Trace metal chemical speciation and acute toxicity to Pacific oyster larvae
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Controlled laboratory studies showed that the toxicity induced by biologically relevant trace metal species of Cu, Cd, Pb and Zn on embryo-larval development occurred at concentrations in excess of those found in the natural environinent, except for Cu in metal perturbed areas. Average free ion concentrations inducing 50% abnormal development( EC50feew) ere determineda s 0.23 nM CU2+8, 8.0 rim Cd2+,1 28 nM Zn2+a nd 3 62 nM Pb 2+ . However, the response to some binary metal combinations indicated enhanced (synergy) toxicity at concentrations relevant for estuarine waters (e. g. EC5of'. for Cu2+i n the presenceo f C(ý: +, Zn2+a nd Pb2 +w as 0.004,0.02 and 0.04 nM, respectively). A comparison of voltarnmetric instrumentation (voltammetric in situ profiling (VIP) system versus Hanging Mercury Drop Electrode with potentiostat) highlighted the advantage of high resolution measurements (ca. 20-60 min intervals) for environmental studies and the minimisation of artefacts associated with discrete sampling methodologies. Field-based studies were carried out in two contrasting estuaries in SW England, one heavily impacted with metal contaminants (Fal Estuary) and another subject to greater variety of anthropogenic influences (Tamar Estuary). High resolution in situ trace metal speciationm easurementsc, arried out over tidal cycles, identified important information on the temporal and spatial distributions of biologically relevant dynamic (<4 nm) metal species of Cd, Pb and Cu. Variation in embryo-larval responses to discrete samples from these estuaries, effectively paralleled the metal speciation measurements showing enhanced toxicity when the marine water influence was at its lowest. In both systems, the results indicated that the combined effect of the metals studied was likely to have provided a significant contribution to the bioassay response. However, the difficulty in de-coupling the speciation measurements with biological responses was evident and supports the need for more comprehensive campaigns to study the impact of contaminants on ecosystem functioning. Bioassay and metal speciation analysis techniques were complementary, exhibiting high sensitivity and rapid responses, and would be considered effective screening tools for waters subject to intermittent inputs of metal contaminants and areas with recognised pressures. The integrated approach has extended our knowledge of trace metal speciation in estuarine environments and their effects on the developing embryos of the Pacific oyster. This approach has the potential for wider application.
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