A mixture was classified as strictly additive if the sum of TUs = 1, and less-thanadditive and more-than additive, respectively, if the sum of TUs > 1 and < 1(Norwood 2003, Faust 2001). It is worth noting that terminology in the literature varies between authors. A synergetic effect is used to describe more-than-additive mixtures while ‘less-than-additive’ is often termed an antagonistic effect. However,the term ‘antagonistic’ suggests that a mixture is less toxic than any of its components taken individually. For instance a mixture can be as toxic as its most toxic component with no added effect of other chemicals, in which case it will be less-than-additive but the term antagonistic would be misleading (Newman 2012, Norwood 2003). In this study, ‘less-than-additive’ was used if the sum of TUs > 1 but TEmix < TEi, while ‘antagonistic’ was used if the sum of TUs > 1 and TEmix > TEi (i.e. the mixture was less toxic than the most toxic metal individually). To give a measure of experimental variation, a range for the sum of TUs was determined using both extreme values of the EC50s 95% confidence interval, obtained from probit regression as described above.