Monitoring sources and processes driving trace metal bioaccumulation in oysters from a European macrotidal estuary...

We present 42 years (1979-2021) of elemental and isotopic data (Cu, Zn, Cd, Pb) from oysters (Crassostrea gigas) in the Gironde Estuary (SW France), integrated with historical river discharge records to disentangle anthropogenic and environmental drivers of trace metal bioaccumulation. Cu and Zn concentrations in oyster tissues show pronounced interannual variability-up to twofold-closely tracking river flow. Their fluctuations likely reflect hydrological controls on the mobilization, speciation, and bioavailability of legacy-bound metals rather than new contaminant inputs. Cd concentrations decline more steadily, consistent with the attenuation of upstream industrial sources, while Pb displays a more complex trend shaped by legacy decline, hydrological variability, and possible changes in source composition. Stable isotope signatures (δ⁶⁶Zn, δ¹¹⁴Cd, δ⁶⁵Cu, ²⁰⁶Pb/²⁰⁷Pb) remained remarkably stable over four decades, indicating a constant source apportionment with a dominance of anthropogenic sources on the observed fingerprints. Nevertheless, metal-specific fractionation processes imprint distinct signatures: Zn is enriched in heavy isotopes due to kinetic adsorption onto suspended particles, whereas Cd is isotopically light, consistent with selective desorption via chloride complexation. Slightly heavier δ⁶⁵Cu values likely reflect vineyard runoff coupled with complexation in the dissolved phase. Pb isotope ratios reveal a gradual shift toward radiogenic values, shaped by diminishing industrial inputs and increasing lithogenic contributions. Together, these findings demonstrate that integrating hydrological data with multi-isotope analysis can disentangle source persistence from estuarine reactivity, offering a framework for interpreting long-term contamination dynamics under changing hydrological regimes.