Virginie Lafon

SPOT shallow water bathymetry of a moderately turbid tidal inlet based on field measurements

The determination of shallow water depth in high-energy tidal inlets is essential to model and to forecast the navigation channel position or the short-term topographic evolution of beaches. Colour satellite imagery provides, at low cost, complete maps of areas that are difficult to map by traditional hydrographic means due to their size and their rough underwater morphology. In order to calibrate SPOT images to derive bathymetric maps, a simple method applied to shallow waters of a moderately turbid tidal inlet has been carried out. Its development is based on a set of field measurements, including the reflectance of the water and of the bottom sediment, the vertically averaged diffuse attenuation coefficient, and the concentrations of inorganic particles in suspension, of chlorophyll a and pheopigments and of dissolved organic carbon (DOC). From these data, it appears, first, that the water reflectance is directly linked to the depth, and second, that the water reflectance varies slowly with the turbidity in this area for total suspended matter (SM) concentrations lower than 9 mg l−1. The extinction of light with depth has been characterized in the inlet. The relationship obtained is slightly different from that observed in clear waters. The bathymetric models established for the clearest waters do not calculate the depth accurately. In the contrary, reflectance code adapted to Case II water type, calibrated with in situ measurements, allows us to retrieve depth down to 6 m and for total SM concentrations ranging from 0.2 to 9 mg l−1. This relationship has been applied to five SPOT images of the mouth of Arcachon lagoon. The accuracy of the derived map has been assessed by in situ depth measurements. The mean difference between measured depths and computed depths is about 20%. This accuracy is adequate to assess the inlet morphodynamics quantitatively, which is necessary for middle-term to short-term mathematical modeling of the 3D inlet evolution.

Roles of regional hydrodynamic and trophic contamination in cadmium bioaccumulation by Pacific oysters in the Marennes-Oléron Bay (France).

The Marennes-Oléron Bay, hosting the largest oyster production in France, is influenced by the historic polymetallic pollution of the Gironde Estuary. Despite management efforts and decreasing emissions in the Gironde watershed, cadmium levels in oysters from the bay are close to the consumption limit (5 μg g(-1) dw, EC). From mid April to mid July 2009, we investigated the role of tidal resuspension and regional hydrodynamics on Cd speciation (seawater, SPM, phytoplankton, sediment, microphytobenthos) and bioaccumulation in 18 month-old oysters (gills, digestive glands, rests of tissues) reared under natural conditions (i) at ∼60 cm above the sediment and (ii) on the sediment surface. Dissolved and particulate Cd concentrations in surface and bottom waters were similar and constant over tidal cycle suggesting the absence of Cd release during sediment resuspension. Temporal dissolved and particulate Cd concentrations were closely related to Gironde Estuary water discharges, showing increasing concentrations during flood situations and decreasing concentrations afterwards. Cd depletion in the water column was associated with increasing Cd in the [20-100 μm] plankton fraction, suggesting Cd bioaccumulation. After 3 months, enrichment factors of Cd in tissues of oysters exposed in the water column and directly on the sediment were respectively 3.0 and 2.2 in gills, 4.7 and 3.2 in digestive glands and 4.9 and 3.4 in remaining tissues. Increasing Cd bioaccumulation in gills, digestive glands and remaining tissues can be related to elevated dissolved Cd in the bay, suggesting gill contamination via the direct pathway and subsequent internal redistribution of Cd to other organs and tissues. Elevated Cd contents in oysters reared on tables could be attributed to different trophic Cd transfer (phytoplankton versus microphytobenthos) or to different oyster metabolisms between the rearing conditions as suggested by metallothionein concentrations.

Méthode d’analyse de l’évolution morphodynamique d’une embouchure tidale par imagerie satellite. Exemple du bassin d’Arcachon (France)

Abstract The channels and sandbanks located in the tidal inlet of Arcachon move with the local strong dynamical conditions. To model the evolution of the inlet, with intent for example to forecast the position of the navigation channel, it is necessary to obtain accurate bathymetric maps. As the hydrographic charts are rare and incomplete, particularly in the shallow waters, we processed satellite images to obtain spatial maps. For this purpose, we set out a SPOT bathymetric calibration methodology based on radiometric field data that is valid for the whole area and for images recorded during a long period (1986–1997).