Structure, fonctionnement, évolution des communautés benthiques ...

tel-00009359, version 1 - 1 Jun 2005
Chapitre 3 - Fonctionnement du réseau trophique benthique de la Grande Vasière
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 could be related with a high phytoplankton production area on the shelf slope (see Fig.
4) and possibly advection onto the shelf. Spatial heterogeneity was also observed at a smaller scale on
the central ‘Grande Vasière’: spring sedimentation rates were higher in the southern part of the area
(Stns B and D), while pigment fluxes were 2.5 to 3 times higher in Stn A in late summer, compared to
B, C and D. These variations were related with a ca. 2‰ difference in δ 13 C values of sedimented
particles between Stn. A (lowest value) and the other central ‘Grande Vasière’ stations (Table 4). One
more time, an inshore-offshore gradient was observed for the carbon stable isotopic ratios, with values
more negative offshore (central ‘Grande Vasière’ stations plus Stn E) overall. These spatial variations
were also recorded for δ 15 N values.
High-frequency temporal variability of the sedimentation process
The variability of the sedimentation process associated with the diurnal tidal cycle is
illustrated for stations A, G and E (Fig. 8). On the central ‘Grande Vasière’ (Stn. A), there is a clear
impact of the tidal cycle on sediment re-suspension (Fig. 3). During the spring-tides of late April, total
particle load in bottom water was the lowest at low tide and the highest at high tide, with a clear
decreasing gradient in particle concentration from high to low tide and conversely. This pattern was
also observed during the September neap tides. The halocline and or the thermocline would constitute
a physical barrier for re-suspended particles, the latter being mainly distributed below 60-70 meters
depth, whatever the season. Particle distribution was associated with salinity higher than 35.15 in late
April, vs. 35.42-35.44 in mid-September. In late summer, particles were mainly concentrated in cold
(11.0-11.5°C) bottom water. The tide-related temporal pattern observed for suspended particle
distribution was also found in particle sedimentation within the traps. The hourly total particulate
matter, POC, PON and pigment fluxes exhibited the same temporal variations as particle load in
relation with tide, that is low sedimentation at low tide and sedimentation rates increasing with the
flood (Fig. 8). Thus, minimum sedimentation rates in bottom-moored traps were associated with
minimum particle loads in bottom water and conversely. The relationship with the tidal cycle was not
totally correlated with bottom-water current velocity during the April spring-tides: sedimentation was
observed to increase with current velocity decreasing at the end of the flood, but the highest
sedimentation rates were measured for both the lowest (5 cm.s -1 ) and the highest (15 cm.s -1 ) current
speeds at the trap aperture (Fig. 8). Overall, a 2- to 3-fold variation was observed for hourly TPM
fluxes in bottom traps on April 27. The δ 13 C values in sedimented particles were more negative at low
tide, with a 1.1‰ difference on a 13-hour cycle (from –21.3‰ to –22.4‰).
Bottom current velocity was lower during the September neap tides, ranging from 2 to 10-12
cm.s -1 . In contrast to spring-tides, the sedimentation process did not show a sinusoidal pattern but a
step by step one. Low TPM sedimentation rates (< 2 g.m -2 .h -1 ) were recorded for mean current velocity
≤ 2.5 cm.s -1 at the trap aperture, while sedimentation rates of 3.1-3.4 g.m -2 .h -1 were measured for
current speeds higher than 7-8 cm.s -1 , from mid-tide on (Fig. 8). One more time, increasing
sedimentation rate was associated with increasing particle load in bottom water (Fig. 3). In September
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