Spatial and temporal variations in trace metal concentrations in surface sediments of the Marennes Oléron Bay. Relation to hydrodynamic forcing

Sediments quality assessment is of priority concern to provide a comprehensible overview of ecological and chemical state of an ecosystem. 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, Cd levels in oysters from the bay are close to consumption limit (5 μg g⁻¹ dw, EC no. 466/2001). In this context, the aim of the study was to assess the priority metal (Ni, Cu, Zn, As, Ag, Cd, Hg, Pb and Th) concentrations in sediment within the Bay, by investigating spatial and temporal distribution variations and the role of hydrodynamic forcing. For that we selected three sites (east, west and south) characterizing different environments of the Bay and we observed metal concentrations, grain size distribution, bed elevation and wave activities during a year survey. The sampling strategy pointed out both spatial and temporal metal concentrations variability in sediment. In general, metal enrichments were close to geochemical background. The eastern part of the Bay, largely influenced by the Charente river particulate deposition, presented constant concentrations over the survey. In contrast, in the western part, bed elevation was strongly influenced by hydrodynamic forcing especially wave activities, and metal distribution showed constant metal concentrations except very located Cd minor enrichment related to the Gironde influence via the Antioche Strait (north). The southern part was disconnected from the rest of the Bay and showed minor to very located moderately severe Cd enrichment, related to the Gironde water discharges via the Maumusson Strait (south). Thus, the multi-disciplinary approach was relevant to characterize the interactions between hydrodynamic forcing on the environment and sediments and their metal quality state which (i) were close to geochemical background over a year for Ni, Cu, Zn, As, Ag, Hg and Pb (i) which presented enrichment of Cd in the western and southern part.