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From an ecotoxicological point of view any substance which substantially modifies population and community characteristics must be considered dangerous. It is evident that a toxic substance may influence the biota if its concentration in the environment is over a certain level. Indeed, some heavy metals, dangerous at high concentrations, are essential to the biota at low concentrations. Unfortunately, it is often difficult to establish this level. Legislation concerning environmental protection is based on these concepts and maximum permissible concentrations are established essentially on the conclusions drawn from short-term experiments carried out under standard conditions. On the other hand, difficulties arising when results from different authors are compared and the great unreliability of extrapolating conclusions from laboratory studies to natural environment are well known. From these considerations it is clear that the ecotoxicological study of a metal must take into account the fate of the metal from its source to its uptake by organisms and its effects on individuals, populations and communities. In addition, indirect effects of the metal on the physical environment, such as variations in pH-value and oxygen concentration in water resulting from toxic effects of the metal on photosynthesis must also be considered. The influence of the physicochemical characteristics of the ecosystem on the metal species (i.e. its physicochemical form) and the influence of biological activity on the metal form and availability, are other important topics of research. This paper will illustrate these aspects as they are related to the problem of cadmium pollution in freshwater.

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Bibliographic Reference: EXPERIENTIA, VOL. 40 (1984), PP. 2-14
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