Structure, fonctionnement, évolution des communautés benthiques ...

tel-00009359, version 1 - 1 Jun 2005
ABSTRACT
Chapitre 3 - Fonctionnement du réseau trophique benthique de la Grande Vasière
Sequential sediment traps, electromagnetic current meters and a profiler composed of a
particle size analyser, a CTD probe, a video-system and a carousel water sampler were deployed on
the continental shelf of the Bay of Biscay to characterise hydrology and measure the total downward
particulate matter fluxes. Both spatial and temporal variability were observed in the bottom-water
SPM concentrations and sedimentation patterns, highlighting tide- and wind-driven processes, the
influence of river plumes and physical properties of the shelf break. Overall, the occurrence of a
decreasing gradient for particulate organic matter from coast to open sea was underlined by bottomwater
pigment, POC and PON concentrations, and by POC, PON and pigment fluxes. However, the
offshore station located near the shelf slope, exhibited higher pigment fluxes than the central ‘Grande
Vasière’ stations, which could be related with a high phytoplankton production area on the shelf break
and possibly advection onto the shelf. The distribution of total particle load in the water column during
a tidal cycle clearly showed sediment re-suspension up to 50-60 m off the bottom in all the ‘Grande
Vasière’ stations. This study allowed to determine critical erosion thresholds from near-bottom current
velocity. The influence of solitary internal waves on particles re-suspension and particle fluxes was
shown on the outer margin of the shelf. High frequency temporal variability of the sedimentation
process was emphasised by 2- to 4-fold variations in the hourly TPM fluxes, whatever the station. The
role of benthic-pelagic coupling on ecosystem functioning was shown by the distribution of benthic
macrofauna total biomass: the latter was 2.5 times higher on the outer margin of the shelf 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’.
KEY WORDS
Bay of Biscay, benthic-pelagic coupling, particulate matter, vertical export, sediment traps
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