Population dynamics of Daphnia magna as modified by chronic bromide stress

Abstract

Two chronic toxicity experiments were conducted with Daphnia magna. In a semi-static experiment with cohorts, the no effect level for bromide in respect of the intrinsic rate of natural increase (derived from age-specific survival and fecundity) was 10 mg l⁻¹. A second test was started with small populations in an intermittent-flow system. These populations had a stable age distribution, were composed of cohorts of different ages, and showed an almost perfect logistic growth. Model calculations showed that bromide reduced the upper numerical limit (carrying capacity). It also increased the time-lag required to attain the maximum reproduction rate. For the first parameter, a no effect level of 14 mg l⁻¹ was calculated. For the latter a threshold could not be detected. The EC₅₀ and EC₁₀ were 27 and 18 mg l⁻¹, respectively. Additional experiments showed that individual growth of D. magna in time could also be described by a logistic equation. The age structure of the populations changed when food became limiting. This was parallelled by a reduction of the mean brood size. In conclusion it is stated that the results of the toxicity studies with populations support Halbach's view (1984), that population dynamics can be used like a ‘magnifying glass’ to detect small sublethal ecotoxicological effects of environmental pollutants.