Structure, fonctionnement, évolution des communautés benthiques ...

tel-00009359, version 1 - 1 Jun 2005
Chapitre 3 - Fonctionnement du réseau trophique benthique de la Grande Vasière
At a second level (figure 2) a group of five taxa was clearly separated from all others. This
group is composed of benthic suspension feeders, zooplankton and suprabenthic crustacea; (TL close
to 2 and δ 13 C close to –20‰). A difference of 3.4‰ in the δ 15 N value is observed between POM
values and values for ascidians and the crustacean C. subterranea, two taxa considered in the literature
as suspension feeders exploiting the water layer above sediment. Such a difference is also found for
zooplankton and suprabenthos which are known to feed on POM of the water column. Moreover this
validates the value of the enrichment factor retained in the equation used to determine trophic level
and confirms that these taxa are the primary consumers.
Subgroup 2b is composed of two bivalves and a burrowing crustacean with a TL close to 2.5
(δ 15 N: 8-10‰). A 3.8‰ δ 13 C gap between the consumers of the pelagic primary production and these
others microphageous species reveals that they feed on different types of organic particles. This range
of δ 13 C differences between POM and benthic consumers, and between benthic suspension and deposit
feeders, has already been observed in offshore (Hobson et al. 1995, 2002) and coastal (Herman et al.
2000) benthic ecosystems.
The bivalve A. opercularis is a filter feeder species which feeds on organic particles in
suspension in the two first centimeters of the water layer just above the sediment surface, while
N. sulcata is a surface deposit-feeder. The position of the Pectinidae species in this “benthic deposit
feeder group” indicates that suspension feeders must be carefully considered in energy budget and
functional analyses and that behavior and nutritional anatomical organs are not always sufficient to
affect species in trophic groups. Furthermore, we can deduce that in the ecosystem of the Great Mud
Bank, either the quality of the suspended particles change drastically in the first centimeters of the
water layer above the sediment and that this change is sufficiently stable in time to induce different
isotopic signatures in the tissue of consumers and/or that these species could differ in particle selection
efficiency. A. opercularis lies on the sediment and could possibly ingest a higher rate of interface
superficial film than ascidians, which have inhalant siphons a few centimeters above the interface. In
the same manner, the polychaeta H. fauveli is positioned to feed at the apex of its tube erected four or
five centimeters above the sediment. Owens (1987) found that the δ 15 N of plant species increased
throughout the decomposition process and when considering specifically phytoplankton the amount of
total N decreased in conjunction with the decomposition of phytoplankton cells, whereas δ 15 N
abundance increased gradually in residual organic matter (Wada and Hattori 1991). As no primary
production can occur at a mean depth of 110 m in the Great Mud Bank ecosystem, the diet of deposit
feeders must be composed exclusively of deposited material both detritic and recycled from the
benthic organisms. This feeding pattern is quite different from that of coastal shallow ecosystems in
which microphytobenthos can be an important food source for deposit feeders (Riera et al. 1996;
Middelburg et al. 2000). Such differences within the benthic primary consumers were shown by
Herman et al. (2000) for a small tidal flat where suspension feeders (mainly on a planktonic algal diet)
showed δ 13 C values 4 to 6‰ less than those of deposit feeders (mainly on a benthic microalgal diet).
Goering et al. (1990) and Hobson et al. (1995) suggested that deposited phytoplankton was enriched in
13
C in the bottom nepheloid layer due to recycling across the microbial loop. The different feeding
behaviors of the species may also be a factor accounting for the observed differences in δ 13 C. As some
species are non-selective and others selective microphages. In the latter case, particle selection may be
based on quality as well as size. Some bivalves have a diet based not only on deposited material,
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