Dursban® 4E, an organophosphorus insecticide (A.I. chlorpyrifos) was applied to two experimental ponds (initial concentrations 1.7 and 16.5 µg/l), while a third pond served as a control. This resulted in an immediate and complete loss of the cladocerans. Concentration-dependent adverse effects occurred on copepod nauplii. From 5 weeks after application, nauplii densities exceeded those of the control, presumably the result of the absence of Daphnia pulex and Daphnia longispina. There were no observable effects on copepodids plus adult copepods; however, the low level of identification may have obscured toxic effects. Chlorpyrifos had no effect on ostracods. As a secondary effect, the number of rotifers increased substantially within 7 to 14 days after application. This coincided with the disappearance of Daphnia. The rotifer Keratella sp. became the dominant zooplankton species. Within 2 months after application, the carnivorous rotifer Asplanchna sp. presumably reduced the herbivorous rotifer populations and copepod nauplii.
Ordination by correspondence analysis proved effective in visualizing the effects of Dursban® 4E and time on the structure of the zooplankton community. The result of this analysis concurs with data from the literature of several field tests with Dursban®. A Monte Carlo permutation test suggested a significant (p = 0.01) reaction of the species to the insecticide treatment.