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 65 Cu 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 (ku , 0.546 L g-1  h-1 to 0.213 L g-1  h-1 ) fitted well with an empirical equation, ku  = 1/(1.35 + 0.116·Salinity), by treating salinity as a pseudo-competitor. The median lethal concentrations (LC50 s) 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 (kk s), 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 (CIT ), ranged from 79 to 133 μg-1  g-1 and showed no clear trend with salinity.