Author Description

The observations described above concern taxa that occur more or less 'naturally' in the model ecosystems. Another technique to assess the impact of toxicants under field conditions is that of using standardized bio-assays. Two aquatic species that are frequently used to test sediment quality are the midge Chironomus riparius and the mayfly Ephoron virgo, however these species were not observed in the model ecosystems. Sediment from our TPT model ecosystem experiment was used to assess the response of both species to pollutants in the sediment. Fifteen weeks after the TPT application, sediment samples were taken from each individual test system, with the exception of the 3 μg/L level in both types of test system, and transferred to the laboratory for use in the bioassays. A previous study had already determined the contaminants in the sediment (PAHs, PCBs and metals), and TPT concentrations were measured in the control and the 30 μg/L test immediately after sampling. Linear regression was used to determine the TPT concentrations in the other sediments. For each of the test systems, 25 mL mixed sediment was put in a glass jar, and 20 larvae of E. virgo or 5 larvae of C. riparius were added. After 10 days, the growth and survival of the organisms was assessed. The bio-assay experiments showed that the type of sediment (containing either small or large quantities of historical background pollution) hardly impacted on the growth of C. riparius. A possible explanation is that the polluted sediment also contained more organic matter, which may have served as a suitable food source and may have compensated for the negative effects of the background pollutants. However, the survival and growth of E. virgo larvae was considerably lower at higher concentrations of PAHs, PCBs and metals. It is possible that the nature of the organic material also influenced the outcome in this case. In both types of sediment, it was impossible to establish a dose-response relationship for sedi