The sensitivity of bluegill (Lepomis macrochirus) to low pH in soft (12 or 18 mg/L CaCO₃) and hard (165 or 197 mg/L CaCO₃) water was compared in five day laboratory toxiclty tests. Embryo-larval bluegill were exposed to pH levels ranging from 3.8 to 7.0 in soft water and from 3.8 to 8.0 in hard water. An on-site toxicity test, using lake water (3.4 mg/L CaCO₃) adjusted to pH levels ranging from 3.5 to 7.3, was conducted to compare laboratory and field results. At low pH, hatching was reduced, the hatching period prolonged, and the incidence of partial hatching increased. Increased water hardness mitigated acid toxiclty, enhanced larval survival, and promoted hatchability. Hatching rates were decreased over those of the controls by 76 percent in soft water and by 23 percent in hard water at pH 4.0, and hatching was negligible at pH 3.8. The length of the hatching period was prolonged by 24 to 48 hours at pH levels ≤4.6. Partial hatching averaged 43 percent in pH 4.4. Increasing acidity resulted in increased embryo-Iarval mortality, averaging >62 percent at pH levels ≤4.6; mortality was 100 percent at pH levels <4.4 in soft water and 3.8 in hard water. Bluegill larvae were more sensitive than eggs. The embryo-larval LC50's were pH 4.67 in soft water and pH 4.06 in hard water; LC1's were pH 5.66 in soft water and 5.04 in hard water. In the on-site field experiment, approximately 50 percent mortality occurred at pH 4.6 which was nearly identical to the LC5O of pH 4.67 in laboratory (soft water).

Growth of larvae was not affected by low pH. As acid levels increased, yolk-sac volumes decreased, physical deformities (fin, eye, and spinal abnormalities) increased and behavioral abnormalities were evident. Yolk-sac volume was decreased at pH levels <5.5 in soft water and <5.1 in hard water, perhaps reflecting higher energy expenditures by larvae held at sublethal pH levels. Spinal curvature and fin erosion of larvae were apparent at pH levels <5.25 in soft water. Larvae were lethargic and swimming ability was impaired at pH levels ≤5.5 in soft water and ≤5.1 in hard water. Exposure of embryo-larval bluegill to low pH levels (<5.7) in soft water may compromise their ability to survive, forage efficiently, and escape predation under natural conditions.