Combined effects of environmental and chemical stressors on a model Daphnia population

The perturbation in dynamics of a Daphnia magna population caused by the combined effects of two environmental stressors (temperature and dissolved 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-induced adaptive mechanism (the synthesis of hemoglobin) in population survival. The methodology addresses stressor effects on physiological processes via investigation of individual-based population models. Simulations 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 combined effects of environmental and chemical stressors can considerably reduce the parameter ranges indicative of population survival as compared to the effects of the single stressors.