Effects of a mixture of two insecticides in freshwater microcosms: II. Responses of plankton and ecological risk assessment.

This paper reports on the chronic effects of a mixture of the insecticides chlorpyrifos and lindane in freshwater microcosms. Chronic treatment levels corresponding to concentrations of 0, 0.005, 0.01, 0.05, 0.1 and 0.5 times the LC50 of the most sensitive standard test organism were evaluated. The zooplankton community structure was altered from the 0.05 * LC50 treatment level upwards. Cladocerans were the most susceptible group, followed by Copepoda and Ostracoda. Rotifera increased in abundance at the higher treatment levels. Increased abundance of some phytoplankton taxa and increased chlorophyll-a levels were found at the two highest treatment levels, most probably a consequence of decreased grazing pressure. Threshold levels for the mixture, both at population and community/ecosystem level, corresponded well with those reported in the literature for the individual compounds. The overall risk assessment indicates no antagonistic or synergistic effects of the mixture at ecosystem level. It was found that the safety factors set by the Uniform Principles for individual compounds also ensure protection against chronic exposure to a mixture of insecticides at community level, though not always at species level.