Biological Effects Of Trace Metals

It may be argued that the most important break-through that has occurred in the past decade in the general understanding of toxic and other adverse effects of trace metals on biota in the aquatic and soil ecosystems is the relatively recent acceptance of the bioavailability concept for metals in the environment. Only by accepting the fact that trace metals are not always - or not under all environmental conditions - readily available for uptake into living organisms, is it possible to fully understand and make estimates of the potential risk that metals may pose for man and the environment. Of course, many (but not all) scientists have been aware - for a long time - of the importance of assessing the actual bioavailability of a metal when determining the toxic potential of that metal to any individual species or to a complex community of organisms. However, enlightened scientists have, for many years, not been very successful in making public decision-makers or environmental regulators accept the bioavailability concept when dealing with trace metals in the environment.

One plausible reason for the scientists' lack of success to convince authorities and regulators about the (somehow self-evident) need to consider the fraction of a trace metal that is really mobile, available and reactive, to be able to establish relationships between the harmful concentration and adverse impact, is that scientists have been slow to develop good conceptual models that are both practical and applicable to every-day pollution problems. When environmental chemists, working together with ecotoxicologists, now finally have come up with such readily understandable concepts, such as the Biotic Ligand Models (BLMs) - cf. chapter 6 - and demonstrated how variations in bioavailability of metals in water can be closely related to a variety of water qualities, it has been a relatively rapid process to get acceptance also by regulatory bodies. Such acceptance was achieved at a technical-scientific meeting convened by a EU Committee responsible for the environmental and health effect evaluation of Existing Chemical Substances within the European Union (Eurometaux, 05/2003). At the same meeting, major progress was also made with regard to the bioavailability of trace metals in another environmental compartment, i.e. in aquatic sediments. It was approved that bioavailability and toxicity of metals in sediments could be assessed by using the SEM/AVS concept (cf. section 5.4.3).

Just by introducing these two new tools, it is now suddenly possible to talk about toxic effects on biota in aquatic and soil ecosystems by means of a much clearer language, facilitating the understanding of some rather complex phenomena in nature. Before, it was not possible to describe the chronic toxic concentrations of, for example, zinc to freshwater fish without giving long lists of results from hundreds of bioassays carried out in an array of different water types, from relatively uniform laboratory waters to surface waters originating in different geological regions. It was not an easy task, for example, to grasp the rather confusing data base, showing that zinc could exert chronic toxicity (30-d ECi0 values) to one single life stage in one specified fish species, i.e. juvenile rainbow trout, at concentrations ranging from 38 to 900 ^g Zn/l.

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