EFFECTS OF THE INSECTICIDE DURSBAN(R) 4E (ACTIVE INGREDIENT CHLORPYRIFOS) IN OUTDOOR EXPERIMENTAL DITCHES .2. INVERTEBRATE COMMUNITY RESPONSES AND RECOVERY

This article describes the long-term effects on the macroinvertebrate and zooplankton community in outdoor experimental ditches after a sing le application of the insecticide chlorpyrifos-Nominal concentrations of 0.1, 0.9, 6, and 44 mu g/L of chlorpyrifos were applied to two meso cosms each, while four served as controls. Both macroinvertebrates and zooplankton were sampled from 4 weeks before to 55 weeks after treatm ent. The macroinvertebrate and zooplankton data sets were combined int o one data set and analyzed using the multivariate ordination techniqu e ''redundancy analysis.'' The method provided a clear description of the effects on the invertebrate community in time while still showing the effects at the species level. Crustacea and Insecta showed a rapid , concentration-dependent decrease in numbers after insecticide applic ation (direct effects). An increase in gastropods and Oligochaeta was found, suggesting indirect effects. The start of recovery of the inver tebrate populations affected was found to depend not only on the susce ptibility of the taxa but also on ecological characteristics, such as the length of the life cycle. A no-observed-effect concentration of 0. 1 mu g/L could be derived both at the species and the community level. Safe concentrations, based on no-observed-short-term-effect levels fo r some characteristic indigenous taxa susceptible to chlorpyrifos, als o appeared to protect the total invertebrate community in the long ter m. The invertebrate community at all treatment levels was considered t o have recovered after 24 weeks posttreatment.