Structure, fonctionnement, évolution des communautés benthiques ...

tel-00009359, version 1 - 1 Jun 2005
Chapitre 3 - Fonctionnement du réseau trophique benthique de la Grande Vasière
Except for higher POC:PON ratio values in Stn G (both seasons) and lower ratio in Stn E (late
summer), no clear pattern was observed for the POC:PON ratio. The latter ranged from 7.98 to 13.80
in spring and from 7.08 to 12.54 in late summer (Table 4). The percentage of organic carbon in
sedimented particles and the POC:PON ratio values indicate a more degraded material in the traps
than in suspended matter (Tables 3 and 4).
Pigment flux (µg.m -2 .h -1 )
400
300
200
100
0
684
F G A B C D F G A B C D E
SPRING SUMMER
192
Phaeo
Chl a
Stations
Figure 7. Pigment fluxes (Chl a and phaeopigments, µg.m -2 .h -1 ) measured in the bottom-moored sediment
traps during spring (April 26 to May 3 rd ) and late summer (September 13 th -19 th ) sampling on the Bay of
Biscay continental shelf. Stations are ranked according to an inshore-offshore gradient.
Spatial and temporal (seasonal) variability
Particulate matter sedimentation clearly exhibited seasonal patterns with a decrease of almost
all the parameters from spring to late summer (Table 4). A 6-fold decrease of the mean TPM flux was
observed between late April-early May and September on the central ‘Grande Vasière’. This lower
total sedimentation rate in late April-early May was related with 8-fold and 4.6-fold decreases of the
PTC and POC fluxes, respectively. Lower seasonal variations were observed inshore (Stn. F) for the
mean TPM flux (2.3-fold difference), in contrast to the mean PTC and POC fluxes (4.7-fold decrease
for each parameter). The northern part of the ‘Grande Vasière’ (Stn. G) showed seasonal trends
opposite to those observed inshore and on the central part of the muddy bank, with TPM, PTC and
POC fluxes increases in September of 19, 53 and 28%, respectively. These higher sedimentation rates
cannot be explained by phytoplankton bloom sedimentation, since pigment fluxes in all stations were
much lower in September 2002 than in late April-early May (Table 4). This study emphasises major
spatial and temporal variations in pigment fluxes within the Bay of Biscay, with 14-fold and 9-fold
inshore-‘Grande Vasière’ decreases in spring and late summer, respectively.