Structure, fonctionnement, évolution des communautés benthiques ...

tel-00009359, version 1 - 1 Jun 2005
Chapitre 3 - Fonctionnement du réseau trophique benthique de la Grande Vasière
Zooplankton was collected for the entire water column with a WP2 zooplankton net (0.2 mm mesh
size) and fish samples were obtained with an otter trawl.
At four stations, Particulate Organic Matter (POM) was collected with a Niskin bottle twice a
day for four days at depths near the fluorescence maximum (about 30 m depending on the station and
the time of day ; these depths are named “POM surface”) and close to the bottom (1 – 2 m ; i.e. “POM
bottom”). As a result of technical constraints of field cruises in these offshore waters all the samples
were not done in the same period. The biais induced by this problem was taken in account in the
analysis of results.
Stable isotopes
POM was collected by filtration of sea water (four liters of seawater were filtered for each
sample) on precombusted Whatman GF/F filters and store frozen. Subsequently, the filters were
exposed to HCl vapor for 4 h in order to remove carbonates before being placed in tin cups. The
samples were analyzed for isotope ratios using a Finnigan Delta S isotope ratio mass spectrometer
coupled to a Carlo Erba NA 2100 Element Analyzer.
Because of the low organic matter content (< 2%) and the high rate of carbonates measured in
the sediment, a total extraction of the Sediment Organic Matter (SOM) without degradation was not
possible. So SOM isotopic analyse was carried out.
Muscle tissue samples of megafauna, macrofauna and whole-body samples (without the
digestive gut) of polychaetes were used for stable isotope analysis. After dissection, the tissue samples
of all the taxa were washed carefully with distilled water in order to prevent any contamination by the
carbonates. Zooplankton and suprabenthos samples were acidified (10% HCl) to remove any residual
carbonates from cuticules and then rinsed with distilled water (Riera et al 2000). All samples were
stored frozen individually at –20°C before freeze-drying. Each dried sample was then ground to a
homogeneous powder, and 1 mg of this powder was weighed in tin capsules for stable carbon and
nitrogen isotopic analyses. 13 C/ 12 C and 15 N/ 14 N ratios of fauna tissue were determined by CF-IRMS
analysis using a Europa Scientific ANCA-NT 20-20 Stable Isotope Analyzer together with an ANCA-
NT Solid/Liquid Preparation Module. As the samples contained more than 10% nitrogen, the CF-
IRMS was operated in dual isotope mode, allowing δ 15 N and δ 13 C to be measured in the same sample.
Analytical precision (Standard Deviation (SD), n=5) was 0.2‰ for both nitrogen and carbon, as
estimated from standards analyzed together with the samples.
Stable isotope ratios were expressed in conventional δ notation as parts per mil (‰) according
to the following equation:
1) δ X = [ (R sample/R standard) – 1 ] x 1,000
where X is 13 C or 15 N and R is the corresponding 13 C/ 12 C or 15 N/ 14 N ratio.
As δ 15 N values provide an indication of the trophic position of a consumer, the following
formula was used to estimate trophic level (TL):
2) TL = (δ 15 N consumer - δ 15 N mean POM) / 3.4 + 1
where 3.4‰ is the assumed 15 N trophic enrichment factor according to Minagawa and Wada (1984). In
a benthic ecosystem such as the Great Mud Bank (depth > 100 m), no primary production can occur
227