Temporal and taxonomic patterns of mouthpart deformities in larval midges (diptera: Chironomidae) in relation to sediment chemistry

Entire benthic communities of larval chironomids were sampled in the spring and fall of 1987 from the trace metals-impacted Empire Lake reservoir system, Cherokee County, Kansas. The frequency and severity of mouthpart deformities were calculated for each sample date at the subfamily level and analyzed for correlation with zinc, cadmium and lead concentrations in sediments of the lake. None of the trace metals was significantly predictive of mouthpart deformation, nor was location within the watershed. However, significant taxonomic and temporal patterns in deformation were observed independent of individual trace metals and sampling location. Larval mouthparts were approximately five times as likely to be deformed and three times as severely deformed in the subfamily Chironominae relative to larvae of the subfamily Tanypodinae (frequency = 14.9% vs. 3.1%; Mouthpart Deformity Score, MDS = 30.2 vs. 9.9) (p < 0.01). Similarly and regardless of taxon, the mouthparts of midge larvae present in the sediments for the greatest duration (i.e., overwintering generations) were more frequently (10.7% vs 2.4%) and severely (MDS = 22.2 vs. 1.8) deformed than summer populations (p < 0.01). These levels of frequency and severity are far above those found in unimpacted habitats elsewhere indicating that while individual metal concentrations were not predictive of either the frequency or severity of deformities, clearly the Empire Lake watershed is impacted by some toxicant. Furthermore, the differential temporal and taxonomic responses suggest that both the frequency and severity of mouthpart deformities at the subfamily level can be successful bioindictor tools and more work needs to be done to elucidate their value.