Structure, fonctionnement, évolution des communautés benthiques ...

tel-00009359, version 1 - 1 Jun 2005
Chapitre 3 - Fonctionnement du réseau trophique benthique de la Grande Vasière
(Glémarec 1971). The Great Mud Bank is an important French demersal and benthic fishery, mainly
compounding Atlantic hake (Merluccius merluccius) and Norway lobster (Nephrops norvegicus), but
also sole (Solea solea), and anglerfish (Lophius sp. ; Dardignac 1988).
The study area (sandy mud 95 - 120 m in depth) was selected in the central part of the Atlantic
hake fishery. The soft bottom macrofauna was defined by Glémarec (1969) as a Nucula sulcata-
Brissopsis lyrifera community.
Figure 1: The western part of continental shelf of the Bay of Biscay, Northeast Atlantic. The “Great Mud
Bank” is located inside the dotted marks in light grey. The square represent sampling stations.
Sampling
All data used for isotopic analysis was collected from ten sites within the central part of the
Great Mud Bank selected to be representative of the variability of the muddy sediment on the basis of
previous data (Chassé and Glémarec, 1976). Sediment analysis of our sampling stations showed that
the pelitic fraction (i.e. the fraction of the sediment with particle size < 63 µm) ranged from 9.65%
(median particle size = 170-200 µm, depth 127 m) to 34.86% (median particle size = 170 – 200 µm,
depth 110 m). A 2-m beam trawl was used to sample the epibenthos, i.e. invertebrate megafauna
(individual mean size > 10 mm) and demersal fish (Kaiser et al. 1994; Jennings et al. 1999; Ellis et al.
2000). This proved to be an efficient method for large and rare species and for the integration of smallscale
seabed patchiness (Frauenheim et al. 1989). The beam trawl was fitted with a chain mat and a
20-mm mesh liner. During sampling (20 min), warp length was three times water depth, and the
distance trawled was measured by the ship differential global positioning system (DGPS). The
macrofauna (individual mean size 1 – 10 mm) was sampled using a Hamon grab (0.25 m²), and a
suprabenthic sledge (MACER-GIROQ) was used for suprabenthos collection (0.5 mm nets mesh size).
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