Laboratory toxicity data contrasting responses of aquatic organisms to insecticides are important for focusing on sensitive species (steepest exposure-response slope) exposed to aqueous concentrations of these insecticides in field studies. These data also allow prediction of expected responses of aquatic species to a range of insecticide concentrations in situ. Aqueous 48-h toxicity tests were performed to contrast responses of Daphnia magna Straus, Hyalella azteca Saussure, Chironomus tentans Fabricius, and Pimephales promelas Rafinesque to acetylcholinesterase-inhibiting insecticides: chlorpyrifos, aldicarb, and chlordane. As expected, invertebrates tested (H. azteca, C. tentans, and D. magna) were >/= 200 times more sensitive than the vertebrate P. promelas to chlorpyrifos exposures. H. azteca was approximately 3.5 times more sensitive to chlorpyrifos (453% mortality/&mgr;g/L) than D. magna (128% mortality/&mgr;g/L). For both aldicarb and chlordane, C. tentans was the most sensitive species tested (2.44 and 2.54% mortality/&mgr;g/L, respectively). Differences in chlordane potency for test species varied only by a factor of approximately 2-3 (0.88% mortality/&mgr;g/L for H. azteca to 2.54% mortality/&mgr;g/L for C. tentans). Although point estimates of population responses such as LC50s, NOECs, and LOECs are of some utility for predicting effects of pesticides in aquatic systems, exposure-response slopes are also useful for extrapolation of laboratory data to diverse field situations, especially where sediment sorption may regulate insecticide exposure or bioavailability.

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Responses of aquatic invertebrates to a C 14-15 linear alcohol ethoxylate (LAE) nonionic surfactant with an average of seven ethylene oxide units per mole of alcohol were evaluated in outdoor stream mesocosms. Two experiments were carried out to evaluate responses of aquatic invertebrates to 28-and 30-d exposures in eight stream mesocosms. Changes in benthic invertebrate densities and in invertebrate drift were monitored during pretreatment, treatment, and posttreatment periods. In Experiment 1, mean measured concentrations of surfactant were 0.08, 0.16, and 0.33 mg LAE/L. In Experiment 2, mean measured concentrations were 0.11, 0.28, and 0.55 mg LAE/L. In each experiment, there were two treated streams at each surfactant concentration and two untreated streams that served as controls. No significant effects (p Ͼ 0.05) on population densities of Copepoda, Cladocera, Chironomidae, Nematoda, or Annelida were observed in either experiment. Moreover, no effects were detected on the numbers of any of these organisms collected in drift nets. However, there was a significant decrease (p Յ 0.05) in the densities of Simuliidae at surfactant concentrations Ն0.16 mg LAE/L. This was accompanied by an increase in the numbers of Simuliidae collected in drift nets. Based on these results, the no-observed-effect concentration for aquatic invertebrates was 0.08 mg LAE/L, and the lowest-observed-effect concentration was 0.16 mg LAE/L.

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