Runoff simulation with particle-bound fenvalerate in multispecies stream microcosms: Importance of biological interactions

Multispecies stream microcosms were used to test the toxicity of the pyreth roid fenvalerate (FV) associated with suspended particles in order to simul ate a typical runoff exposure scenario. Stream microcosms were exposed for 1 h in triplicate to 0.0, 13.6, 136, or 1,365 mug/kg FV and effects were mo nitored for 93 d. Experimental design allowed for detection of interspecifi c effects on the emergence and thus survival of the caddisfly species Limne philus lunatus Curtis and of intraspecific effects on the spatial distribut ion of adult and juvenile Gammarus pulex L. (Amphipoda). Exposure at 136- a nd 1,365 mug/kg resulted in a significant acute increase in drift. Survival of L. lunatus was significantly reduced in the 1,365- mug/kg treatment dur ing single-species exposures. When other species were present, survival of L. lunatus was significantly reduced at 136 mug/kg. A similar increase in t est system susceptibility was observed in relation to the spatial distribut ion of G. pulex. Juvenile individuals avoided areas with high numbers of ad ult amphipods, which may prey on the juveniles. This avoidance was signific ant in the control and the 13.6- mug/kg treatment but did not occur at high er levels of exposure. This study highlights the ecotoxicological importanc e of field-relevant levels of particle-associated hydrophobic chemicals tha t transiently increase during runoff events. Interspecific and intraspecifi c interactions can alter the test results; i.e., significant lethal and sub lethal effects are measurable at FV levels approximately an order of magnit ude lower than when biological interactions are excluded.