A sensitive and accurate method is described for the measurement of the apparent complexing capacity of lake waters. It is based on the measurement of labile copper by differential pulse anodic stripping voltammetry after a number of ionic copper spikes have been allowed to equilibrate with the complexing materials in a water sample. Coefficients of variation for the determination of EDTA at a level of 0.50 μmole/liter in Hamilton Harbor water were 6 and 8%. Analysis of seven replicates of a Hamilton Harbor water sample gave an apparent complexing capacity of 0.52 μmole/liter Cu equivalent with a relative standard deviation of 7.7%. Analyses of a number of samples from Lake Erie and from lakes near Sudbury, Ontario gave a range from nondetectable to 0.70 μmole/liter Cu equivalent of complexing capacity.

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The toxic effect of copper on phytoplankton production is investigated in waters having different complexing capacities. It is demonstrated that a water's complexing capacity does not guarantee that an equivalent amount of copper could be tolerated without adversely affecting algal production. Possible explanations for these findings axe offered and discussed.It is deduced that ionic copper probably is already toxic to planktonic algae at concentrations of about 10 -t~ mole/1.

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