Structure, fonctionnement, évolution des communautés benthiques ...

tel-00009359, version 1 - 1 Jun 2005
Chapitre 3 - Fonctionnement du réseau trophique benthique de la Grande Vasière
microheterotrophs and bacteria (Kang et al. 1999; Kreeger and Newell 2001), but also on dissolved
organic carbon (Roditi et al. 2000). Such feeding preferences could lead to variations in the δ 13 C of
species considered broadly as part of the deposit-feeder group. These results can be compared with
those obtained by Fry (1988) for Georges Bank, an exploited benthic ecosystem in the same depth
range. No marked variability of δ 15 N was found there between benthic and pelagic primary consumers
(copepods, Euphausiacea, bivalves, benthic amphipods), but δ 13 C values for pelagic copepods and
Euphausiacea were 2-3‰ lower than for benthic primary consumers. It was concluded that these
differences reflect the influence of two different food sources or a modification of food quality.
The third group is composed of secondary consumers consisting of species from the pelagic food web
component, primarily because of food affinities (all species had δ 13 C < -16.7‰), and secondarily for
their trophic level. The trophic level of the analyzed species ranged from 3 to 3.7, suggesting a mixed
diet based on several different trophic levels.
The third trophic level of the food web is composed of benthic and pelagic Crustacea,
Polychaeta and bentho-pelagic fish (δ 15 N averaging 11‰). Among these species, analysis of the
known behavioral features identified two groups: endo- and epibenthic species (group 2a of the
dendrogram, Figure 2) demersal and suprabenthic species (group 3a of the dendrogram, Figure 2).
Group 2a; the benthic web component ( δ 13 C close to – 16‰) is composed of surface and
subsurface deposit feeders, carnivorous crustacean decapods (N. norvegicus, M. rugosa, L. depurator,
G. rhomboides), carnivorous crustacean peracarids (C. allmani) and polychaetes (N. caeca, G. rouxii).
Group 3a; the pelagic web component is composed of species with δ 13 C averaging -19 to -
18‰. These are the “true” pelagic feeders, i.e., the fish T. trachurus, the cephalopods Sepia sp., and
some Crustacea, Mysidacea and peracarid shrimp (C. echinulatus, P. nouveli).
The top predators, group 3b, which reached the fourth TL (3.6 to 4.0), had δ 15 N levels of up to
14‰ in adult muscle tissues. These predators are not only demersal fishes such as the Atlantic hake
adults, but also benthic crustaceans such as S. demaresti.
In general, the trophic positions predicted by δ 15 N were in good agreement with data in the
literature concerning fish and invertebrate diet and feeding behavior. However, three polychaete
species which showed unexpected values require further investigation. Notomastus latericeus, with
extensive distribution in coastal and intertidal muddy sediments, is considered to be a subsurface
deposit feeder, like most members of the family Capitellidae. Terebellides stroemi, a tubiculous of the
family Terebellidae, is regarded as a surface deposit feeder, which catches particles of the organic
superficial film with its palps and cirra (Fauchald and Jumars 1979). The results however have showed
great variations in the δ 13 C values of this species, which suggests considerable variability of the food
source, whereas δ 15 N values were higher than expected from its trophic behavior (i.e., primary
consumers) as described in the literature. Another species, Lumbrinereis impatiens, is a small surface
carnivorous Lumbrinereidae that supposed feeds on benthic preys. Its δ 13 C values were lower than
expected, which explains why it was grouped in the cluster of “pelagic component” species. Further
isotopic measurements are required over a large environmental gradient to confirm these values.
However, it is likely that the feeding behavior of these species is based on a selection of ingested food
mainly composed of animal detritus and/or micro- and meio- fauna. As remarked previously for the
bivalve Aequipecten opercularis these results emphasize another advantage of stable isotopic
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