POPULATION-SPECIFIC TOXICITY RESPONSES BY THE FRESH-WATER OLIGOCHAETE, STYLODRILUS-HERINGIANUS, IN NATURAL LAKE-MICHIGAN SEDIMENTS

Sediment reworking rate, mortality and organism dry weight were measured for Stylodrilus heringianus in laboratory microcosms. The experiments were designed to identify potential population‐specific response differences to mixed (stirred to obtain a more uniform particle size distribution over depth) and unmixed (passively settled) microcosm sediments. Lake Michigan sediments and worms were collected offshore Benton Harbor, Michigan and Grand Haven, Michigan. The mixed Benton Harbor sediments were toxic to S. heringianus collected from Grand Haven, whereas there were no significant differences in measured responses between mixed and unmixed sediment microcosms for Grand Haven‐collected worms exposed to Grand Haven sediments or Benton Harbor‐collected worms exposed to Benton Harbor sediments. Note that the mixing of sediments resulted in increased availability of contaminants sorbed to the fine sediment fraction. Because contaminant and oligochaete population density data suggest that Grand Haven sediments are less contaminated, the population‐specific response suggests that S. heringianus may adapt to the low level long‐term stressful conditions (chemical or otherwise). Results also suggest caution and consideration of the history of test organisms in the design and interpretation of toxicity tests. Copyright © 1990 SETAC