Biomonitoring of coastal Cu contamination sources in the French Mussel Watch Program: An isotope perspective

Copper (Cu) is an essential trace metal for marine organisms but becomes toxic when anthropogenic activities increase its concentrations in coastal environments. Mussels are central to biomonitoring programs like Mussel Watch, however their physiological regulation of essential elements complicates the interpretation of Cu bioavailability using concentration data alone. Stable Cu isotopes offer a promising tool for tracing anthropogenic sources, yet their application in long-term coastal monitoring remains limited. This study integrates controlled exposure experiments with field observations from five French coastal sites-Loire Estuary, Toulon Bay, Antifer, Yport, and Grazel Lagoon-to evaluate how mussel Cu isotope compositions respond to external inputs. Laboratory results show that mussel isotope signatures shifted toward the dissolved Cu source, indicating that external Cu inputs influence δ⁶⁵Cu values in tissues, with minor influece of the biological isotope fractionation. In field settings, Loire Estuary mussels exhibited stable isotope compositions, reflecting no substantial changes in environmental Cu sources. Conversely, Toulon Bay mussels showed progressive shifts suggesting the attenuation of legacy anthropogenic contamination. Episodic Cu enrichment in mussels from Antifer, Yport, and Grazel underscored the impact of localized pollution, often accompanied by moderate isotopic shifts. Seasonal analyses suggest that reproductive cycles may also influence isotopic variability in these organisms. Ultimately, this study shows that Cu isotope composition in mussels integrate signals of environmental pollution and potentially internal biological processing. These findings underscore the potential of Cu isotopes as complementary tracers for monitoring coastal metal pollution, provided biological regulation and temporal dynamics are explicitly integrated into the interpretative framework.