Structure, fonctionnement, évolution des communautés benthiques ...

tel-00009359, version 1 - 1 Jun 2005
Chapitre 3 - Fonctionnement du réseau trophique benthique de la Grande Vasière
The physical properties of the study area, including the wind-induced general surface water
circulation, lead to geographical differences in distribution patterns of the microphytoplankton and
nanophytoplankton populations on the continental shelf of the Bay of Biscay (Chapelle et al., 1994;
Lampert et al., 2002; Gohin et al., 2003). On one hand, high microphytoplankton (diatom) cell
concentrations on the northern half of the shelf have been shown to originate from the Loire and
Vilaine estuaries (Chapelle et al., 1994; Lampert et al., 2002) and then advected offshore. This study
shows that, in addition to Stn. F, the stations G and A are strongly influenced by the river plumes
(Tables 3 and 4). On the other hand, maximum microphytoplankton (dinoflagellates) or
nanophytoplankton (the coccolithophorid Emiliana huxleyi) cell concentrations regularly observed on
the shelf break (e.g. on 25 March 2002; Fig. 4) would advect onto the shelf due to surface water
circulation. The onset of the dissipative phase of coccolithophorid blooms would be induced by
internal waves at the shelf break during spring tides, in the period of thermohaline stratification. The
semidiurnal vertical mixing induced by diffusion and shearing at the pycnocline interface would be
strong enough during spring tides to result in a semidiurnal injection of deep-water originated nutrients
into the surface layer (Mazé, 1987; Lampert et al., 2002). The long duration of E. huxleyi blooms in
the study area and wind-induced surface water circulation would explain the high pigment fluxes
measured at Stn. E during this study and the associated POC:PON ratio and carbon stable isotope ratio
(Table 4). The carbon-to-nitrogen ratio of particulate matter may give some insight into the proximate
composition and origin of the material: low ratios (3 to 7 by weight) have been reported for
phytoplankton whereas high ratios may result from influx of terrigenous material or from increased
detrital organic matter (Banse, 1974; Slawyck et al., 1978; Clavier et al., 1995). The low mean carbonto-nitrogen
(weight) ratio of the bottom-water SPM (< 7.6) and trapped material (7.1-14.6) suggest
plankton-derived labile organic matter for sedimented particles overall.
Bottom-water SPM concentrations and particle fluxes in sequential traps showed high
seasonal variability (4- to 6-fold decreases in particle fluxes from spring to late summer) emphasising
the major role of phytoplankton spring bloom in benthic-pelagic coupling.
Role of benthic-pelagic coupling on the benthic macrofauna
The role of benthic-pelagic coupling on the ‘Grande Vasière’ ecosystem functioning is shown
by the distribution of benthic macrofauna total biomass: the latter is 2.5 times higher on the outer
margin of the shelf (Stn. E) than the one recorded in the northern part of the muddy bank (Stn. G), and
3.7 times higher than the one recorded on the central ‘Grande Vasière’. The lack of spring data for
macrofauna at station E does not help to interpret the spatial patterns observed. Total benthic
macrofauna biomass appeared to be partly correlated with particulate matter inputs from the water
column on the ‘Grande Vasière’: in late summer, total macrofauna biomass in station G was 45%
higher than the one recorded on the central ‘Grande Vasière’, which agree with higher particle fluxes
on the northernmost pure mud area. However, the high macrobenthic biomasses sampled in September
2002 on the outer margin of the shelf can’t be explained only by the flux values measured in the traps
at that time (Fig. 11 and Table 4). It is likely that the spring phytoplankton bloom on the shelf break
results in mass cell sedimentation on the shelf slope and nearby at the end of the bloom. The
abundance of suspension feeders on the medium biogenic sands is in favour of high plankton-derived
trophic inputs at the sediment-water interface. By contrast, suspension feeders exhibited very low
205