The effect of salinity on the acute toxicity of total and free cadmium to a Chesapeake Bay copepod and fish

The objective of this study was to determine the influence of a range of salinities (5, 15 and 25 ppt) on the acute toxicity of total and free cadmium to the sheepshead minnow (Cyprinodon variegatus) larvae and the copepod Eurytemora affinis nauplii. Data were analysed to determine if the acute toxicity (96 h LC50) was different among salinities for the test species. Total cadmium was measured in selected test conditions and the proportion of total cadmium as Cd2+ (free ion or toxic form) was determined at each salinity. Ninety-six hour LC50 values for C. variegatus were 180.3, 312.4 and 495.5 μg l−1 total cadmium at 5, 15 and 25 ppt, respectively. A significant increase in LC50 values with salinity was most likely related to a decrease in the free ion as salinity increased. Ninety-six hour LC50 values for E. affinis were 51.6, 213.2 and 82.9 μg l−1 total cadmium at 5, 15 and 25 ppt, respectively. A comparison of LC50 values for the copepod between salinities showed a significant difference between 5 and 15 ppt and between 15 and 25 ppt. There was no difference in LC50 values between 5 and 25 ppt. The physiological characteristics of E. affinis were most likely responsible for the higher tolerance at the middle salinity. Cadmium speciation in the various test salinities was dominated by association with inorganic binding ligands; organic complexation was negligible. The speciation at all salinities was dominated by CdCl+ and CdCl02. The free ion accounted for 20, 8 and 4.5% of the total cadmium at 5, 15 and 25 ppt, respectively. These data have important implications for estuaries such as Chesapeake Bay because the presence of the toxic form of cadmium will increase as salinity decreases.