Bioaccumulation and effects of metal contaminated soil on Great Plains toads, Bufo cognatus.
Bryer, Pamela Jean
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Smelting and mining sites create local contaminated areas due to atmospheric release and deposition of metals and by increasing metal-enriched soil runoff. One of the proposed mechanisms of amphibian declines is contamination of habitats by chemical pollutants. Due to the number of mining and smelting sites worldwide, there is a risk to numerous amphibian populations from metal contamination. Currently, little is known about the effects of metal contaminated soil on amphibians. While it is generally understood that certain metals are highly toxic at low concentrations to most any organism, the exposure potential between organisms varies greatly. Metal uptake from soil is difficult to predict due to a complex metal-soil-water-toad relationship. Because of this complex relationship the use of models and passive sampling devices are not possible. This study consists of a series of soil exposures to toads Great Plains toads, <i>Bufo cognatus</i>. Each study follows the same basic design to assess metal bioaccumulation in toads housed individually on contaminated soil. The soils used in this study all came from or were created to mimic the soil conditions at the Anaconda Smelter Superfund Site, Deer Lodge County, MT, USA. The site is characterized by elevated levels of arsenic, cadmium, copper, lead, and zinc. Following their exposure, toads were tested through a series of behavioral, physiologic, and biochemical assays to asses the effects of the exposure. Between the studies, soil origin, soil temperature, size/age of the toads, and exposure duration were varied. Over the course of the studies, all metals showed increased bioaccumulation as soil metal concentration or exposure duration increased. Arsenic was not detectable in small, young-of-the-year toads. Cadmium concentrations, numerically, increased the most across each of the studies. Lead concentrations increased in most cases of increased exposure, however, variation in lead uptake in the adult toads studied was large as exposure duration increased. Zinc and copper tissue concentrations, both regulated essential metals, each increased with increasing exposure, however, zinc showed only slight to moderate increases overall. Partitioning within the body was highly variable between metals and between the studies. Overall, for juvenile toads, skin accumulated the bulk of the metals while in adults most metals accumulated in liver and kidney. One study compared the uptake of metals from soil collected at the Anaconda Smelter site to a series of spiked soils meant to mimic the ratio of metals in the Anaconda Smelter soil. Interestingly, we found that bioavailability was different between these soils, but not in the expected pattern. Cadmium and lead were both more bioavailable in the Anaconda Smelter soil than the spiked soil. Metal contaminated soil exposure had clear effects on the health of the toads. Time taken to bury was measured as a potential indicator of detecting contaminated soil avoidance, however, no differences were seen between soils. Although not consistent between each study the following endpoints were found to change in response to increasing soil metal concentration: prey orientating reflex, hop length, righting reflex, body mass, delta-aminolevulinic acid dehydratase (ALAD) activity, urine specific gravity, and mortality. Consistently across studies, organ (liver, kidney, and spleen) morphometrics did not change. It is clear, from this study and the works of others, that metal contaminated soil poses potential harm to amphibian populations. The effect of metal contaminated soil on toads can have two important ecological affects: 1) toads suffer directly from the presence of metal contaminated habitats and die, and 2) toads can accumulate metal concentrations that are then passed on via predation to other organisms in the food web. Protecting toads from metal contaminated soil is a challenge given their ability to bury deeply in the soil and their free ranging nature.