The toxicity of copper to Staurastrum chaetoceras (Schr.) G. M. Smith
and Staurastrum manfeldtii Delp. has been evaluated using lake water a
nd synthetic media. It appeared that the EC50 values of growth inhibit
ion in synthetic medium with chelators were overestimated, compared to
natural water. The omission of chelators in the synthetic medium led
to EC50 values distinctly lower than in lake water cultures. The speci
es Staurastrum manfeldtii isolated from an oligomesotrophic environmen
t proved to be significantly more sensitive to copper than Staurastrum
chaetoceras originating from a eutrophic environment; this difference
was observed in all media tested. It is concluded that the use of syn
thetic media is likely to give distorted information on metal toxicity
so that the use of lake water is to be recommended for ecologically r
elevant tests.