PARENT SESSION

2I - High-tiered Studies
Hall 7
8:30 AM - 12:30 PM, Monday, 28 April 2003
Chair: Streloke, M.¹, ¹
Co-chair: Schutz, R.², ²

(MO7/2) Comparison of responses to chlorpyrifos by microcosm communities under varying experimental conditions.

Van Wijngaarden, Rene¹, Brock, Theo¹, Douglas, Mark², ¹ Alterra, Wageningen, Gelderland, The Netherlands² Dow AgroSciences, Wantage, Oxfordshire, United Kingdom

ABSTRACT- The study aimed to determine whether environmental conditions influence effects of the insecticide chlorpyrifos on algal densities in aquatic ecosystems via reduced zooplankton-grazing pressure. Experiments were carried out in plankton-dominated microcosms. In one experiment, microcosms were placed under cooler conditions (18 °C, lower light intensity). In a second and third experiment, microcosms were placed under warmer conditions (28 °C, higher light intensity). Plankton growth was stimulated by regularly adding nutrients. Nutrient levels in the third experiment were a factor of 10 to 20 times higher than in Experiment 1 and 2. Treatment levels were 0, 0.01, 0.1 and 1 ug/L chlorpyrifos for all three experiments. Under the cooler conditions, consistent NOECs for structural endpoints (zooplankton and phytoplankton abundance) were at the 0.1 ug/L-treatment level. No consistent significant eutrophication symptoms were found. Clear effects on the zooplankton occurred at 1 ug/L, but recovery took place within the experimental period. In both warmer experiments, NOECs for structural endpoints also were at the 0.1 ug/L-treatment level. Significant reductions of zooplankton caused clear signs of eutrophication at the 1 ug/L-treatment level. In Experiment 2, effects lasted 2 to 4 weeks. In Experiment 3 effects lasted the entire post-treatment period. Clear differences in threshold levels of direct effects on zooplankton due to chlorpyrifos treatments did not occur between the three system types. Although signs of eutrophication occurred in the warmer systems at higher concentrations (i.e., 1 ug/L) and not in the cooler ones, the overall NOEC for the three different system types was 0.1 g/L. Treatment related signs of eutrophication can be explained by reduced densities of cladocerans. Results are in alignment with other micro- and mesocosm studies that simulated ecologically more complex communities.

Key words: chlorpyrifos, model ecosystems, eutrophication, plankton