This is an article I wrote that published in the Charlotte Observer and Raleigh News & Observer’s Sci-Tech pages on Nov. 14, 2011.
A sure sign that winter has arrived is when drivers spot chunks of road salt in their lanes. It’s safe to say drivers appreciate ice-free roads, but … ever wonder where all that salt ends up?
In North Carolina, the Department of Transportation spreads, on average, 256,249,901 pounds of salt on state-managed roads each year.
UNC Asheville biologist James Petranka decided to investigate what this seasonal onslaught means for our native amphibians. Because amphibians breathe through their skin and are highly susceptible to environmental contaminants, Petranka wondered if flushes of road salts to their breeding ponds kill them.
The salt, he learned, didn’t kill the amphibians outright, though it does harm their growth as juveniles. Perhaps more alarming, he found the road salt is causing problems in the food web.
The effect of road salts on lakes and streams is documented, but it’s understudied in pools that form seasonally, and seasonal pools are where amphibians prefer to breed in late winter and early spring. After reading a scientific report on road salt effects upon wood frogs and spotted salamanders in the Adirondack Mountains of upstate New York, Petranka couldn’t get his mind off what might be unfolding in the mountainous woods of Western North Carolina that surround his office.
The southeastern U.S. lays claim to harboring the greatest diversity of salamanders in the world. Of the 102 salamander species known in the Southeast, 60 are found in North Carolina. Most of this diversity occurs in the western part of the state, where mountain folds create a mosaic of geographically-restricted amphibian populations. Some species look identical, but due to isolation are highly distinct genetically. Some salamanders may never travel more than 200 to 300 feet from where they hatch, Petranka says.
As an amphibian guru, he should know. He authored “Salamanders of the United States and Canada,” considered a major reference source. He was also the first researcher who, in the late ’80s, established that amphibians use chemical cues in their environment to decide where to lay their eggs. They avoid laying eggs where they sense egg- and larvae-predators, like fish.
To test the effect of road salts, Petranka set up a series of low-tech experiments on UNC Asheville’s campus. You know those blue kiddie wading pools you see on your neighbor’s lawn? Turns out they are perfect for simulating natural seasonal pools: just the right diameter and depth.
Petranka placed nearly 60 plastic wading pools outdoors and filled them with aged tap water. Next he added algae and protozoans collected with fine nets from natural seasonal pools, plus small crustaceans, like water fleas and copepods. He used leaf litter to mimic the most common type of natural pool bottom. Then he added wood frog and spotted salamander larvae, collected from the wild. Finally, he introduced a road salt mix at levels found in polluted wild pools.
“We couldn’t do the experiments in a natural setting because there simply aren’t enough seasonal pools left locally,” Petranka said. North Carolina is estimated to have lost 90 percent of its wetlands, including seasonal pools, since European contact, mainly due to ditching, draining and damming rivers.
Salt, salamanders, skeeters
Petranka discovered amphibians didn’t fare well in the salted pools: he found that even low levels of road salts reduced spotted salamander larvae growth because the salt decimated the water fleas and other invertebrates that they eat.
Because seasonal pools eventually dry, reduced larvae growth can prevent or delay metamorphosis, putting them at risk for higher mortality rates. And those stunted larvae that do survive may reach sexual maturity up to a year later than normal, Petranka says.
He also learned that mosquitoes fared the experimental salt treatment much better – that salted pools can trigger mosquito plagues.
In a natural setting, mosquitoes avoid laying their eggs in ponds where there are amphibian larvae, because the amphibian larvae gobble up the mosquito larvae. But when road salts were applied, mosquitoes were more likely to blossom, as were shore flies and midges. Those insects colonized Petranka’s wading pools naturally. After experimenting with different species compositions, Petranka says he’s getting a good signal that mosquitoes strongly avoid laying their eggs in pools containing a full array of native species.
“The mosquito larvae have this thick outer cuticle, and they sit just below the water’s surface, with a breathing tube sticking up,” Petranka said. “It’s almost like they are custom-built to withstand a saline environment.”
One of the two species of mosquito Petranka documented is a known vector for West Nile virus. “What this means is that in areas where salty runoff reaches seasonal pools, we may be unintentionally selecting for mosquitoes and other salt-tolerant insects,” Petranka says.
Nancy Karraker, lead author to the Adirondacks study that inspired Petranka, says his work is the first to examine the effect of de-icing agents on food webs, not just single species. Karraker, a professor of environmental science at the University of Hong Kong, also expressed hope that because Petranka had found a risk of human disease linked to salt-contaminated wetlands, people and policy makers would be more willing to rectify the problem.
“I think most conservation biologists would agree that it is very important to find a balance between public safety and protecting the environment, when it comes to road de-icing salts,” Karraker says, noting that alternative compounds derived from sugar beets are being tested. “The problem with alternative compounds is always price. Rock salt costs about $50 per ton, and other substances including calcium chloride or sugar beet products cost hundreds of dollars per ton. That is why we continue to rely principally on rock salt when better alternatives exist.”