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 other hand, re-suspension rates of the carbon (POC), as measured by the method of
Gasith (1975) adapted by Bhaskar et al. (2000), were lower for the central ‘Grande Vasière’ (49.33 ±
16.25%; range: 31-62%) than those deduced from the trap fluxes, and could not be estimated for the
pure mud station (G). The POC re-suspension rate calculated in September at the offshore station was
of 33%.
3- Sediment
Mean grain size
Station G appeared to be the most silty, particles under 63 µm ranging from 56 to 91%
sediment dry weight (Table 1). The central ‘Grande Vasière’ stations (A, B, C and D) exhibited a silt
fraction comprised between 10 and 20% sediment DW. Mean grain size varied from 122 to 187 µm
(Table 1). Except for B which showed smaller mean grain size (123 µm), the central ‘Grande Vasière’
stations and the offshore station (E) had similar mean grain size (ca. 180 µm) but different silt contents
(Table 1). The silt content in a given station did not show great variations according to the season,
except for G. According to Chassé and Glémarec (1976) classification, the sampling stations belong to
5 different sediment types, from pure mud to medium biogenic sand (Table 1).
POC content (%)
Sediment POC content on the ‘Grande Vasière’ ranged from 0.15% (Stn. A) to 1.40% (Stn. G)
in late April-early May, and from 0.21% (Stn. D) to 1.32% (Stn. G) in mid September 2002 (Table 6).
The inshore station (F) exhibited lower organic matter content (POC= 0.04% of sediment dry weight)
in September than the central ‘Grande Vasière’ (0.24 ± 0.03%) and the outer margin of the shelf
(0.49%). The central ‘Grande Vasière’ exhibited low seasonal variations in sediment POC content
(0.22 ± 0.05% in late April-early May). This result was also found in pure mud of the northern station
(G).
Benthic Biomass
The total biomass of the benthic macrofauna in late summer (September) varied from 665.4
(Stn. A) to 1221.5 mg AFDW.m -2 (Stn. G) on the ‘Grande Vasière’. Total benthic biomass was higher
on the outer margin of the shelf (Stn. E), reaching up to 3099.9 mg AFDW.m -2 on average (Fig. 11).
Mean total benthic biomass on the central ‘Grande Vasière’ was 764.6 ± 84.75 mg AFDW.m -2 in late
April-early May, vs. 844.5 ± 195.7 mg AFDW.m -2 in mid-September 2002. Thus, total benthic
macrofauna biomass on the outer margin of the shelf was 2.5 times higher than the one recorded in the
northern part of the muddy bank, and 3.7 times higher than the one recorded on the central ‘Grande
Vasière’ in late summer 2002. Total benthic biomass increase between late April-early May and mid-
September appeared more pronounced on the northern part of the bank (+47.6%) than on the central
‘Grande Vasière’ (+10.4%).
The spatial variability observed for total benthic biomass was also found when considering
trophic groups, with : (i) outstanding suspension feeder biomass at Stn. E (63.8% of total biomass in
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