THE EFFECTS OF AMBIENT PH ON NITROGEN-EXCRETION IN EARLY-LIFE STAGES OF THE AMERICAN TOAD (BUFO-AMERICANUS)

Acidification of breeding ponds has been identified as a potential thr eat to the survival and health of North American amphibian populations . The effects of acid exposure on ion and acid-balance are well known, but there is little information on how environmental water pH influen ces nitrogen balance in amphibians. The aim of this study was to deter mine the effects of moderately acidic water (pH 6.0) on nitrogen excre tion in early life stages of the toad, Bufo americanus. Acid exposure (pH 6.0, 54 h) resulted in a 20-80% increase in ammonia-N excretion ra tes in embryos and early, middle and late tadpoles stages, whereas the re was no significant effect on urea-N excretion. Tissue ammonia conce ntrations were significantly higher (+ 33%) in the embryos and 35-65% lower in the three groups of tadpoles exposed to water of pH 6.0 compa red to control animals (pH 8.5). In embryos, ammonia excretion account ed for greater than 90% of total nitrogen excretion (ammonia-N + urea- N), but by the late tadpole stage this value had decreased to approxim ately 65%. These findings indicate that exposure of embryonic and larv al B. americanus to moderately acidic water disrupts nitrogen balance by increasing nitrogen loss as ammonia, with no compensatory decrease in urea excretion.