Effect of declining toxicant concentrations on algal bioassay endpoints

Sorption, degradation, volatilization, and uptake by test organisms cause concentrations of many toxicants to decline during toxicity testing. Despite the recognition of this occurring, nominal, measured initial or time-averaged concentrations are commonly used for the calculation of inhibitory or effect concentrations from toxicity test data. Because a premise of constant exposure is assumed but not met in these calculations, the toxicity of the test water may be significantly underestimated. Laboratory experiments using a 72-h algal growth inhibition bioassay and copper as a toxicant are used to demonstrate that calculated inhibitory concentrations will often be underestimated twofold if losses of copper occurring over the 72-h test duration are not considered. A simple model is presented for toxicant concentration decline and for the relationship between algal growth rate and toxicant concentration. This model is used to demonstrate the magnitude that calculated inhibitory concentrations may be underestimated if concentration declines are not considered for a series of different concentration decline scenarios. For a toxicant whose concentration declines exponentially to less than 5% of its original value within 36 h of a 72-h test, the inhibitory concentration is shown to be underestimated by a factor of 50.