TOXICITY OF AMMONIA TO EARLY LIFE STAGES OF THE SMALLMOUTH BASS AT FOUR pH VALUES

Abstract

Early life stages of smallmouth bass (Micropterus dolomieui Lacepede) were exposed to ammonia concentrations at four different pH values (from 6.5 to 8.7). Acute lethal tests were conducted with juveniles, and partial chronic tests were initiated at the late embryo stage and extended through 32 d of exposure. The 96-h LC50 ammonia concentrations at pH 6.53, 7.16, 7.74 and 8.71 were 371, 117, 39.5 and 7.43 mg/L total ammonia-N (NH3-N) and 0.694, 1.00, 1.20 and 1.78 mg/L un-ionized NH3, respectively. The acute toxicity of ammonia in terms of NH3-N was directly related to test pH, and in terms of NH3 was inversely related to pH. Embryo through preswim-up larvae were the most resistant early life stages studied during chronic exposures. Survival of embryos to larvae 7 d post hatch was unaffected by ammonia concentrations ranging from 0.236 to 0.865 mg/L NH3 at pH values of 6.60 to 8.68. Adverse effects on early developmental stages demonstrated that survival to 14 d and growth to 32 d were among the most sensitive indicators of ammonia toxicity. The estimated 32-d no-observed-effect concentrations, defined as the geometric means of the maximum no-effect and minimum effect concentrations determined from statistically significant decreases in growth (wet weight), were calculated to be 17.4, 14.4, 14.6 and 2.40 mg/L total NH3-N and 0.0437, 0.148, 0.599 and 0.612 mg/L un-ionized NH3 at pH values of 6.60, 7.25, 7.83 and 8.68, respectively. The acute to chronic ratios, as calculated from NH3 concentrations and acute results adjusted to the above pH values for chronic tests, were 16.7, 7.03, 2.07 and 2.89, respectively.