Road Salt Results In Metamorphosis Of Wood Frog Tadpoles, Early Mortality

Every year, millions of tons of road salt are applied to the streets and highways across the United States to help make travel safer. However, what scientists still can't fully explain is how it affects our wildlife.

Researchers at Case Western Reserve University have found that exposure to road salt may increase the size of wood frogs, but also shorten their lives. The study authors discovered that wood frog tadpoles that were exposed to road salt grew larger at metamorphosis when compared to other frogs who did not come in counter with the contaminant. And even more surprisingly, researchers found that mortality rate was higher among juvenile frogs exposed to salt. 

"This study really shows that you need to look across multiple life stages to get a good look at what environmental pollutants are doing," said Kacey Dananay, a Case Western Reserve PhD student who led two experiments in the study, in a news release.

In this recent study, researchers sampled public and private lands, including many sites in Cleveland Metroparks, to determine where amphibians breed naturally in Northeast Ohio. They tested whether the frog's presence, abundance, stage of development and size were associated with road salt contamination at 30 wetlands.

When dipping nets into the water over a certain period of time and collecting up to 30 specimens, researchers then measured for growth and developmental stage by examining the water's conductivity, which provides an estimate of road salt contamination, and overhead forest canopy that is associated with reduced larval growth and development.

From there, the researchers conducted a statistical analysis in which they found wood frog tadpoles were less abundant in wetlands with higher salt concentrations and higher body mass. 

As these results were rather surprising, the researchers decided to test whether salt affects larval wood frog development, adding road salt in different concentrations: none, 100, 500 and 900 milligrams per liter--concentrations they'd found in the survey. Egg masses were collected and, four and five days after hatching, the tadpoles were then placed in the ponds.

The findings revealed that the salt appeared to restrict the growth of zooplankton, which, like tadpoles, feed on pond algae. This likely allowed algae to grow more abundantly, providing more food and resulting in larger tadpoles. 

Then, a second experiment revealed that half the tadpoles were raised in clean water while the other half were put in 500 milligrams of salt per liter. Furthermore, the second group grew larger and at metamorphosis transformed into larger frogs. 

The researchers assigned the juvenile frogs from both groups randomly to pens and placed either in pairs or groups of five in a total of 20 enclosures. Survival rates in the high-concentration groups were the same for juveniles that had been exposed to salt as tadpoles and those that had not. Among juveniles in low-concentration pens, those that had been exposed to salt had a significantly lower survival rate compared to those that had not.

More information regarding the findings can be seen via the journal Environmental Toxicology and Chemistry.

Related Articles

Growing Tadpole Disease Threatens Earth's Dwindling Frog Population

For more great science stories and general news, please visit our sister site, Headlines and Global News (HNGN).