Sediment bioassays with Chironomus riparius: Understanding the influence of experimental design on test sensitivity

3,4-dichloroaniline in a formulated sediment has been used as a model system to investigate how test design can influence test outcome. The first variable tested was the size of Chironomus riparius larvae used to start a 10-day growth test. Three sizes of larvae were used (early 1st, late 1st or early 2nd instar) and two endpoints (dry weight and length). For both endpoints the LOEC for the smallest and largest starting sizes was the same. In a second experiment density and food levels were varied. Increasing the density five fold reduced larval sizes but had minimal impact on test sensitivity. Reducing food levels to 10% of normal did, however, severely reduce growth and test sensitivity.