Researchers in Oregon have discovered that a level of nitrogen-based
compounds which the U.S. Environmental Protection Agency (EPA) says is
safe for human drinking water - a level often found in agricultural areas
as a result of using crop fertilizers - is enough to kill some species of
amphibians. A study at Oregon State University (OSU) has shown that
several frog, toad and other amphibian species, especially at their more
vulnerable larval stages, can be highly susceptible to fairly low levels
of nitrate and nitrite exposure.
When exposed to moderate amounts of nitrates and nitrites, some
tadpoles and young frogs reduced their feeding activity, swam less
vigorously, experienced disequilibrium, developed physical abnormalities,
suffered paralysis and eventually died. In control tanks with normal
water, none died.
"I think this is clearly a significant problem," said Andrew
Blaustein, a professor of zoology at OSU and expert on global amphibian
declines. "Right here in the Pacific Northwest we're having localized
extinctions of some amphibians and widespread declines in others. We now
have clear evidence that nitrate and nitrite exposure at levels considered
safe for humans or fish is enough to kill amphibians."
In their study, the OSU scientists worked with five species of
amphibians, including the Oregon spotted frog, red-legged frog, western
toad, Pacific treefrog and northwestern salamander. In the past 40 years
the Oregon spotted frog has largely disappeared from most of its known
historical range, an area of lowlands with intensive agricultural use.
The scientists tested the sensitivity of the amphibians to
environmental levels of nitrates and nitrites. The Oregon spotted frog was
the most sensitive - three to four times more vulnerable to nitrates and
nitrites than red-legged frogs and Pacific treefrogs. Not by coincidence,
the scientists believe, the more-sensitive spotted frog is the species
that has almost totally disappeared from these areas.
Levels of nitrite considered safe for human drinking water killed over
half of the Oregon spotted frog tadpoles after 15 days of exposure. All
five species showed a similar level of mortality at levels of nitrites
that were higher, but still well below those that the EPA considers safe
for warm water fishes.
The study results indicate that water quality criteria set up by the
EPA does not guarantee the survival of some protected and endangered
amphibians. According to Blaustein, health effects such as those caused by
nitrates and nitrites may also work in concert with other environmental
insults, such as acid rain or UV-B exposure, to compound problems.
"Many people are looking for the one single thing that is causing
all these amphibian declines, but in reality it's almost certainly a
combination of causes," Blaustein said. "It's clear there can be
a synergistic effect that causes higher mortality when you have different
problems all occurring at once."
For instance, Blaustein said the furor that has arisen over frog
deformities such as extra legs has been linked to a parasite known as a
fluke. "But it's probably not that simple," he said. "These
flukes have been around forever and we never observed the level of problem
we're now seeing with deformed frogs. It's quite possible this fertilizer
issue relates to that, along with killing tadpoles directly."
The flukes that can cause amphibian deformities live part of their life
cycle in a snail, Blaustein said. Snails eat algae. And higher levels of
nitrogen-based fertilizers can cause increased algal growth, increasing
the snail populations.
"At one pond, we found 67 percent of the frogs had multiple
legs," Blaustein said. "And this was in a wildlife management
area, which was not intensively farmed but was only surrounded by
agricultural lands." Measurements of water there showed highly
elevated levels of nitrate - up to 11 milligrams per liter - which is just
above the EPA legal level for drinking water.
"As we look for the cause of declining amphibians, we're going to
find a lot of these types of interactions," Blaustein said. "But
the fact remains that nitrogen fertilizers by themselves, used at levels
considered safe in drinking water, are enough to kill some amphibians. So
clearly that's part of the answer and a fairly serious concern in its own
right. And it's pretty good evidence that we need to think again about the
levels of these nitrate compounds that we say is safe."