Structure, fonctionnement, évolution des communautés benthiques ...

tel-00009359, version 1 - 1 Jun 2005
Chapitre 3 - Fonctionnement du réseau trophique benthique de la Grande Vasière
measurement: the identification of species whose feeding habits need further investigations. These
species, which show no coherence between their feeding habits as deduced from their morphology and
their behavior, and the isotopic values for carbon and nitrogen in their tissues, should be reconsidered
in different types of environmental situations in order to specify more accurately their position in food
web.
Variations of isotopic values during growth of target species
Diets are modified throughout the life cycle of many species, particularly as their increasing
size facilitates adaptation to other prey. In some studies, stable carbon and nitrogen isotope ratios have
been used as tracers of the diet changes of a species according to its life cycle and the variability of the
ecosystem exploited for food. These studies have focused on fishes (Wainright et al. 1993), pelagic
invertebrates (Riera et al. 2000) or benthic invertebrates (Kang et al. 1999).
Our results indicate that M. merluccius juveniles (8-15 cm) with δ 15 N of 11.8‰ may be
considered as third-level consumers and with δ 13 C around –18‰ that their diet is mainly of pelagic
origin. Guichet (1995) showed that their preys initially consist mainly of pelagic crustaceans.
Micromesistius poutassou, Trachurus trachurus are direct competitors at this size class. Large
M. merluccius adults (39-47 cm) show large differences in stable isotopes when compared with their
juveniles (2.3‰ and 1.1‰ respectively for δ 15 N and δ 13 C). Micromesistius poutassou and Trachurus
trachurus are the main preys of these adults (together with Engraulis encrasicolus Guichet (1995)).
With δ 15 N values averaging 12‰ (TL 3.4 and δ 13 C = -19 to -17‰), Trachurus trachurus shows no
clear changes in isotopic ratios between 17.5 and 26 cm in length. Its diet remains composed mainly of
Euphausiacea (Murta et al. 1993). Micromesistius poutassou follows the same isotopic trend as
T. trachurus, showing very similar isotopic values. Sorbe (1980) noted in the Southern Bay of Biscay
that the stomach of the same size classes of this species contained mainly pelagic Crustacea (copepods
and Euphausiacea).
The 2‰ enrichment of δ 15 N and the stable δ 13 C values for Trisopterus minutus indicate
changes in diet without any major changes in food origin. Armstrong (1982), who studied the stomach
contents of this species, found that prey size increased with fish size, but that preys consisted of
pelagic shrimps for all size classes. Riera et al. (2000) showed that changes in the diet of shrimps (fish
preys) throughout their life cycle are correlated with changes in their δ 15 N isotopic signature. Our
results confirm this observation, i.e., T. minutus feeds on the same kind of preys, but from different
size classes that induces change in their isotopic signature throughout their lives.
At a lower trophic level, M. merluccius juveniles (total length around 10 cm) and
N. norvegicus juveniles (cephalothorax length around 15 mm) had very close isotopic values for both
δ 13 C and δ 15 N. This finding reveals that these species could compete for the trophic resources in their
early stage of life.
The two benthic crustaceans, N. norvegicus and M. rugosa, showed the same pattern in terms
of stable carbon and nitrogen isotope ratios evolution with size. Thus, it can be deduced that they have
similar diets and compete in the same trophic niche. Both remain at the third trophic level (secondary
consumers) throughout their benthic life, but the isotopic values of carbon indicate that changes in diet
occur as they increase in size. The δ 13 C values (-18‰) of juveniles suggest a mixed diet of pelagic
and benthic preys, whereas a strict benthic diet is likely during the adult period. These results confirm
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