Hatching Success, Growth and Development of Larval Wood Frogs (Lithobates sylvaticus) Exposed to Naphthenic Acids in Outdoor Microcosms
Abstract
Wood frogs (Lithobates sylvaticus) inhabiting the Athabasca oil sands region in northern Alberta, Canada, are at risk of exposure to contaminants from oil sands mining. The extraction of bitumen from oil sands produces large volumes of waste water whose main toxic fraction contains naphthenic acids (NAs). This study seeks to determine if exposure to NAs from oil sands waste waters affects the hatching success, growth and development of early life stages of the wood frog using a unique field-lab hybrid design. Wild-collected embryos were first exposed to a range of NA concentrations (1.0 to 43.0 mg/L) in natural pond water microcosms (20 L) situated outdoors at the Queen’s University Biological Station (Ontario, Canada). After an 18-day exposure to NAs, hatchlings (Gosner Stage 22) were transitioned to clean water and then monitored over 5 weeks in mesh cages in their wetland of origin for potential latent effects on larval growth and development. We observed that hatching success (range: 3.5-40%) and time to first hatch (range: 9-16 days) were not influenced by exposure to NAs, but that hatchling growth was negatively affected; on day 18 of the experiment, hatchlings exposed to NAs were smaller than controls and the dose-response relationship between NA concentration and total length of hatchlings was statistically significant (p<0.01). Interestingly, the opposite relationship was observed at the end of the post-exposure monitoring phase, when total length was greater in metamorphs exposed to higher concentrations of NA as embryos (p<0.01). No effects of NA exposure on development (as measured by Gosner stage on days 68-71) were observed during the exposure or the post-exposure phases of this experiment. Our findings suggest that embryonic exposure to NAs under semi-natural conditions can affect the growth of early life stages of the wood frog, but they have the capacity to recover under clean-water conditions . This study showed less severe impacts of NA exposure to early life-stage frogs than laboratory-based studies, illustrating the value of more realistic experimental conditions for toxicity testing of environmental contaminants.