A critical review

If we go back to the stated objective of the investigations published by Lithner et al. (2003), as it was specified in chapter 2 of their report, we can recall that the authors emphasized the importance of knowing the degree of bioavailability of the pollutants in order to be able to make a correct analysis of the risk of biological effects. This is because "bioavailability constitutes an important link between the environmental concentration and the risk of biological effects, being of central significance in the ecological risk analysis. In order to be able to assess the bioavailability it is often convenient to analyse organisms from the area with regard to the actual substances, and relate these to ambient concentrations, which preferably should be determined at the same time."

Knowing that the authors are well aware of how to proceed in theory, and what strategy to choose when designing a field investigation, it is somewhat surprising to learn how it was done in reality:

• The only systematic and reliable part of all the bioavailability studies included was the experiment with transplanted zebra mussels (Dreissena polymorpha). The mussels baskets were placed at 9 different sites, all at a distance of 1 m above the sediment surface. One or two samples of six defecalised individuals with a composite weight of 20 mg DM from each site were subjected to chemical analysis. Thus, results from this part of the study are reliable and can be used in an evaluation of the presence of bioavailable metals (and organic substances) in the water. The major uncertainty in the experimental design used is that it was not possible to discriminate between contaminants currently released from the anthroposphere and historical contaminants residing in the top layer of the sediment, and periodically being resuspended back to the water column.

• The studies of additional animal species with the aim of broadening of the survey of bioavailable metals was, however, not fulfilling basic scientific requirements and their interpretability was therefore unacceptably poor. To get representatives of the littoral ecological niche, two different crustacean species were collected, Asellus aquaticus from two reference sites and one contaminated site in Lake Malaren as well as from three smaller lakes, and Gammarus pulex from two sites in Lake Malaren. Moreover, to reflect the soft bottom profundal habitat, midge larvae, Chironomus plumosus, were collected from three contaminated sites in Lake Malaren and from two sites in small lakes. The sample sizes of Chironomus varied from 0.8 to 7.7 mg DM and the number of individuals from 6 to 55.

• The possibilities to make meaningful comparisons between sites, or between different media at the same site, were not facilitated by the fact that sampling of water for metal analysis or of particles in water were made on other occasions than the biological sampling.

• A great deal of the discussion in the report is devoted to the possible relationships between metals in the water mass and metals in crustaceans or midge larvae, in order to get a basis for calculating BAFs. However, it must unfortunately be concluded that most of this discussion has a very low scientific credibility, due to the fact that:

- the basic material is very limited, few sites and only one sample per site;

- samples of the same species from different sites are not comparable;

- there is no correspondence in time between samplings of water and biota;

- sampling of water and biota (if simultaneous) was made from different water masses, not necessarily having the same water quality.

In fact, even with respect to the transplanted zebra mussels, it must be questioned whether the calculated BAFs have any real meaning, because sampling of water was generally made twice at each site (at a distance in time of 1-2 months, and not necessarily at the same date as the mussel exposure ceased), sometimes showing that concentrations varied by a factor of 2 to 3, at least for Cr and Zn. Unfortunately, water was sampled from a water layer (0.1-0.2 m below the surface) quite separate from the water layer where the mussels were exposed (1 m above the bottom, i.e. at 4-5 m water depth).

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