Structure, fonctionnement, évolution des communautés benthiques ...

tel-00009359, version 1 - 1 Jun 2005
Chapitre 3 - Fonctionnement du réseau trophique benthique de la Grande Vasière
Inconsistent results with this method led us to estimate re-suspension from SPM
concentrations in the water column according to the tidal cycle. This second ratio method was based
on SPM concentrations increasing with the tidal current in the last 50-60 meters off the seabed, while
particles were mainly concentrated near bottom at slack water. Subsurface-to-bottom differences in
SPM concentrations were generally calculated at mid tide and at slack waters. The subsurface mean
concentrations were calculated from the first 3 metres under the pycnocline; the bottom values were
the concentrations recorded at the trap depth (1.6 m off the seabed).
Finally, a third attempt was made from the total particulate matter and POC fluxes measured
in September in the mid-depth and bottom-moored traps. Re-suspension was calculated as the
difference in fluxes recorded at the two depths, assuming settling particles all originating from the
‘Grande Vasière’ surface waters (no current-driven alien inputs under the upper traps) and after
checking that (i) the upper trap was not influenced by re-suspension, and (ii) the chlorophyll maximum
biomass was situated over this trap.
Statistical analyses
Temporal and spatial fluctuations were assessed by a two-way analysis of variance (ANOVA)
with season (spring or summer) and station as fixed factors, after the homogeneity of variances was
tested (Cochran test). When the homogeneity hypothesis was rejected by ANOVA, Newman and
Keuls a posteriori multiple-comparison tests were used to separate possible sets of homogeneous
means (Clavier et al., 1995).
RESULTS
1- Water column
Physical parameters
Between-site comparisons are sometimes irrelevant since sampling may have occurred in
different tide regimes, depending on the station and season, particularly in spring (e.g. during springtides
at A, B and F in April 2002, but during neap tides at G; see Table 2). Mean bottom-water current
on the ‘Grande Vasière’ varied according to tide coefficient, ranging from 6.01 cm.s -1 during neap
tides (coeff. 38; TROPHAL, station B) to 12.46 cm.s -1 during spring-tides (coeff. 111-109;
GASPROD, station B). However, general trends were observed: (i) bottom-water salinity and
temperature appeared consistently higher in summer at all stations, except for G (Table 2); (ii) station
F, under the river plumes influence, exhibited the lowest bottom-water salinity and the highest
stratification index (SI) for both seasons, i.e. the highest water column stratification (either in
temperature and/or salinity). The freshwater inputs in F were also underlined by colder surface-water
temperature in early spring but warmer temperature in late summer, compared to the ‘Grande Vasière’
stations; and (iii) water column stratification appeared quite similar in A, B, C, D and G and was
higher in late summer than in spring for all the central ‘Grande Vasière’ stations (Fig. 2). The lack of
spring data for the deepest station (E) does not allow to make any seasonal comparison between the
180