Effects of low pH and aluminum on amphibians at high elevation in the Sierra Nevada, California

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At high elevation in the Sierra Nevada of California, surface waters are extremely low in acid-neutralizing capacity and thus may be vulnerable to changes in water chemistry due to acid deposition. The present study assesses the sensitivity of embryos and hatchling larvae of two Sierran amphibians, Pseudacris regilla (Pacific chorus frog) and Ambystoma macrodactylum (long-toed salamander), to low pH and an elevated level of dissolved aluminum. The populations of these two species are not known to be declining at present. These findings are compared with results for two other Sierran amphibians, Rana muscosa (mountain yellow-legged frog) and Bufo canorus (Yosemite toad), both of which reportedly have declined substantially in numbers in recent years. Embryos and hatchlings of P. regilla and A. macrodactylum were kept for 7 d in reconstituted soft water at pH 4.0–6.0 (inorganic monomeric aluminum effectively 0 or 39–80 μg/L at pH ≥ 4.9), and subsequently for a post-treatment period of up to 16 d in reconstituted soft water at pH 6.0 (no aluminum). LC50 pH values for post-treatment survival averaged 4.3 for embryos and tadpoles of both species. The estimated extreme pH for Sierra Nevada surface water, 5.0, did not cause a significant reduction in survival for either life stage of either species, and sublethal effects on body size and hatching time were not evident at pH ≥ 5.0. Aluminum also did not affect survival of either species, although sublethal effects were evident as reduced body size of A. macrodactylum larvae and earlier hatching in P. regilla. Pseudacris regilla and A. macrodactylum were not consistently more tolerant of low pH than R. muscosa and B. canorus. However, the latter two species show sublethal effects of low pH at pH ≥ 5.0, whereas the former two do not.