Testing sediment biological effects with the freshwater amphipod Hyalella azteca: the gap between laboratory and nature.

The freshwater amphipod, Hyalella azteca, is widely used in laboratory sediment toxicity and bioaccumulation tests. However, its responses in the laboratory are probably very different from those in the field. A review of the literature indicates that in its natural habitat this species complex is primarily epibenthic, derives little nutrition from the sediments, and responds primarily to contaminants in the overlying water column (including water and food), not sediment or porewater. In laboratory sediment toxicity tests H. azteca is deprived of natural food sources such as algal communities on or above the sediments, and is subjected to constant light without any cover except that afforded by burial into the sediments. Under these constraining laboratory conditions, H. azteca has been reported to respond to sediment or porewater contamination. In nature, contamination of overlying water from sediment is less likely than in the laboratory because of the large, generally non-static sink of natural surface water. H. azteca does not appear to be the most appropriate test species for direct assessments of the bioavailability and toxicity of sediment contaminants, though it is probably appropriate for testing the toxicity of surface waters. Toxic and non-toxic responses will be highly conservative, though the latter are probably the most persuasive given the exposure constraints. Thus H. azteca is probably a suitable surrogate species for determining sediments that are likely not toxic to field populations; however, it is not suitable for determining sediments that are likely toxic to field populations.