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 impact of benthic trawling on macrofaunal communities has been studied in terms of
changes in abundance and biomass (Jennings and Kaiser 1998; Lindeboom and de Groot 1998; Kaiser
et al. 2000) and in the structure of the marine food web (Pauly et al. 1998, Jennings et al. 2001).
Benthic trawling leads to modifications of trophic interactions (predation and competition) through
species removal (target and by-catch species), direct and indirect mortality (stimulation of scavengers
and opportunistic predators) and mechanical disturbances of benthic habitats (reviewed by Jennings
and Kaiser 1998).
The continental shelf of the Bay of Biscay is an important demersal and benthic fishery for
France. In its northern part, from South Brittany to the Loire estuary, the “Grande Vasière” (Great
Mud Bank) constitutes a main fishery for Atlantic hake (Merluccius merluccius) and Norway lobster
(Nephrops norvegicus). Benthic and demersal fish communities of the Bay of Biscay have been
described recently (Poulard and Boucher 1997; Blanchard 2000), but no work since that of Glémarec
(1969) has investigated macroinvertebrate benthic communities on a large spatial scale. The aims of
this study was to better understand the trophic position of two species of economical importance
(Merluccius merluccius and Nephrops norvegicus) in their respective communities and in particular to
identify their main trophic competitors and prey. To investigate this question we used stable nitrogen
isotope ratios as an indicator of trophic level and δ 13 C as a tracer of the origin of the food consumed.
Stable isotope (carbon and nitrogen) analysis is now regarded as a classical approach for
studying trophic levels and tracing food sources in marine ecosystems (Peterson and Fry 1987;
Michener and Schnell 1994). Indeed, heterotrophic stable isotopic signature depends on the stable
isotopic signatures of its preys and on isotopic fractioning during food processes. Stable nitrogen and
carbon isotope ratios (expressed respectively as δ 15 N and δ 13 C) are typically increased from preys to
consumers by 3-4‰ for δ 15 N and 1‰ for δ 13 C (DeNiro and Epstein 1981; Minawaga and Wada 1984;
Cabana and Rasmussen 1994). Thus, δ 15 N can indicate the trophic position of an organism within the
food web on the basis of the organic matter and the primary producers. The particular δ 13 C values
found in primary producers (phytoplankton and microphytobenthos) are also responsible for
differences in the isotopic signatures of their consumers (Riera et al. 1999), allowing distinctions to be
made between pelagic and benthic food webs (Hobson et al. 2002). Consequently, δ 13 C can be used as
a tracer of organic trophic sources for benthic consumers (Peterson 1999).
Materials and methods
Area
A large sedimentary bank known as the “Grande Vasière” characterizes the northern
continental shelf of the Bay of Biscay (French coast, Northeast Atlantic) (Figure 1). This Great Mud
Bank is situated between a depth of 80 m and 130 m over a distance of 150 nautical miles from south
to north and 30 - 40 nautical miles from east to west. It consists of a large expanse of muddy
sediments parallel to a coastal area composed of pre-littoral depressions, gravelly plains and hydraulic
dunes. To the west, at around 130 m of depth, hard bottoms separate the Great Mud Bank from the
outer edge of the continental shelf which is constituted by Ditrupa sands down to 160 meters
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