The carry-over effects of road salt on wood frogs (Rana sylvatica): Investigating aquatic-terrestrial linkages associated with environmental pollutants

Background/Question/Methods

The effects of pollutants are often investigated in short-term laboratory experiments that potentially miss effects mediated through the food web or persisting over multiple life-stages. Furthermore, pollutants may alter how organisms interact with their environment. We tested whether road salt pollution has such diverse effects on wood frogs (Rana sylvatica). Road salt is a common de-icing agent applied to roads in winter. After snow melt, salt is washed off roads where it may impact the environment. Laboratory studies demonstrated high concentrations of road salt negatively affected amphibian eggs and larvae. However, focusing on larvae in laboratory conditions may miss indirect effects of salt on larvae or carry-over effects of larval salt exposure. To investigate these issues, we conducted two experiments on wood frogs using outdoor mesocosms and terrestrial pens. First, we raised larvae under four salt concentrations and measured larval survival, growth, development and resource availability. Second, we raised larvae to metamorphosis in two salt concentrations. After metamorphosis, juvenile frogs were placed in terrestrial pens at high (5 frogs/m²) and low (2 frogs/m²) densities. After 18 weeks, we measured juvenile survival, growth and leaf decomposition rates to test for carry-over effects of salt on the terrestrial environment. 

Results/Conclusions

In the salt concentration experiment, salt exposure did not affect larval survival or development but increased larval growth. This counterintuitive outcome may have been due to an indirect positive effect. Increased road salt decreased zooplankton abundance but increased algal growth. Increased algal growth in road salt polluted mesocosms suggests larvae are indirectly benefiting from increased food resources; however, algal growth may be a result of reduced zooplankton abundance rather than a direct effect of salt exposure. If we ended our experiment at the larval stage, we would have concluded salt was beneficial to wood frogs based on increased mass and food resources. In our carry-over experiment, exposure to road salt as larvae increased mortality among terrestrial, juvenile frogs in high-density terrestrial environments. Road salt did not have an effect on juvenile growth or leaf decomposition. High densities of juvenile frogs, however, lowered juvenile frog growth and leaf decomposition rates. Expanding our experiment to the post-metamorphic life-stage revealed a carry-over effect of road salt that reduced juvenile frog survival. Our results suggest that the effects of environmental pollutants may be underestimated when focusing only on one life stage or conducting experiments solely in laboratory settings.