Deformities in the menta of chironomid larvae from the Experimental Lakes Area, Ontario

Chironomus spp. larvae in Experimental Lakes Area Lake 382, which received experimental additions of cadmium, were exposed to cadmium concentrations up to approximately 200 ng . L(-1) in water and to about 4 mu g . g(-1) in dry sediment, resulting in body burdens of about 3.5 mu g .(g dry weight)(-1). Mentum deformities in Chironomus spp. larvae in Lake 382 and five reference lakes ranged from 0 to 16%, which was within the range reported for other contaminated lakes (2 to 83%). The frequency of deformities in Lake 382 was not detectably different either in larvae collected between 1989 and 1992, or in those from reference lakes. Incidence of deformities was higher in four of the five reference lakes than in Lake 382. The constancy of the deformities (confined mainly to the median tooth, commonly with cleft or forked median teeth) is consistent with an inherited genetic effect; therefore, we consider the deformities to be natural abnormalities. We conclude that the low cadmium levels added to Lake 382 did not induce mentum deformities and that background levels of deformities are high (up to 16%) in Experimental Lakes Area lakes. The common occurrence of natural abnormalities must be considered when using chironomid mouthpart deformities as an index of environmental degradation.