Structure, fonctionnement, évolution des communautés benthiques ...

tel-00009359, version 1 - 1 Jun 2005
Chapitre 3 - Fonctionnement du réseau trophique benthique de la Grande Vasière
with data obtained using different scientific approaches (reviewed in Knauer and Asper, 1989; see also
Pedersen et al., 1995; Pejrup et al., 1996). There is evidence that sediment traps provide reasonable
flux information for specific particles, chemical compounds, time periods or ecosystems. In particular,
an obvious relationship exists between estimates of total primary production and POC sedimentation
out of the euphotic zone, both spatially and seasonally (eg. Suess, 1980; Deuser et al., 1981; Ittekkot et
al., 1984a, b; Betzer et al., 1984a, b; Lee and Cronin, 1984; Deuser, 1986a, b; Pace et al., 1987;
Wassmann, 1988; Bathmann et al., 1991). However, the relationship between the size and settling rate
of large particles contains considerable scatter and particle distribution cannot be always converted
into sedimentation rates (see Knauer and Asper, 1989). In particular, this relationship may be all the
more complex in coastal zones facing macrotidal regime and high turbulence, such as the Bay of
Biscay. The present study investigates deposition and re-suspension of particulate matter on the
“Grande Vasière” by using two complementary approaches: in situ measurements of sedimentation
rate and theoretical calculation of re-suspension.
MATERIALS AND METHODS
Study area
The study area extends from 46°10 to 47°50 N and 1° to 6°W, representing ca. 50 000 km 2 .
Mean depth ranges from 0 to 170 m. In the central part of the Bay of Biscay, the muddy sediments lie
on sands and shell gravels derived from river deposits, reworked during the Holocene transgression
(Allen and Castaing, 1977). These fine-grained deposits do not generally exceed a few decimetres in
thickness, and the extent of this cover varies according to the season (Pinot, 1974) and to the
hydrodynamical conditions. The ‘Grande Vasière’ is parallel to a coastal area composed of pre-littoral
depressions, gravel plains and hydraulic dunes. To the west, at ca. 130 m depth, hard bottoms separate
the mud bank from the outer edge of the continental shelf constituted by Ditrupa arietina sands down
to 160 m (Glémarec, 1969). Rivers are the main sources of fine sediments that enter the Bay of Biscay
(Jouanneau et al., 1999). At present, 2.5 x 10 6 t.yr -1 of continental-born fine sediments are delivered by
rivers onto the Bay of Biscay. Eighty-five percent of this input is originating from the two main rivers
(Gironde and Loire). About 65% of this sediment flux is ‘stored’ onto the continental shelf (Jouanneau
et al., 1999).
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