A field bioassay was developed to study the in situ effects of sediment contamination on Chironomus riparius larvae. The survival, development rate and increase in Chironomus larvae biomass was compared between laboratory bioassays with the field cages. The incidence of mentum deformities was compared not only between laboratory and field bioassays but also with observations on field populations of Chironomus larvae. Survival in the field bioassays was slightly higher than the laboratory, except at locations with known contamination of the surface water. The influence of surface water quality in field bioassays was demonstrated in translocation experiments, in which clean sediment was placed in a polluted site, and vice versa. In a field bioassay carried out in the autumn, an inverse relationship between the rate of development and the initial larval density in the field cages was apparent. In addition, in the field bioassays with C. riparius considerable seasonal variation in the survival and incidence of mentum deformities was found. Field bioassays performed during the winter season indicate that low temperatures can interact with or add to the effects of sediment contamination on chironomid populations.

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