Abstract:
Acute (10 day) semi-static toxicity tests in which the midge, Chironomus tepperi, were exposed to atrazine and molinate
were conducted in laboratory water and in river water, in the absence and presence of sediment. The bioavailability
measured as median lethal concentrations (LC50) and 95% fiducial limits (FLs) of atrazine to C. tepperi in laboratory
water in the absence and presence of sediment were 16.6 (14.3–19.4) and 21.0 (18.2–24.1)mgl-1, respectively while the
corresponding values in river water were 16.7 (14.7–19.0) and 22.7 (20.3–25.4)mgl-1, respectively. For molinate, the
LC50 and FL values in laboratory water in the absence and presence of sediment were 8.8 (6.8–11.4) and 14.3 (12.4–
16.4)mgl-1, respectively and the corresponding values in river water were 9.3 (7.6–11.3) and 14.5 (12.4–16.9)mgl-1,
respectively. Atrazine has low toxicity (LC50 > 10mgl-1) while molinate has moderate toxicity (1mgl-1 <
LC50 < 10mgl-1) to C. tepperi. River water did not significantly (P > 0.05) reduce the bioavailability of either chemical
to C. tepperi. However, the presence of sediment did significantly (P < 0.05) reduce the bioavailability of both atrazine
and molinate to C. tepperi.
