An Evaluation of Benthic Macroinvertebrate Community Structure and Sediment Toxicity in Lake Taylorville

Sediments act as a "sink" for xenobiotics which can be accumulated by a variety of organisms and thereby enter the food chain contaminating fish, wildlife and humans. Although the chemical nature of these substances can be known by direct chemical analysis of the sediments, the significance of the contaminants and their bioavailability should be determined through bioassay techniques. A survey of the benthic macroinvertebrates of Lake Taylorville, a reservoir in central Illinois which is fed by two rivers that drain a primarily agricultural area, revealed a rather depauperate fauna. Subsequent studies were conducted in order to identify the underlying cause of the relatively low density and poor species diversity which were observed. Acute (survival) and chronic (life cycle) tests were conducted using Daphnia magna as a test organism in liquid phase elutriate. Acute tests were performed on neonates (< 24-hr-old) exposed to the elutriate for 96 hours, chronic tests were determined by 5-d-old D. magna exposed to elutriate for 15 days with change in the elutriate every 24 hours. No significant toxicity was observed in either test indicating that any toxic substances present are not water soluble. Bulk sediment toxicity tests were performed on Chironomid larvae in order to detect the presence of water insoluble substances that could be bound to sediment particles. Chironomid larvae of three size classes were exposed to the sediments for 48 hours. The acute tests however did not reveal any toxicity. A spiked sediment toxicity test using a commercial pesticide (Diazinon) demonstrated that the Chironomids used were sensitive to toxic chemicals. Although the macrobenthic studies showed a high Oligochaete to Chironomid ratio I have no evidence based on the data to indicate that the sediments have any toxic effects on organisms. If there are any toxicants present in the sediments then they are not bioavailable. It is unlikely that xenobiotics are important in structuring the benthic macroinvertebrate community in Lake Taylorville.