The effects of a pulsed application of chlorpyrifos on macroinvertebrate communities in an outdoor artificial stream system.

The results of an experiment examining the effects of pulsed application of a pesticide, chlorpyrifos, in an outdoor replicated artificial stream system are described. Two levels of chlorpyrifos were used, 0.1 microgram.liter-1 (low dose) and 5.0 micrograms.liter-1 (high dose), and applied for 6 hr. Low-dose streams showed little impact from the treatment and were indistinguishable from control streams. Significant reductions in invertebrate density occurred in the high-dose streams and were mainly due to reductions in density of chironomid larvae. There were no significant reductions in taxon richness associated with the treatments indicating no localized extinctions of species. Diversity measures were insensitive to the changes observed in the streams. Ordination and classification procedures were more illuminating and indicated that the major effect of pesticide application was to interfere with the normal pattern of community change occurring within the system. Recovery following treatment was rapid. The results are discussed with reference to the use of indicator species and biological monitoring strategies intended to identify human-mediated disturbance.