Acute toxicity of copper to three life stages of Chironomus tentans as affected by water hardness-alkalinity

Abstract

The acute toxicity of copper to first-and fourth-instar larvae of the midge Chironomus tentans was determined in soft, medium and hard water (43, 110 and 172 mg litre −1 as CaCO 3 total hardness, respectively). Percentage hatch of C. tentans eggs as influenced by copper concentration was studied in soft and hard water. Fourth instars were 12–27 times more resistant to copper stress than were first instars. Eggs were much more resistant to copper than either larval stage. Water hardness-alkalinity had a significant effect on copper toxicity to both larval instars. The concentration of copper necessary to immobilise 50% of the test population after 96 h (96-h EC 50) was not significantly different in soft and medium water (16·7 and 36·5 μg Cu litre −1, respectively) for first-instar larvae, but both were significantly less than in hard water (98·2 μg Cu litre −1). Mean 96-h EC 50s for fourth-instar larvae were not significantly different in medium (997·5 μg Cu litre −1) and hard (1184·6 μg Cu litre −1) water, but both were significantly higher than in soft (211·8 μg Cu litre −1) water. The influence of hardness-alkalinity on hatching success was less apparent. Percentage hatch in soft water was significantly reduced only at 5000 μg C litre −1, while in hard water, percentage hatch was not affected even at that copper concentration. If sensitivity to copper is representative of sensitivity to other chemicals, the practice of using fourth-instar larvae in toxicity tests may grossly underestimate the sensitivity of the species to toxicants.