Forty populations of Daphnia galeata mendotae were maintained for 22 wk (or until they became extinct) in a medium consisting of filtered Lake Michigan water to which Chlamydomonas reinhardi and different concentrations of a mixture of labelled (109Cd) plus stable CdCl2 were added 24 h before use. The medium was changed daily Monday through Fridays. Weekly determinations were made of total numbers, average individual dry weight, mean brood size, egg–female ratio, aborted eggs and embryos, and average whole-body 109Cd content. Chronic cadmium stress reduced the populations' average numbers and biomass, while it increased the populations' turnover rates, temporal variability, probability of extinction, and proportion of ovigerous females. For individuals there was increased prenatal mortality, reduced life expectancy, increased average weight, and increased brood size. Due to compensatory increases in the proportion of ovigerous females and mean brood size, the cadmium sensitivity of population attributes was considerably less than that of the most sensitive individual attribute, prenatal mortality. A linear relationship between population carrying capacity (relative mean numbers) and added cadmium concentration indicated that the effective concentration for a 50% reduction of carrying capacity (EC50) is 7.7 μg Cd/L and that for a 1% reduction (EC1) is 0.15 μg Cd/L. An increase of 0.15 μg Cd/L in Lake Michigan itself, barring synergistic interaction with other factors in the lake, would probably not have a detectable effect on the population dynamics of D. galeata mendotae. Key words: zooplankton dynamics, stress ecology, Daphnia galeata mendotae, cadmium, heavy metals, toxic substances, Laurentian Great Lakes
