Every year in the United States, road crews dispense more than 24 million metric tons of de-icing salts. In the snowy Northeast, where plows and salt trucks ply the roads for months on end, sodium chloride can accumulate in ponds and other water bodies. Because of lower stream flows and evaporation, some of the greatest salt concentrations occur in late spring and summer. Highly salinized water can harm or kill tadpoles and other larval amphibians.
Previous studies on wood frog tadpole responses to salinized water have had contradictory results, a discrepancy that suggests that some wood frog populations may have adapted to higher salt concentrations. A team from Rensselaer Polytechnic Institute in Troy, New York, investigated the possibility of this adaptation by comparing the salt tolerances of wood frog tadpoles from different populations. They also conducted experiments to determine whether greater salt tolerance came at a cost in terms of growth (increase in mass), development (movement through tadpole development stages), or activity level (frequency of movement).
As reported in the March 2024 issue of Ecology and Evolution, the researchers collected partial masses of newly laid wood frog eggs from nine ponds in upstate New York, all within 40 meters (44 yards) of roads but differing significantly in salinity at the time of egg collection. The least salinized of these ponds contained only 1 milligram of chloride per liter, and the most salinized pond (coded in the study as “APHS”) had a salt concentration of 774 milligrams of chloride per liter – approximately 100 times higher than in a typical pristine pond.
The researchers placed the egg masses from each pond in separate wading pools and raised the emerging tadpoles in uniform, carefully controlled conditions. They conducted tests on tadpoles from each pond.
They discovered that the APHS tadpoles were significantly more tolerant of high salt concentrations than tadpoles from the other pond populations. When exposed to lethal concentrations of 8 grams per liter of sodium chloride, the APHS tadpoles survived twice as long as did the other tadpoles.
The researchers did not, however, find evidence of a cost associated with this tolerance. Tadpoles from all nine ponds showed little difference in growth or development, and all demonstrated the same level of reduction in activity when raised in water that contained unnaturally high but sublethal amounts of chloride.
The experiment results suggested that wood frog populations are capable of evolving to have greater salt tolerance, although the researchers emphasized the need for more data to confirm this finding. “Focusing [future research] on highly impacted ponds will not only help determine whether our observation is part of a broader pattern, but also provide insight into the mechanism that might drive higher tolerance to chloride among wood frog populations,” they noted.