The perturbation in dynamics of a Daphnia magna population caused by t
he combined effects of two environmental stressors (temperature and di
ssolved oxygen) and a toxicant (a nonpolar organic lipophilic chemical
) on individual physiology is investigated in a model setting with the
primary objective to determine indicators of stress at the population
level. Another objective is to investigate roles of a stressor-induce
d adaptive mechanism (the synthesis of hemoglobin) in population survi
val. The methodology addresses stressor effects on physiological proce
sses via investigation of individual-based population models. Simulati
ons demonstrate that population health is reflected in population age
and size structure, that population effects can be more damaging than
expected from known levels of effects on the individual, and that comb
ined effects of environmental and chemical stressors can considerably
reduce the parameter ranges indicative of population survival as compa
red to the effects of the single stressors. (C) 1997 Elsevier Science
B.V.