Copper isotope ratios in oysters from the French Mussel Watch program confirm antifouling paints as a major source of copper in Arcachon Bay

The ratio of copper stable isotopes ( 65 Cu/ 63 Cu, expressed as δ 65 Cu) in bivalves offers a promising tool to identify coastal anthropogenic sources, yet its reliability remains uncertain. This study examines Cu isotopes in oyster samples from the chronically Cu-contaminated Arcachon Bay across three stations over a four-decade period. Cu concentrations rose significantly at nearshore stations, with a smaller increase at the offshore site. δ 65 Cu values increased alongside Cu levels, consistent with a binary mixing between natural and anthropogenic Cu derived from antifouling paints. This finding confirms that antifouling paints are the major local anthropogenic source of Cu and demonstrates that Cu isotopes are an effective tool for tracking these compounds in coastal environments. Further investigation of additional compartments and the integration of new isotope systems can better constrain diffuse secondary inputs—such as riverine, atmospheric, and runoff—and refine source apportionment in coastal areas, like Arcachon.