Structure, fonctionnement, évolution des communautés benthiques ...

tel-00009359, version 1 - 1 Jun 2005
DISCUSSION
Chapitre 3 - Fonctionnement du réseau trophique benthique de la Grande Vasière
Trap efficiency
Proper use of sediment traps and correct interpretation of results require that potential
collection of biases be recognized, minimized, and, where possible, eliminated. Traps in any current
alter the flow field, resulting in the generation of eddies at the top of the trap (see Knauer and Asper,
1989). The size and frequency of eddy formation and the flow structure within a trap varies with trap
geometry. These complex flow patterns generate vertical velocities much greater than the typical fall
velocities of most marine particles, making it difficult to predict the behavior of particles in the
vicinity of a trap. Past laboratory and field studies of sediment-trap biases identified six dimensionless
parameters which influence collection efficiency (Hargrave and Burns, 1979; Bloesch and Burns,
1980; Butman et al., 1986; Knauer and Asper, 1989). Of these, only three were shown by dimensional
analysis to be important under typical trapping conditions in the ocean: the trap Reynolds number, the
trap aspect ratio and the ratio of flow speed to particle fall velocity. In addition, symmetry of the trap
is required to maintain uniform flow dynamics in the trap and a minimum of three trap diameters
cross-stream and ten diameters downstream would be required to eliminate trap-trap flow interactions
(Butman, 1986). Though the biases demonstrated cannot be generalized beyond the parameter range
tested, a number of recommendations made by the “ U.S. GOFS working group on sediment trap
technology and sampling ” (in Knauer and Asper, 1989) were followed in this study. Cylindrical traps
with aspect ratio > 5 (A = 5.3) and diameter > 4 cm (D = 15 cm) were selected to maintain a tranquil
layer of fluid at the trap bottom (see also Bloesch and Burns, 1980; Blomquist and Hakanson, 1981;
Heussner et al. 1990; Gust et al., 1992; Bale, 1998), moorings were designed to minimize tilt of the
traps and the rotating sample collectors were interfaced with a current meter so that samples could be
fractionated according to intervals of current speed.
The low mean tilt (< 5°) and current speed (< 13.5 cm.s -1 ) at the bottom trap aperture indicate
that collection efficiency was unlikely to be biased near bottom (see Gardner, 1985; Baker et al.,
1988). This result was all the more valid for Stn. G (current speed < 8 cm.s -1 at both seasons) and for
the September sampling on the central ‘Grande Vasière’ (current speed < 8.2 cm.s -1 , except for Stn.
D). In comparison, Baker et al. (1988) found that the agreement between 20 cm diameter drifting and
moored traps was within 10% when mean current speed was < 15 cm.s -1 and the accumulated duration
of speeds < 12 cm.s -1 was > 60% of the deployment period. With respect to trap tilt, Gardner (1985)
showed that the collection rate of cylinder traps increased when tilted either into or away from the
current; increases to 125% were noted at 5° tilt and reached 250-300% at 45°. In this study, the
differences in sedimentation rates between mid-depth and near-bottom traps are unlikely to result from
different tilts: mean trap tilts at mid-depth and near bottom were 2.15° and 1.40° on the central
‘Grande Vasière’ stations, respectively, vs. 2.21° and 0.36° at Stn. G.
In contrast to bottom traps, collection efficiency could have been biased (under-collection) in
the mid-depth traps facing the highest flow speeds. However, mean current speed at the mid-depth
trap aperture did not exceed 18.32 cm.s -1 for the three stations (A, C and G) taken into account for resuspension
estimates. Moreover, mean current speeds were similar for these three stations at mid
depth (range: 17.55-18.32 cm.s -1 ) on one hand and near bottom (range: 7.13-8.17 cm.s -1 ) on the other
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