EVALUATION OF POTENTIAL CONFOUNDING FACTORS IN SEDIMENT TOXICITY TESTS WITH 3 FRESH-WATER BENTHIC INVERTEBRATES

We examined the effects of natural sediment physicochemical properties on the results of lab tests with the amphipod Hyalella azteca, the mi dge Chironomus tentans, and the oligochaete Lumbriculus variegatus. Te n-day exposures with the three species were conducted with 50 uncontam inated sediment samples from Lakes Erie, Huron, Superior, and Ontario, which differed markedly with regard to characteristics such as grain- size distribution, organic carbon content, and mineralogical compositi on. Tests were conducted both with and without the addition of exogeno us food. Survival of Hyalella azteca, survival and growth of Chironomu s tentans, and survival/reproduction and growth of Lumbriculus variega tus were significantly greater in tests in which the animals were fed vs. those in which they were not. Approximately 10% of the tests in wh ich Hyalella azteca was not fed and 80% of tests in which the amphipod s were fed resulted in >80% survival, a common criterion for defining the acceptability of tests with Hyalella azteca in clean control sedim ents. Similarly, a relatively high percentage of the tests in which Ch ironomus tentans was not fed would have failed a control survival crit erion of 70% for the midge. Hence, there is significant potential for false positive results if Hyalella azteca or Chironomus tentans is not fed during sediment tests. Predictive modeling of the assay results i n relationship to sediment physicochemical characteristics failed to r eveal any additional factors that influenced survival of Hyalella azte ca and Chironomus tentans, or reproduction and growth of Lumbriculus v ariegatus in tests in which the organisms were fed. However, linear mo deling did suggest that growth of fed as well as unfed Chironomus tent ans may have been influenced by grain-size distribution of the test se diments.