Approximately two-thirds of the copper in lake water remains in the nonfilterable residue of the following two-step procedure: 0.45 μm filtration followed by Diaflo UM-2 membrane (passing mol wt 1000) ultrafiltration. Examination of the effect of this residual copper on the photosynthesis rate of natural phytoplankton has shown that this residual copper is physiologically not available to the organisms. This fact can be utilized in a bioassay (as demonstrated with EDTA as a test substance) to gain information on the concentration of a ligand and its apparent association constant with copper. It is concluded that nonultrafilterable ligands present in lake water form copper complexes as stable as the CuEDTA complex and can thus play an ecologically significant role in the regulation of copper availability and therefore its toxicity to phytoplankton.
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Gaechter et al. (1978) studied this question.