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Global decline of amphibians has
been a hot issue in recent years among both the scientists who study them and
the general public. A paper by University of Georgia researchers out in the
December issue of Environmental Toxicology and Chemistry suggests that atmospheric
deposition of mercury in aquatic habitats has the potential to have significant
impacts on amphibians in the larval stage of development. Mercury may cause
increased incidence of death and malformations and affect the regulation of
growth, development, and timing of metamorphosis.
Previous laboratory studies looked at unrealistically high concentrations
of mercury in water, but Jason Unrine and others from UGA’s Savannah
River Ecology Laboratory (SREL) studied environmentally realistic exposures
in the diet. This was studied under controlled conditions to simulate contamination
of the diet by atmospheric deposition of mercury. Human activities, such as
burning of fossil fuels and waste incineration, have approximately doubled
the amount of mercury in the atmosphere over the last 150 years. Recent debate
surrounding mercury contamination has focused on potential effects on the unborn
fetus in pregnant women who consume contaminated fish, but much less attention
has been focused on effects in wildlife.
In this first study to demonstrate effects of environmentally realistic dietary
mercury in amphibian larvae, the purpose of the study was to determine if dietary
mercury concentrations relevant to aquatic habitats contaminated by atmospheric
deposition have the potential to cause adverse effects in amphibian larvae.
The scientists thought they would see some sublethal effects, such as a decrease
in growth rate and smaller size at the time of metamorphosis, not significantly
increased rates of death. Their subject was the southern leopard frog.
The results of the experiment included increased rates of mortality, malformation,
growth and development and premature metamorphosis as a result of increased
dietary mercury exposure. These results suggested that mercury concentrations
in the diet of amphibians in habitats contaminated by atmospheric deposition
may be sufficient to disrupt normal growth and development of amphibians. This
could decrease the number of normally developed individuals surviving to reproductive
age and may lead to declines in populations.
“Dietary exposure in sites receiving mercury only through atmospheric
deposition may be sufficient to cause adverse effects on amphibian development
and decrease survival through metamorphosis,” said Unrine.
In the control group 10 to 15 percent of the animals died or were affected
with mild deformities. In the medium and high concentrations of mercury contamination,
50 to 60 percent of the animals died or were severely malformed.
“That malformations, decreased survival and changes in normal growth
and developmental rates occurred at dietary mercury concentrations relevant
to habitats contaminated solely by atmospheric mercury suggest that mercury
pollution from atmospheric deposition has the potential to adversely impact
amphibian populations,” explained Unrine.
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