Optimization of in situ bioassays withHyalella aztecaandChironomus tentans

Uncertainty is always associated with extrapolations of laboratory toxicity test responsesto in situ conditions. Exposing toxicity test organisms in situ reduces uncertainty by removing sampling related artifacts and better simulating natural exposures. The amphipod Hyalella azteca and the midge Chironomus tentans, were enclosed in plastic containers with 149- or 74-mm mesh openings. These chambers were anchored to stream substrates allowing continual water exchange and contact with sediments. Hyallela azteca survival was greater than 80% after 4 weeks of exposure. In situ testing with H. azteca in cold water (<15°C) resulted in survival consistently below 60%, possibly due to limited acclimation periods. Handling stress during laboratory to field transport reduced C. tentans survival. High survival was not consistently achieved until a protective ground paper towel substrate was used. A smaller mesh size (74 mm) on the chambers reduced the occurrence of indigenous organisms inside the chambers, making C. tentans identification unmistakable. In situ testing with C. tentans using refined methods resulted in greater than 80% survival after 2 weeks of exposure. In situ exposures of these two benthic invertebrates can effectively evaluate short- to long-term conditions of water and sediment quality.