Among- and within-population variability in tolerance to cadmium stress in natural populations of Daphnia magna: implications for ecological risk assessment.

Previous attempts to test the hypothesis that laboratory selection of isogenetic populations can produce test organisms with a significantly increased mean tolerance to toxic substances have failed. One possible explanation for such failure is that the tolerance of laboratory populations is largely constrained by their origins (were the source populations composed of tolerant genotypes?). To address this question, among- and within-population variability in stress tolerance was assessed by calculating the variance in individual fitness and longevity across a cadmium gradient (0-10 microg/L).