Structure, fonctionnement, évolution des communautés benthiques ...

tel-00009359, version 1 - 1 Jun 2005
Chapitre 3 - Fonctionnement du réseau trophique benthique de la Grande Vasière
For isotopic analysis, SPM and TPM were filtered on pre-combusted Whatman GF/F filters
before storage at –20°C. Subsequently, the filters were exposed to HCl fume for 4 h in order to remove
carbonates before being placed in tin cups. The samples were analysed for isotope ratios using a
Finnigan Delta S isotope ratio mass spectrometer coupled to a Carlo Erba NA 2100 Element Analyzer
to allow POC and PON measures. Stable isotope ratios were expressed in conventional δ notation as
parts per mil (‰) according to the following equation:
δ X = [ (R sample/R standard) – 1 ] x 1,000 (1)
where X is 13 C or 15 N and R is the corresponding 13 C/ 12 C or 15 N/ 14 N ratio.
Pigments were extracted in 90% acetone, chlorophyll a and phaeopigments were measured on
a Turner 112 fluorometer according to Lorenzen (1967).
Benthic macrofauna organisms were identified to species level for most taxonomic groups
(except for nematods, nemerteans, phoronids and chelicerates), counted and weighed (ash-free dry
weight) after burning at 450°C for 4 hours. Benthic macrofaunal biomasses were then displayed in
three major trophic groups, i.e. suspension feeders, deposit feeders and carnivores, according to
Fauchald and Jumars (1979) and Bonsdorff and Pearson (1999).
In order to characterise the degree of water column stability, a stratification index (SI) based
on the water volumic mass (σt) differences by 5-m water cross-sections (Bustillos-Guzman et al., 1995
modified by Lampert et al., 2002) was computed, using the following equation:
n
∆σ
∑
i=
1 ∆z
SI =
n
t
x 10 4
where i = number of 5 m cross-sections in the water column and z = depth.
Re-suspension estimate
Re-suspension of bottom sediments may influence the sedimentation rates of particulate matter
measured by traps in macrotidal environments. To estimate the amount of particles re-suspended and
re-sedimented into the traps, three different methods were used. The first one was a ratio method
taking the chemical composition of particulate matter in the water column, in the traps and in the
sediment into account (Gasith, 1975; Tagushi, 1982; Clavier et al., 1995). The concentration of any
chemical compound in the trapped material can be expressed as a function of its value in the sediment
and in the suspended material:
Ct = XCs + (1-X)Cr
Where Ct, Cs and Cr are the percentages of the given element in trap, SPM and sediment,
respectively and X is the proportion of sedimented material in the trap. In this study, percent organic
carbon has been used to calculated X, following Bhaskar et al. (2000).
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