Integrating bioavailability and toxicity of copper in estuarine sediments using whole-sediment exposure with medaka...

Estuarine sediments play a vital role in supporting marine ecosystems by delivering nutrients, forming habitats, filtering pollutants, and storing carbon, but copper (Cu) contamination in these sediments poses serious ecological risks due to its persistence, bioavailability, and toxicity. This study employed a whole-sediment exposure system using medaka fish (Oryzias latipes) embryos, enabling simultaneous chemical exposure through overlying water, pore water, and sediment under environmentally realistic conditions. The bioavailability and toxicity of four Cu-contaminated sediments with distinct geochemical characteristics were assessed including artificial (AT), freshwater (MH) and two estuarine (GD and DS) sediments. Among three environmental sediments, MH released more bioavailable Cu due to its lower pH and organic content, resulting in higher toxicity, while GD and DS showed reduced toxicity due to high salinity and Cu complexation, as confirmed by sequential extraction showing organic/oxide-bound Cu in MH and carbonate/oxide-bound forms in GD and DS. Embryo toxicity from the whole sediment exposure followed the order AT > MH > GD ≈ DS, and embryo mortality correlating more strongly with dissolved and Chelex-100-extractable Cu than with total sediment Cu or body burdens in hatchalings. These differences in Cu speciation, mobility, and bioavailability contributed to varied toxicity outcomes across sediments, highlighting dissolved and resin-extractable Cu are more reliable predictors of sediment toxicity, especially for esturine sediment with higher salnity. The whole-sediment exposure system with medaka embryos offers a practical and effective method for evaluating the bioavailability and ecological risks of heavy metal-contaminated sediments.