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The response of phytoplankton communities to copper, cadmium, zinc, and pentachlorophenolate in 24-factorial experiments were investigated. Seven experiments lasting from three to five days were carried out. During each experiment, 16 bottles were incubated in situ. Different species had different responses to the toxicants. For evaluation of the response of the whole community the desirability function was used. The densities of abundant species in the control bottle were used as a reference and considered as desirable. All exceptions more than one standard deviation from the control were considered detrimental. The value of the overall desirability function D was calculated as a geometrical mean of the values of the desirability function for n single species D = (d1d2…dn)1/n. The overall desirability function values in the factorial experiments were treated by regression analyses. In all experiments, the responses of the phytoplankton community to the toxicants were similar. In general, copper caused the greatest change in phytoplankton communities.

A method of the statistical estimation of the threshold of “no effect” and “100% effect” levels of a test compound has been suggested. The experimental data of this method concern the survival of organisms exposed to a range of concentration of toxic chemicals.