Sublethal toxic effects in a generic aquatic ecosystem.

The dynamical behaviour of an aquatic ecosystem stressed by limiting nutrients and exposure to a conservative toxicant is investigated. The ecosystem downstream of a pollution source consists of: nutrients, biotic pelagic and benthic communities, and detritus pools in the water body and on the sediment. The long-term dynamic behaviour of this system is analysed using bifurcation theory. A reference state is defined and our aim is to quantify the effects of toxicological (toxic exposure), ecological (feeding, predation, competition) and environmental stressors (nutrient supply, dilution rate). To that end we calculate the ranges of stress levels where the long-term dynamics (equilibrium, oscillatory or chaotic behaviour) is qualitatively the same. In this way we obtain levels of toxicological loading where the abundances of all populations are the same as in the reference case, the no-effect region. We will also calculate toxic exposure levels that do not lead to a change in the composition of the ecosystem, and therefore its structure, with respect to the reference unexposed situation, but where population abundances and internal toxicant concentrations may have been changed quantitatively. The model predicts that due to indirect effects even low sublethal toxic exposure can lead to catastrophic changes in the ecosystem functioning and structure, and that the long-term sensitivities of oligotrophic and eutrophic systems to toxic stress are different.

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