Structure, fonctionnement, évolution des communautés benthiques ...

tel-00009359, version 1 - 1 Jun 2005
Spatial and temporal trends
Chapitre 3 - Fonctionnement du réseau trophique benthique de la Grande Vasière
Bottom-water particulate matter clearly exhibited seasonal patterns (Table 3). SPM, PTC,
POC et PON concentrations were lower in late summer than in spring at all stations, except for G.
Nevertheless, pigment concentrations were highest in September on the ‘Grande Vasière’ (in contrast
to F), with Chl a:Phaeo ratios emphasizing the presence of phytoplankton less degraded. The carbon
stable isotopic ratio did not show great variations, both at a spatial and temporal scale, except for the
coastal station (F) which exhibited a 2.5 to 3.5‰ difference with the central ‘Grande Vasière’ stations
in spring. At the contrary, δ 15 N values were higher in spring in all stations, except for F.
The central ‘Grande Vasière’ stations (A, B, C and D) exhibited similar values or trends
overall, though the northernmost stations (especially A) had SPM, PTC, POC and PON concentrations
consistently higher in late April-early May. The POC:PON ratios were slightly higher in September,
but values < 7.4, whatever the station and season, emphasized fresh organic matter. There was almost
no chlorophyll a in bottom water of the central ‘Grande Vasière’ in spring, with Chl a:Phaeo ratios of
ca. 0.5. Pigment concentrations were somewhat higher in late summer, with Chl a:Phaeo ratios more
variable (from 0.6 to 1.5). The δ 13 C values exhibited little if any variations at the spatial (≤ 1.3‰) and
temporal scales (≤ 0.5‰).
Station F, located in the river plumes, showed the highest values for bottom-water PTC, POC,
PON and pigment contents in spring, and for pigment contents in late summer (Table 3). Indeed,
station F exhibited significantly higher organic matter contents in bottom water than the ‘Grande
Vasière’ stations. In particular, the high POC:PTC (92.8%), POC:SPM (1.93% on 26 April) and Chl
a:Phaeo (1.8-2.1) ratios were associated with δ 13 C values less negative than those of the ‘Grande
Vasière’.
The northernmost ‘Grande Vasière’ station (G) showed seasonal trends opposite to those of
the central ‘Grande Vasière’, i.e. bottom-water particulate matter increases between spring and late
summer. Station G exhibited the highest concentrations in SPM, PTC, POC and PON in September, as
well as the highest seasonal variation for δ 13 C (2‰ difference).
On the other hand, the deepest offshore station (E) showed particulate matter (SPM, PTC,
POC, PON and pigments) contents in bottom water quite low and similar to those recorded in C and
D. 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.
2- Particulate matter sedimentation
Trap tilt and current speed at the trap mouths
The mean tilt of bottom-moored traps varied between 0.36 ± 0.12° (Stn. G in September) and
4.24 ± 0.86° (Stn. B in September) from the vertical, with a mean tidal current velocity ranging from
5.4 ± 2.6 cm.sec -1 (Stn. G in spring) to 13.3 ± 4.6 cm.sec -1 (Stn. F in September) at the trap mouth
(Table 2). The mean tilt of mid-depth moored traps varied between 1.15 ± 0.85° (Stn. B in September)
and 2.31 ± 0.82° (Stn. D in September) from the vertical, with a mean tidal flow speed ranging from
17.6 ± 4.2 cm.sec -1 (Stn. A in September) to 35.9 ± 8.2 cm.sec -1 (Stn. B in September).
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