Effects of the insecticide Dursban 4E (Active ingredient chlorpyrifos) in outdoor experimental ditches. I. Comparison of short-term toxicity between the laboratory and the field

Using the insecticide Dursban 4E (active ingredient chlorpyrifos) as the test compound, results of laboratory acute single-species toxicity tests with indigenous and standard test species were compared with short-term direct effects in outdoor experimental ditches (mesocosms). In the mesocosms a regression experiment was performed with nominal initial chlorpyrifos concentrations of 0.1, 0.9, 6, and 44 microgram/L. The mesocosms were sprayed once. Effects were investigated by sampling macroinvertebrates and zooplankton and by doing in situ cage experiments with several species. Chlorpyrifos concentrations showed highest spatial and temporal variation within 2 d of treatment. Acute effects were observed on arthropods only and essentially were manifest on day 0. Short-term direct effects in the mesocosms could be quantified by a regression method for seven of 120 species. For these species, 48- and 96-h median effective concentrations (EC50s) ranged from 0.1 to 3.4 microgram/L and were in the same order of magnitude as their laboratory EC50s. Susceptibility of the most sensitive standard test species (Daphnia magna; 48-h median lethal concentration [LC50], 1 microgram/L) was more or less representative of susceptible indigenous species. In the mesocosms effects were negligible at the 0.1-microgram/L treatment level. A safety factor of 0.1 (48-h LC50 of Daphnia magna) may have protected almost all of the species in the community in the mesocosms against short-term direct effects. A safety factor of 0.01 probably protected the most susceptible taxa we found (laboratory 96-h EC10 for Gammarus pulex, 0.02 microgram/L; no-observed-effect concentration for Copepoda, <0.1 microgram/L). The question remains, however, of whether long-term (in)direct effects on the populations or the community may occur at the 0.1-microgram/L treatment level.