Structure, fonctionnement, évolution des communautés benthiques ...

tel-00009359, version 1 - 1 Jun 2005
Chapitre 3 - Fonctionnement du réseau trophique benthique de la Grande Vasière
hand. Between-site differences in sedimentation and re-suspension rates cannot then be explained by
different collection efficiency of the traps. It is also known that replicate sampling funnels on the Pro-
Trap systems return very consistent values during deployments and individual values deviate from the
mean figure by no more than 7% despite the fact that no attempt is made to align the two funnels
symmetrically in the currents (Bale, 1998). Finally, the TPM fluxes measured in the mid-depth traps
were never observed to decrease as flow speed increased, even for stations B and D which
experienced mean current speeds of 35.9 and 30.3 cm.s -1 at mid-depth, respectively. This result would
signify little impact of tidal streams on trap collection efficiency during this study according to Baker
et al. (1988). On the ‘Grande Vasière’, the highest sedimentation rates in the bottom traps clearly
result from particle re-suspension in bottom water (as suggested by positive correlation between
sedimentation rate and suspended particle load overall). The same results were found by Bale (1998)
in the southern North Sea, with the same traps. In particular, Bale showed that the Pro-Trap systems
collected sediment in direct proportion to the horizontal particle flux (defined as the product of mean
current velocity and the mean concentration of SPM over periods corresponding with each trap
sampling interval) without biasing the composition of the material. This study allowed to determine
the critical thresholds for near-bottom current speed (7-9 cm.s -1 ) implying sediment erosion and resuspension
on the ‘Grande Vasière’.
By contrast, the effect of internal waves on collection efficiency of traps could not be
predicted from this study. It is known that traps held vertically in the water column still can experience
an effective tilt due to the passage of internal waves (e.g. Gardner, 1985). In this study, the three
current peaks observed in Stn. E in September were not associated with tilt increases in near-bottom
trap, while the absence of mid-depth trap precluded to go any further.
In waters such as the sites occupied in this work, particle transport is dominated by oscillating
tidal currents inducing mobilization, advection and deposition of sediments. Re-suspended material
were shown to extend well into the water column (up to 50-60 m off the seabed), but under the height
at which mid-depth traps were set. It should be therefore possible to make deductions about the
settling rate or vertical fluxes of SPM from the data obtained in this work. However, different current
velocities at mid depth and near bottom, and large uncertainties about the real re-suspension rates,
when comparing the results of the three estimation methods, do not allow us to conclude.
Spatio-temporal variability of particle flux and origin of sedimented material
This study emphasises clear spatial patterns for suspended particulate matter, particle
sedimentation and re-suspension rates and benthic macrofauna biomass on the continental shelf of the
Bay of Biscay. Four zones can be defined from our 7 study sites: the inshore area (Stn. F), under the
influence of the river (Vilaine, Loire, and Gironde) plumes, the central ‘Grande Vasière’ stations (A,
B, C and D), the pure mud area in the northern part of the shelf (Stn. G), and the outer margin of the
shelf (Stn. E). Overall, the occurrence of a decreasing gradient for particulate organic matter from
coast to open sea is underlined by the pigment, POC and PON concentrations in bottom water.
Particulate matter sedimentation, as suspended particles, showed a decreasing gradient from coast to
open sea, especially for POC, PON and pigment fluxes. However, the offshore station (E) located near
the shelf slope, exhibited higher pigment fluxes than the central ‘Grande Vasière’ stations, which
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