Air Quality Criteria for Lead Volume II of II - (NEPIS)(EPA) - US ...

potential toxic effects in other organisms. Non-living organic material (e.g., components of
sloughed-off scales, mucus, carcasses, and other decomposing, humic material) can similarly
combine with Pb from the water column, rendering it unavailable. This section will review the
literature on biological environmental factors and their influence on the bioavailability, uptake,
and toxicity of Pb.
Van Hattum et al. (1996) studied the influence of abiotic variables, including DOC on Pb
concentrations in freshwater isopods (Proasellus meridianus and Asellus aquaticus). They found
that BCFs were significantly negatively correlated with DOC concentrations. Thus, as DOC
concentrations increased, BCFs decreased in P. meridianus and A. aquaticus, indicating that
DOC acts to inhibit the availability of Pb to these isopods.
Kruatrachue et al. (2002) investigated the combined effects of Pb and humic acid on total
chlorophyll content, growth rate, multiplication rate, and Pb uptake of common duckweed.
When humic acid was added to the Pb-nitrate test solutions (50, 100, and 200 mg Pb(NO3)2/ L),
toxicity of Pb to duckweed was decreased. The addition of humic acid to the Pb-nitrate solution
increased the pH. The authors suggested that there was a proton dissociation from the carboxyl
group in the humic acid that complexed with Pb, resulting in a decrease in free Pb ions available
to the plant.
Schwartz et al. (2004) collected natural organic matter (NOM) from several aquatic sites
across Canada and investigated the effects of NOM on Pb toxicity in rainbow trout
(Oncorhynchus mykiss). They also looked at toxicity effects as they related to the optical
properties of the various NOM samples. The results showed that NOM in test water almost
always increased LT50 and that optically dark NOM tended to decrease Pb toxicity more than did
optically light NOM in rainbow trout.
In summary, non-living constituents of biological origin in the environment have been
shown to reduce Pb availability and, therefore, toxicity in some aquatic organisms. It is
generally thought that this occurs through complexation or chelation processes that take place in
the water column.
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