Structure, fonctionnement, évolution des communautés benthiques ...

tel-00009359, version 1 - 1 Jun 2005
Chapitre 3 - Fonctionnement du réseau trophique benthique de la Grande Vasière
On the offshore margin of the continental shelf (Stn. E), the higher near-bottom current mean
velocity (ca. 12 cm.s -1 on September 13; Table 2) compared to the central ‘Grande Vasière’, led to
sedimentation patterns closely associated with the tidal cycle (Fig. 10). The TPM flux in bottom traps
followed a dome-shaped distribution centred on mid ebb-tide (maximum sedimentation rate between
00:24 and 1:30 PM) on 13 September. Temporal variations in total sedimentation rate were low (from
2.5 to 3.7 g.m -2 .h -1 ) during the study period. Three high-speed current events were superimposed on
the tide-driven dome-shaped distribution of current velocities at the trap aperture (Fig. 10). These
current peaks, known as solitary internal waves on the continental shelf (Pingree and Mardell, 1985;
Mazé, 1987; B. Le Cann, LPO Brest and A. Serpette, EPSHOM Brest, pers. com.), were associated
with light scattering increases 1.6 m off the seabed. In particular, the major turbidity event recorded
between 2:00 and 3:00 PM occurred just after the main internal wave passing over. However, total
particulate matter sedimentation was not correlated with these internal waves and the associated resuspension
events: the main sedimentation peak occurred before and there was no increase in bottom
trap sedimentation rates related with the increase of water turbidity after the internal waves passing
over. Thus, these high-frequency events did not seem to disturb the tide-driven sedimentation process
on the outer margin of the continental shelf.
Re-suspension estimate
Re-suspension mostly concerns mineral matter because of the low organic matter content in
sediments (Table 6). This is confirmed by the composition of the particulate matter sedimented in the
traps (eg. The POC:TPM ratios in Table 4). From the three methods used to estimate re-suspension,
the one using the trap fluxes gave the highest rates. In September 2002, total particulate matter
sedimentation rates in mid-depth and bottom traps were compared, assuming the absence of resuspension
signal in the upper trap and settling particles all originating from the ‘Grande Vasière’
surface waters. Re-suspension estimates were calculated only when the upper trap was located under
the chlorophyll biomass maximum, that is stations A, C and G. At each station, more than 77% of the
total sedimentation (TPM) in bottom traps would be linked to deposition of re-suspended benthic
material. This feature was particularly conspicuous for the pure mud station (G) with a 94% rate. The
mean TPM re-suspension rate would be of 82.5% (±7.8) on the central ‘Grande Vasière’. POC resuspension
in the latter was lower (57.7%).
Table 6. POC content (% dry weight) in superficial (0-0.5 cm) sediments of the study sites.
Station A B C D E F G
% POC Spring 0.15 0.26 0.21 0.24 - - 1.40
% POC Summer 0.24 0.28 0.23 0.21 0.49 0.04 1.32
Particulate matter re-suspension rates estimated from SPM concentrations in the water column
were lower than the ones estimated from the trap fluxes. Mean values at stations A, G and E were
56.72 ± 6.53% (range: 48.8-64.5%), 60.22 ± 2.96% (range: 58.2-63.6%), and 26.83 ± 9.85% (range:
16.8-36.5%), respectively.
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