Long-term Effects of Hydrogen Sulfide onHexagenia limbata(Ephemeroptera)1

Nymphs of the mayfly Hexagenia limbata (Serville) were subjected to hydrogen sulfide in test chambers with a mud substrate into which they could burrow. Acute and chronic tests were in flow-through apparatus. At 17.8–18.3°C and O 2 of 4.5–6.6, 96-h LC 50 was 0.165 mg/liter H 2 S and LC 50 at 12 days was 0.060 mg/liter H 2 S. In chronic tests running 138 days with 0.029 mg/liter H 2 S, 37% mortality occurred, and at 0.0762 mg/liter H 2 S, none survived. No subimagos emerged at concentrations of 0.0348 mg/liter H 2 S but below this level 30–70% emerged.