CHIRONOMUS-TENTANS LIFE-CYCLE TEST - DESIGN AND EVALUATION FOR USE INASSESSING TOXICITY OF CONTAMINATED SEDIMENTS

The development and standardization of toxicity test methods for asses sing toxicity of contaminated freshwater sediments has focused predomi nantly on short-term exposures and lethality. In many situations, howe ver, toxicity is more likely to become manifest over long periods of t ime so there is need for standardized testing procedures by which subl ethal sediment toxicity can be adequately assessed. In this study, we present and evaluate a new life cycle test, using the midge Chironomus tentans, which enables the assessment of sublethal toxicity of contam inated sediments. In designing the life cycle test, our goal was to ma ke the assay relatively straightforward and, to the extent possible, m ethodologically compatible with the standard 10-d test. The test is in itiated with newly hatched (<24 h) larvae and uses four effects-based endpoints to assess toxicity: survival, growth, emergence, and reprodu ction. Survival is determined at 20 d and at the end of the test by ba ck-calculating through emergence data. Growth can be determined at 20 d, which corresponds to the 10-d endpoint in the 10-d C. tentans growt h rest initiated with 10-d-old larvae. From day 23 to the end of the t est, emergence and reproduction are monitored daily. The number of for each egg mass, which is then incubated for 6 d to determine hatching success. Each treatment in the life cycle test is terminated separatel y after 7 consecutive days without emergence. We evaluated the life cy cle test by following one generation of C. tentans in sediments collec ted from the upper Mississippi River. Survival of larvae exceeded 90% at 20 d. Of these larvae, between 60 and 70% successfully emerged; sur vivorship among pupae and adults exceeded 85%. Mean egg production ran ged from 906 to 1,107 eggs per female. The test required 65 d to compl ete, including pretest preparation. These data show that the C. tentan s life cycle test can be used to assess sublethal toxicity accurately in contaminated sediments and provides a suitable compliment to the st andard C. tentans 10-d test.