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tel-00009359, version 1 - 1 Jun 2005<br />

Spatial and temporal trends<br />

Chapitre 3 - Fonctionnement du réseau trophique benthique de la Grande Vasière<br />

Bottom-water particulate matter clearly exhibited seasonal patterns (Table 3). SPM, PTC,<br />

POC et PON concentrations were lower in late summer than in spring at all stations, except for G.<br />

Nevertheless, pigment concentrations were highest in September on the ‘Grande Vasière’ (in contrast<br />

to F), with Chl a:Phaeo ratios emphasizing the presence of phytoplankton less degraded. The carbon<br />

stable isotopic ratio did not show great variations, both at a spatial and temporal scale, except for the<br />

coastal station (F) which exhibited a 2.5 to 3.5‰ difference with the central ‘Grande Vasière’ stations<br />

in spring. At the contrary, δ 15 N values were higher in spring in all stations, except for F.<br />

The central ‘Grande Vasière’ stations (A, B, C and D) exhibited similar values or trends<br />

overall, though the northernmost stations (especially A) had SPM, PTC, POC and PON concentrations<br />

consistently higher in late April-early May. The POC:PON ratios were slightly higher in September,<br />

but values < 7.4, whatever the station and season, emphasized fresh organic matter. There was almost<br />

no chlorophyll a in bottom water of the central ‘Grande Vasière’ in spring, with Chl a:Phaeo ratios of<br />

ca. 0.5. Pigment concentrations were somewhat higher in late summer, with Chl a:Phaeo ratios more<br />

variable (from 0.6 to 1.5). The δ 13 C values exhibited little if any variations at the spatial (≤ 1.3‰) and<br />

temporal scales (≤ 0.5‰).<br />

Station F, located in the river plumes, showed the highest values for bottom-water PTC, POC,<br />

PON and pigment contents in spring, and for pigment contents in late summer (Table 3). Indeed,<br />

station F exhibited significantly higher organic matter contents in bottom water than the ‘Grande<br />

Vasière’ stations. In particular, the high POC:PTC (92.8%), POC:SPM (1.93% on 26 April) and Chl<br />

a:Phaeo (1.8-2.1) ratios were associated with δ 13 C values less negative than those of the ‘Grande<br />

Vasière’.<br />

The northernmost ‘Grande Vasière’ station (G) showed seasonal trends opposite to those of<br />

the central ‘Grande Vasière’, i.e. bottom-water particulate matter increases between spring and late<br />

summer. Station G exhibited the highest concentrations in SPM, PTC, POC and PON in September, as<br />

well as the highest seasonal variation for δ 13 C (2‰ difference).<br />

On the other hand, the deepest offshore station (E) showed particulate matter (SPM, PTC,<br />

POC, PON and pigments) contents in bottom water quite low and similar to those recorded in C and<br />

D. Overall, the occurrence of a decreasing gradient for particulate organic matter from coast to open<br />

sea is underlined by the pigment, POC and PON concentrations in bottom water.<br />

2- Particulate matter sedimentation<br />

Trap tilt and current speed at the trap mouths<br />

The mean tilt of bottom-moored traps varied between 0.36 ± 0.12° (Stn. G in September) and<br />

4.24 ± 0.86° (Stn. B in September) from the vertical, with a mean tidal current velocity ranging from<br />

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<br />

(Table 2). The mean tilt of mid-depth moored traps varied between 1.15 ± 0.85° (Stn. B in September)<br />

and 2.31 ± 0.82° (Stn. D in September) from the vertical, with a mean tidal flow speed ranging from<br />

17.6 ± 4.2 cm.sec -1 (Stn. A in September) to 35.9 ± 8.2 cm.sec -1 (Stn. B in September).<br />

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