Organisms born into the same contaminated environment as their parents can be exposed both maternally and environmentally to contaminants, potentially placing them at greater risk of adverse effects than when exposed via either of the two pathways independently. We examined whether embryonic exposure to maternally derived mercury (Hg) interacts with dietary exposure to negatively influence larval development in American toads ( Bufo americanus ). We collected eggs from breeding pairs at reference and Hg-contaminated sites and monitored performance, development, and survival of larvae fed three experimental Hg diets (total Hg, 0.01, 2.5, and 10 μg/g). The negative sublethal effects of maternal and/or dietary Hg manifested differently, but maternal Hg exposure had a greater overall influence on offspring health than dietary exposure. However, the combination of sublethal effects of the two exposure routes interacted with lethal consequences; larvae exposed to maternal Hg and high dietary Hg experienced 50% greater mortality compared to larvae from reference mothers fed the control diet. This study is the first to demonstrate that the latent effects of maternally transferred contaminants may be exacerbated by further exposure later in ontogeny, findings that may have important implications for both wildlife and human health.

Regulatory jurisdictions worldwide are increasingly incorporating bioavailability-based toxicity models into development of protective values (PVALs) for freshwater and saltwater aquatic life (e.g., water quality criteria, standards, and/or guidelines) for metals. Use of such models for regulatory purposes should be contingent on their ability to meet performance criteria as specified through a model-validation process. Model validation generally involves an assessment of a model's appropriateness, relevance, and accuracy. We review existing guidance for validation of bioavailability-based toxicity models, recommend questions that should be addressed in model-validation studies, discuss model study type and design considerations, present several new ways to evaluate model performance in validation studies, and suggest a framework for use of model validation in PVAL development. We conclude that model validation should be rigorous but flexible enough to fit the user's purpose. Although a model can never be fully validated to a level of zero uncertainty, it can be sufficiently validated to fit a specific purpose. Therefore, support (or lack of support) for a model should be presented in such a way that users can choose their own level of acceptability. We recommend that models be validated using experimental designs and endpoints consistent with the data sets that were used to parameterize and calibrate the model and validated across a broad range of geographically and ecologically relevant water types.

Amphibian population declines have been documented worldwide and environmental contaminants are believed to contribute to some declines. Maternal transfer of bioaccumulated contaminants to offspring may be an important and overlooked mechanism of impaired reproductive success that affects amphibian populations. Mercury (Hg) is of particular concern due to its ubiquity in the environment, known toxicity to other wildlife, and complex relationships with other elements, such as selenium (Se). The objectives of the present study were to describe the relationships between total Hg (THg), methlymercury (MMHg), and Se in three amphibian species (Plethodon cinereus, Eurycea bislineata cirrigera, and Bufo americanus) along a Hg-polluted river and floodplain, and to determine if B. americanus maternally transfers Hg and Se to its eggs in a tissue residue-dependent manner. Total Hg and MMHg concentrations in all species spanned two orders of magnitude between the reference and contaminated areas, while Se concentrations were generally low in all species at both sites. Strong positive relationships between THg and MMHg in tissues of all species were observed throughout. Both Hg and Se were maternally transferred from females to eggs in B. americanus, but the percentage of the females' Hg body burden transferred to eggs was low compared with Se. In addition, Hg concentrations appeared to positively influence the amount of Se transferred from female to eggs. The present study is the first to confirm a correlation between Hg concentrations in female carcass and eggs in amphibians and among the first to describe co-transference of Se and Hg in an anamniotic vertebrate. The results suggest future work is needed to determine whether maternal transfer of Hg has transgenerational implications for amphibian progeny.

Abstract. Concern that environmental contaminants contribute to global amphibian population declines has prompted extensive experimental investigation, but individual-level experimental results have seldom been translated to population-level processes. We used our research on the effects of mercury (Hg) on American toads (Bufo americanus) as a model for bridging the gap between individual-level contaminant effects and amphibian population viability. We synthesized the results of previous field and laboratory studies examining effects of Hg throughout the life cycle of B. americanus and constructed a comprehensive demographic population model to evaluate the consequences of Hg exposure on population dynamics. Our model explicitly considered density-dependent larval survival, which is known to be an important driver of amphibian population dynamics, and incorporated two important factors that have seldom been considered in previous amphibian modeling studies: environmental stochasticity and sublethal effects. We demonstrated that decreases in embryonic survival and sublethal effects (e.g., reduced body size) that delay maturation have minor effects on population dynamics, whereas contaminant effects that reduce late-larval or post-metamorphic survival have important population-level consequences. We found that excessive Hg exposure through maternal transfer or larval diet, alone, had minor effects on B. americanus populations. Simultaneous maternal and dietary exposure resulted in reduced population size and a dramatic increase in extinction probability, but explicit prediction of population-level effects was dependent on the strength of larval density dependence. Our results suggest that environmental contaminants can influence amphibian population viability, but that highly integrative approaches are needed to translate individual-level effects to populations.

Mercury (Hg) is an important environmental contaminant due to its global distribution, tendency to bioaccumulate, and toxicity in wildlife. However, Hg has received little attention in amphibians compared to other vertebrates. Amphibians vary widely in life history strategies and feeding ecologies, which could influence Hg exposure and accumulation. To determine whether species and life stage affects Hg bioaccumulation, adults from three species (Plethodon cinereus, Eurycea bislineata, and Bufo americanus) and larvae from the latter two species were collected along a contamination gradient on the South River (VA, USA). Total Hg (THg) concentrations in the contaminated site were 3.5 to 22 times higher than in the reference site. Differences were found in THg concentrations in amphibians that were consistent with their habitat requirements and feeding preferences. In general, adults (3,453 +/- 196 ng/g, dry mass) and larvae (2,479 +/- 171 ng/g) of the most river-associated species, E. bislineata, had the highest THg concentrations, followed by B. americanus tadpoles (2,132 +/- 602 ng/g), whereas adults of the more terrestrial B. americanus (598 +/- 117 ng/g) and P. cinereus (583 +/- 178 ng/g) had the lowest concentrations. In addition, nondestructive sampling techniques were developed. For the salamander species, THg concentrations in tail tissue were strongly correlated (r>or= 0.97) with the remaining carcass. A strong positive correlation (r = 0.92) also existed between blood and whole-body THg concentrations in B. americanus. These results suggest that amphibians and their terrestrial predators may be at risk of Hg exposure in this system and that nondestructive methods may be a viable sampling alternative that reduces impacts to local populations.