Structure, fonctionnement, évolution des communautés benthiques ...

tel-00009359, version 1 - 1 Jun 2005
Particulate matter sedimentation
Chapitre 3 - Fonctionnement du réseau trophique benthique de la Grande Vasière
Mooring components and operation. Multiple sample programmable Pro-Trap sediment traps
(Thouzeau and Gros, 1993; Bale, 1998) were deployed on the study sites (moored traps). The Pro-Trap
allows for sequential sampling of particles settling out of the water column over pre-determined time
intervals. There is also the capability to log other selected parameters (temperature, pressure, angle of
the frame, and light scattering) and interface to electromagnetic current meters (Thouzeau et al.,
submitted). Each system consists of four PVC cylindrical funnels, each of 0.0184 sq.m. exposed area
(aspect ratio 5.3), mounted in a stainless steel frame. Carousels mounted underneath two of the funnels
are each fitted with 11 sample jars which are advanced by rotation of the carousels at pre-set intervals
to take 2 x 11 samples. No sample jars are fitted in the first sample positions, i.e. the two apertures
(one on each carousel) open to the sediment tubes when the trap is deployed. This is to avoid capture
of sediment possibly stirred up during deployment and, if there is to be a long delay before the first
advance, to prevent a sample jar being over-loaded with sediment with consequent carry-over to the
next one. The other two funnels take a single pair of samples over the entire deployment. The two fullterm
sample jars are located in pivoting holders beneath the base plate. At the end of sampling, when
the carousels move from the final sampling position to the Start/Stop position, the pivoting holders are
moved out of line allowing the water to drain freely out of the tubes on recovery.
Depth (Druck 35 bar pressure transducer PDCR 810) and angle (Spectron systems Technology
SP5003) sensors allow to define the position of the traps in the water column. A 500 m depth-rated
VALEPORT 800-0 series electromagnetic current meter is also fitted to each PRO-TRAP system. A
solid state 5.5 cm spherical sensor provides 2-axis flow velocity measurements. Finally, a light
scattering sensor (Sea-Tech LS sensor) allows to measure water turbidity at the trap aperture.
Sampling strategy. Bottom-moored traps were used to assess the total downward flux and
particulate organic matter (POM) sedimentation. The moorings were deployed during the GASPROD
and TROPHAL cruises. The mooring consisted of a subsurface spherical float (425 liter buoyancy), 5
22-liter VINYFLOAT buoys at a nominal depth of 15-30 m, and one (April-May cruise) or two
(September cruise) Pro-Trap/Valeport systems positioned under or just over the thermocline and 1.6 m
off the bottom. The bottom system was located within a 2.5 m high steel benthic frame. Bottle
advances occurred every 66 min during each experiment (total sampling duration of 12 h 6 min; 70
scans per sample, 6 sensor readings per scan). Sampling duration and partitioning were chosen to
record short-term variations in sedimentation events in relation to water mass advection. Thus,
particulate matter sedimentation was measured during one tidal cycle. Given the sampling durations,
poison or preservative were not added to the sample jars.
Trap samples were treated on board in order to measure mass flux and POM content (PTC,
POC, PON, chlorophyll pigments, and stable carbon and nitrogen isotopic ratios, δ 13 C and δ 15 N) of
sedimented material at the laboratory.
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