study are to characterize hydrological parameters and vertical
export of particulate matter within the muddy bank, the sedimented materials constituting potential
food sources for benthic suspension and deposit feeders. Both spatial and temporal variability of
sedimentation processes are considered; in particular, high frequency (daily) patterns linked to
hydrodynamical conditions and seasonal (spring vs. late summer) variations were studied. Spring and
late summer conditions were selected due to the occurrence of major phytoplankton blooms at these
periods in the Bay of Biscay (Lampert, 2001; Loyer, 2001; Lampert et al., 2002). Re-suspension was
also estimated, in order to determine the net particle fluxes originating from the water column and
sedimenting on the muddy bank. The results obtained in this study will be used later on to determine if
pelagic compartments may constitute significant food sources for the benthos living on the continental
shelf of the Bay of Biscay (food web modelling; coll. O. Le Pape, IFREMER Nantes).
At the european scale, major studies have been conducted in the North Sea, in the English
Channel, in the Skagerrak and in the Baltic Sea to quantify the role of benthic-pelagic coupling in food
web structure and long-term changes (Reise, 1982; Kroncke, 1995; Lindeboom and de Groot, 1998;
Frid and Clark, 2000), as well as in carbon export (Van Haren et al., 1998; Van Raaphorst et al.,
1998). This study is the first one addressing vertical export of particulate matter on the continental
shelf of the Bay of Biscay in relation to benthic macrofauna biomass. Multiple sample programmable
sediment traps and electromagnetic current meters were deployed at each site to measure the total
downward sediment flux. Over the past three decades, the use of sediment traps to measure coastal and
oceanic particle fluxes and composition has proliferated. The sediment trap is currently the main
existing tool for the direct measurement of settling particles (see Knauer and Asper, 1989). Various
studies of the vertical transport of material using sediment traps have provided results which agree
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