Geochemical survey and metal bioaccumulation of three bivalve species (Crassostrea gigas, Cerastoderma edule and Ruditapes philippinarum) in the Nord Médoc salt marshes (Gironde estuary, France)
Introduction
The Gironde estuary (southwestern France) is characterized by a historic cadmium (Cd) contamination of the water and the sediment, which originated from a former ore treatment plant in the industrial basin of Decazeville (Latouche, 1988, Jouanneau et al., 1990; Fig. 1). The Cd pollution was discovered in the early 1980s by the National Observation Network (RNO), as the Cd concentrations measured in oysters from the Gironde estuary were close to 100,000 ng g−1 (dry weight). This value represents 20 times the consumption safety level currently authorized in France for marine bivalves (5000 ng g−1, dry weight or 1000 ng g−1 fresh weight; CE No. 466/2001; Boutier et al., 1989, Latouche, 1992). As a consequence, the Gironde estuary has been classified as “zone D”, i.e. interdiction of oyster recovery for consumption, production or purification. After an accidental polymetallic pollution (Cd, Zn, Cu, Hg, Ba, As) in 1986, important work was done (e.g. groundwater treatment) limiting Cd emission into the Lot River (Roux and Simonet, 1987, Blanc et al., 1999, Audry et al., 2003). After a distinct decrease of Cd emission in the early 1990s, Cd transport in the Lot–Garonne River system, still accounts for 40–80% of the annual gross fluxes into the Gironde estuary (Schäfer et al., 2002, Blanc et al., 2004, Audry et al., 2004). Important sediment stocks (9.47±0.9 106 m3; ∼200 t of Cd; Lapaquellerie et al., 1995) accumulated in the Lot valley between the Riou-Mort and the Garonne River (Fig. 1) may represent additional Cd sources due to remobilization by floods or dredging (Blanc et al., 1999, Audry et al., 2004). Although Cd is mainly transported in the particulate phase, most of it (∼90%) becomes highly bioavailable by dissolution due to estuarine biogeochemical processes and management (e.g. Elbaz-Poulichet et al., 1996, Kraepiel et al., 1997, Robert et al., 2004). As a result, Cd concentrations in the Gironde's oysters show a progressive decrease of 30–40% between 1979 and 1993 (e.g. Jouanneau et al., 1990, Michel et al., 2000), but average values remain very high (National Observation Network—RNO, France).
During the past 15 years, several fish farms have developed production of kuruma prawn (Penaeus japonicus) in the Nord Médoc salt marshes, adjacent to the Gironde estuary (Fig. 1). In these breeding basins, periodically alimented by water from the Gironde estuary, an important proliferation of savage cockles (Cerastoderma edule) of economic interest was observed each year. Filter-feeding bivalves filter large quantities of water, resulting in bioaccumulation of ambient metals (Andrès et al., 1999, Baudrimont et al., 2003, Hamza-Chaffai et al., 2000).
The aim of the present 15-month survey (May 2000 to December 2001) in a Nord Médoc breeding basin under realistic conditions, was to characterize the geochemical behavior and ecotoxicological impact of selected heavy metals (Cd, Cu, Zn and Hg) on three bivalves of economical interest (Crassostrea gigas, Cerastoderma edule and Ruditapes philippinarum). The impact of physico-chemical conditions and heavy metal contamination levels in water, suspended particulate matter (SPM) and sediment on bioaccumulation rates and kinetics is analyzed and discussed.
Section snippets
Study site
The present large-scale experimentation was conducted in one of the breeding basins of the “Bleu Médoc” fish farm at St Vivien de Médoc, France. The farm consists of 11 basins (each of 2.3 ha of surface and ∼0.8 m depth), which were dredged 30 years ago (Fig. 1). The main activity of this farm is kuruma prawn (P. japonicus) production. The basins are directly supplied by the Gironde estuary waters via a small channel, controlled by a gate. During the 15-month experiment, the water in the basin
Physico-chemical parameters
Temperatures measured throughout the experiment reached maximum values of 28 °C in summer and minimum values of 5 °C in winter. There were no systematic differences between temperatures obtained by the multiprobe and manual measurements (Fig. 2). Salinity also showed seasonal trends with maximum values of S=25 at the end of summer and minimum values of S=10 in winter. From July to September 2000 salinities were systematically higher (up to 3 points) at ∼50 cm depth than in surface water (Fig. 2
Geochemical scenario
Variation of physico-chemical parameters throughout the experiment reflects seasonal changes and basin management. Temperature reflects length and intensity of exposition to sunlight at both, seasonal and daily time-scale (Fig. 2, Fig. 3). Additionally, rapid decreases in temperature and salinity are due to important precipitation, i.e. dilution by rainwater. In contrast, decreasing temperature coinciding with increasing salinity was due to basin alimentation by estuarine water. On a seasonal
Conclusion
The present study was part of a larger research program realized in four basins on three aquaculture farms in the Nord Médoc salt marshes, to evaluate the impact of geochemical conditions on metal bioaccumulation by shellfish under realistic conditions of prawn production. The basin presented in this work corresponded to the reference site without perturbations by prawn production. The results show the high potential of Cd, Cu and Zn bioaccumulation in oysters, compared to other species and
Acknowledgements
The authors wish to thank Y. Lapaquellerie, N. Maillet, H. Bouillard, V. Roques-Duflo, G. Durrieu, G. Lavaux, E. Maneux, A. Palvadeau and B. Etcheverria for their important support in field work, analysis and data treatment. We thank also the Bleu Médoc fish farm for their important contribution. This work is part of the GIS ECOBAG program and was financially supported by the Agence de l'Eau Adour-Garonne, Conseil Général de la Gironde, the European Community (FEDER-PESCA), District de la
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