Revealing the complex effects of salinity on copper toxicity in an estuarine clam Potamocorbula laevis with a toxicokinetic-toxicodynamic model

The effects of salinity on metal toxicity are complex: not only affecting metal bioaccumulation, but also altering the physiology and sensitivity of organisms. In this study, we used a toxicokinetic-toxicodynamic (TK-TD) model to separate and quantify the dual effects of salinity on copper (Cu) toxicity in a euryhaline clam Potamocorbula laevis. The toxicokinetics of Cu was determined using the stable isotope ⁶⁵Cu as a tracer at concentrations (10–500 μg L⁻¹) realistic to contaminated environments and at salinities ranging from 5 to 30. At low Cu concentrations (ca. 10 μg L⁻¹), Cu bioaccumulation decreased monotonically with salinity, and the uptake rate constant (k_u, 0.546 L g⁻¹ h⁻¹ to 0.213 L g⁻¹ h⁻¹) fitted well with an empirical equation, k_u = 1/(1.35 + 0.116·Salinity), by treating salinity as a pseudo-competitor. The median lethal concentrations (LC₅₀s) of Cu were 269, 224, and 192 μg L⁻¹ at salinity 5, 15, and 30, respectively. At high Cu concentrations (ca. 500 μg L⁻¹), elevating salinity were much less effective in decreasing Cu bioaccumulation; whereas Cu toxicity increased with salinity. The increased toxicity could be explained by the increases in Cu killing rates (k_ks), which were estimated to be 0.44–2.08 mg μg⁻¹ h⁻¹ and were presumably due to the osmotic stress caused by the deviation from the optimal salinity of the clams. The other toxicodynamic parameter, internal threshold concentration (C_IT), ranged from 79 to 133 μg⁻¹ g⁻¹ and showed no clear trend with salinity. - Highlights: • The geochemical effects of elevated salinity decrease Cu bioaccumulation. • The physiological effects of elevated salinity increase Cu toxicity. • A toxicokinetic-toxicodynamic (TK-TD) model is developed to quantify the dual effects of salinity. • The biotic ligand model can be integrated into the TD-TK model for predicting salinity effects. - A toxicokinetic-toxicodynamic model is developed to separate and measure the geochemical and physiological effects of salinity on Cu toxicity to estuarine organisms.