EFFECT OF INDIGENOUS ANIMALS ON CHRONIC END-POINTS IN FRESH-WATER SEDIMENT TOXICITY TESTS

Sediment toxicity tests were conducted using three species of benthic invertebrates, Chironomus riparius, Hyalella azteca, and Hexagenia lim bata, with various densities of the oligochaete worm Tubifex tubifex. It was shown that indigenous animals, simulated by the presence of Tub ifex tubifex, did not affect survival of the test species (P greater-t han-or-equal-to 0.05) but did reduce growth in all three test species and in two species at the lowest tested densities, equivalent to 1,460 worms per square meter. At densities of Tubifex tubifex equivalent to 20,000 m-2, the growth of Chironomus riparius was reduced by >90%, Hy alella azteca by >60%, and Hexagenia limbata by almost 50%. The densit ies of oligochaetes are equivalent to those found in many contaminated sites. Therefore, it is concluded that the presence of indigenous org anisms can confound the interpretation of toxicity test results, based on chronic end points. It is recommended that removal of organisms be considered before toxicity tests are conducted with freshwater sedime nts from sites with large populations of benthic invertebrates, especi ally oligochaete worms.