Structure, fonctionnement, évolution des communautés benthiques ...

tel-00009359, version 1 - 1 Jun 2005
Chapitre 3 - Fonctionnement du réseau trophique benthique de la Grande Vasière
could be related with a recurrent high phytoplankton production area on the shelf break (Fig. 4; see
also Lampert et al., 2002 and Gohin et al., 2003) and possibly advection onto the shelf.
In the study area, the northward geostrophic spreading of river plumes is known to favour the
offshore advection of coastal waters as far as the continental slope during the spring transition (Lazure
and Jégou, 1998 ; Hermida et al., 1998). At the end of April 2002, the main river plumes extended
over the continental shelf as shown by figures 8 and 9 and by the surface salinity distribution (Fig. 12)
derived from the IFREMER 3-D multi-layer model (Lazure and Jégou, 1998). The surface seawater
exhibited a well-marked vertical haline stratification characteristic of the spring situation in that area
(Fig. 8 and 9; see also Lampert et al. 2002). While the Gironde plume was restricted to the coastal
zone near the mouth of the estuary, the Loire and Vilaine plumes tended to spread towards the entire
shelf section, especially south of Belle-Ile and at the northernmost boundary of the study area (Fig.
12). Stations F, G, A and B, and C and D to a lesser extent were clearly under the influence of the river
plumes during the GASPROD cruise. In the coastal area, the presence of the river plumes resulted in a
thermal stratification superimposed on the haline stratification (Fig. 2). The seasonal thermocline was
starting to develop inshore (Stn. F) with a temperature gradient of ca. 2°C between surface and 15-20
m (Fig. 2). The density structure (data not shown) followed closely that of the salinity distribution,
which demonstrated the major role of freshwater inputs in determining the spring physical structure
over the study area (Lampert et al. 2002). In late summer, the thermal stratification was maximum on
the central ‘Grande Vasière’ and outer margin (temperature gradient of 6-7°C between surface and 40-
60 m), while turbulent mixing in stations F and G led to a lower temperature gradient (3.5 to 4.5°C) on
the vertical. The shelf thermal stratification favours advection of estuarine inputs in the surface water
over the entire area and even sometimes beyond the shelf break limit (Lazure and Jégou, 1998 ;
Hermida et al., 1998).
Figure 12. Surface salinity distribution on 21 th April 2002 derived from the IFREMER 3-D multi-layer
model (Lazure and Jégou, 1998).
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