Structure, fonctionnement, évolution des communautés benthiques ...

tel-00009359, version 1 - 1 Jun 2005
Hydrology
Chapitre 3 - Fonctionnement du réseau trophique benthique de la Grande Vasière
For each sediment trap deployment, a specific profiler was used 5 times a day to measure
hydrological parameters and vertical particle size distribution and quantities. This profiler is composed
of a CILAS (Marcoussis, France) particle size analyser (PSA; Gentien et al., 1995), a SBE25 CTD
probe (Sea-Bird Electronics, Washington, USA), a video-system (Lunven et al., 2003) and a SBE32
carousel water sampler (Sea-Bird Electronics, Washington, USA). Operated in real time, this system is
adapted to the characterisation of the water column. From the detected layers, finest measurements are
performed at depth steps when the probe is raised. Video observations on marine particles are
displayed and recorded on deck. Water samples are therefore performed for biological or chemical
laboratory analyses.
For each profile, water samples were taken in the fluorescence maximum and near bottom (1 to 3
m above). In addition, 5 water depths were sampled once a day, according to the position of the
thermocline (surface and bottom water plus 3 water depths located within the thermocline).
Temperature, salinity, fluorescence, total particle load and particle size were measured directly.
Suspended Particulate Matter (SPM), Particulate Total Carbon (PTC), Particulate Organic Carbon
(POC), Particulate Organic Nitrogen (PON), stable carbon and nitrogen isotopic ratios (δ 13 C and
δ 15 N), and chlorophyll a (Chl a) and pheophytin a (Pheo) were measured on the water samples
collected.
Sediment and macrofauna
Sediment samples were collected using a Bowers and Connelly Mini Corer Special (BCMID-SP).
Core diameter was 9 cm, representing a 0.0064 m 2 surface area per sample. Sediment samples were
frozen at –20°C, except for grain size analysis. Parameters to be measured on the core samples were
sediment grain size,% organic matter, POC, and PON.
Benthic macrofauna samples were collected during the trap experiments with a 0.25 m 2 Hamon
grab (5 replicates per station, except for F not sampled). Once onboard, samples were washed, sorted
on a 1 mm mesh size and preserved in 7% formaldehyde solution.
Data analyses
The material collected in each cup of the sediment traps was re-suspended and homogenised
before subsamples were taken for various analyses. The samples collected either by sediment traps or
Niskin bottles have undergone the same kinds of treatments.
SPM and TPM were filtered on pre-combusted and pre-weighted (Whatmann GF/F) filters (47
mm diameter) and stored at –20°C prior to analysis. The filters were rinsed with demineralised water
to remove salt and dried at 60°C for 48 h before weighting. Then, the samples were analysed for PTC
content by using a Carlo Erba NC 2500 Element Analyser (Majeed, 1987).
Sediment organic carbon and nitrogen analyses were performed on a LECO 800 CHN gas
chromatography analyzer after removal of carbonates according to Cauwet (1995).
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