Geochemical investigations of metal bioaccumulation in oysters along the land-sea continuum: Sources, fluxes and reactivity in the Seudre Estuary (France)

Since 2009, monitoring of Crassostrea gigas oysters tissues in the Seudre estuary, France, has revealed elevated bioaccumulation of several metals, notably cadmium near regulatory limits, at the estuary's entry compared to its mouth. Suspected metal sources from the upper continental watershed prompted specific geochemical investigations to identify potential origins of this bioaccumulation. Data were collected through targeted sampling campaigns of water and sediment in both continental and estuarine regions, supplemented by statistical analyses of long-term regulatory monitoring datasets (groundwater, surface water, sediment, and oyster tissues) provided by Water Framework Directive. Analyses indicated that dissolved concentrations of nutrients (NO₃^-) and certain metals (Ba, Cu, Cr, Zn, and Pb) originated from continental waters, while others (Cd, Li, As) were linked to seawater influx. Results showed significant sediment accumulation for Cr, Ni, and Zn, but not for As, Cd, Cu, or Pb. No significant Cd source or anthropogenic contributions, such as agricultural runoff, were identified in the continental watershed. Dissolved Cd concentration is 20 ng/L at the estuary mouth. Geochemical modeling suggested that clay precipitation near the mixing zone could explain sediment metal accumulation. Additionally, salinity effects on free metal cation concentrations and reduced complexation with chloride ions at the estuary's entry might enhance Cd bioaccumulation in oysters. Exposure to continental nitrate fluxes was identified as another potential factor influencing metal bioaccumulation. These findings highlight the importance of biogeochemical reactivity within estuarine environments, in addition to sources and fluxes of elements, in assessing metal behaviour and accumulation in such systems.