EFFECTS OF LOW PH AND ALUMINUM ON 2 DECLINING SPECIES OF AMPHIBIANS IN THE SIERRA-NEVADA, CALIFORNIA

In the Sierra Nevada of California at high elevation, two species of amphibians, Rana muscosa and Bufo canorus, have dramatically declined in numbers during the past two decades. The surface waters in which these species spawn typically are- extremely low in acid-neutralizing capacity, and thus may be vulnerable to changes in water chemistry due to acidic deposition. Such changes have been hypothesized to be a cause of the recent population declines. The present study assesses the sensitivity of embryos and hatchling tadpoles of these species to low pH and an elevated level of dissolved aluminum. Embryos and hatchlings were kept for 7 d in reconstituted soft water (RSW) at pH 4.0 to 6.0 (inorganic monomeric aluminum effectively 0 or 39-80 mug/L), and subsequently for a post-treatment period of 4 to 14 d in RSW at pH 6.0 (no aluminum). LC50 pH values for post-treatment survival of embryos and tadpoles of R. muscosa averaged 4.4 and <4.0, respectively, and 4.7 and 4.3, respectively, for B. canorus. The estimated extreme pH for Sierra Nevada surface waters (5.0) did not cause a significant reduction in survival for either life stage or species. However, sublethal effects at pH greater-than-or-equal-to 5.0 were evident as reduced body size in R. muscosa embryos at pH 5.0 and 5.25, and earlier hatching of B. canorus embryos at pH 5.0. The concentrations of aluminum used were greater than any recorded levels in the field, although the field concentrations that may occur during snowmelt are currently unknown. Experimental exposure to aluminum did not affect survival of embryos or tadpoles of either species. However, sublethal effects were evident as reduced body size of B. canorus tadpoles and earlier hatching in B. canorus embryos.