The Effects of Duration of Exposure to Acidic Conditions on Survival of Bufo americanus Embryos

The potentially lethal effect of acid deposition on larval amphibians has stimulated increasing interest in acid tolerance and toxicity (Freda and Dunson, 1985; Pierce and Sikand, 1985; Pierce and Harvey, 1987; Pierce and Montgomery, 1989). Amphibian tolerance to low pH can be influenced by both environmental and genetic factors (Pierce, 1985), and varies both among and within species (Gosner and Black, 1957; Freda and Dunson, 1985). Although many species can tolerate chronic exposure to pH 4.0, some species appear to be much more sensitive to pH depressions (Pierce, 1985). Small, ephemeral ponds provide critical breeding habitats for many temperate zone amphibians (Pough, 1976; Pierce et al., 1984). Dunson and Connell (1982) believed that temporary pond breeders may be particularly susceptible to acid deposition because these habitats are created during storm events. Small ponds may only experience pH depressions for a brief period of time following acid precipitation events. These depressions may vary from several hours to several weeks (Clark and Hall, 1985) depending on the buffering capacity of the system. Gosner and Black (1957) postulated that the adverse effects of low pH on amphibians may require continuous or prolonged exposure. Clark and Hall (1985), however, documented a decrease in Bufo americanus and Rana sylvatica embryo survival after 1 d of exposure to pH 4.3. In this study we examine the effects of 1, 2, and 3 d exposures of acidic solutions on the hatching success of B. americanus embryos. To determine how duration of exposure to low pH affects larval survival, we conducted a 3 x 6 factorial experiment using a randomized block design. The two factors were exposure time (1, 2, or 3 d) and pH (3.0-5.5). Experimental solutions of pH 3.0, 3.5, 4.0, 4.5, 5.0, and 5.5 were made by diluting nitric acid with aged tapwater. Bufo americanus eggs were incubated in plastic containers (10 cm in diameter and 6 cm deep) that held 160 ml of acidic water. The containers were randomly assigned positions on a laboratory bench in 10 blocks. Each block contained three replications of each of the six experimental solutions (total number of replicates = 30). Blocking was used to compensate for a slight temperature gradient across the bench which could possibly affect pH-embryo interactions. Because non-buffered acidic solutions