In a hopeful sign, mercury levels decline in Everglades wading birds

December 14, 2000

GAINESVILLE, Fla. --- In a rare piece of good news about mercury contamination in the Everglades, a University of Florida researcher has found that levels of the pollutant in wading birds have dropped significantly since 1994.

Scientists and state officials charged with reducing mercury pollution aren't sure of the cause, but the declines may reflect the removal of mercury from commercial products and industrial processes, a trend that began in the late 1980s, they say. The declines also come at a time when efforts by the Florida Department of Environmental Protection to limit mercury emissions from incinerators in Florida are just beginning to bear fruit.

Peter Frederick, a UF associate professor in the department of wildlife ecology and conservation, was the lead investigator in the seven-year study sponsored by the DEP. For the study, he and several graduate students monitored mercury levels in great egret chicks in seven Everglades colonies, or communal nesting sites.

As predators at the top of the food chain, great egrets are good barometers for the herons, ibises, storks and spoonbills that also live in the Everglades, Frederick said. Chicks' parents, which bring them food, hunt within about 15 miles of nests, so chicks are ideal study subjects because the mercury they accumulate comes from surrounding areas.

When birds ingest mercury, they release some of it in their growing feathers. This gave Frederick and other researchers a harmless way to track the chicks' mercury uptake: They caught 20 chicks in each colony each year, plucked a few of their feathers for analysis, and put the chicks back into their nests.

The results surprised and pleased the researchers. The most significant: Between 1994 and this year, average levels in the chicks' feathers dropped 73 percent, Frederick said.

Some natural processes can affect mercury uptake in wading birds. For instance, levels may rise in dry years because big fish -- which contain more mercury because they've had more time to accumulate it -- become trapped in small pools and are easier for birds to catch.

(Conversely, mercury levels are expected to drop in wet years, because big fish are hard to catch.) Mercury levels also are predicted to rise in years immediately following droughts because of a phenomenon known as the reservoir effect. During dry years, submerged areas become exposed, drying out and releasing mercury stored in sediments. Upon reflooding, this mercury becomes available to the aquatic food web and, eventually, the birds.

1999 and 2000 were dry years, which would suggest mercury levels would increase. But levels have declined both years, continuing a steady decline that began in 1997.

"The decline overall has been dramatic, and it's been occurring in the face of other environmental changes that are likely to push values up rather than down," Frederick said.

In 1994, the highest concentration of mercury in any of the colonies studied was 25 parts per million, while the lowest was 5 parts per million. In 2000, the highest concentration was 10 parts per million and the lowest was 2 parts per million, Frederick said. Normal background levels range from one-half one part per million to 2 parts per million, he said.

The findings bode well for the egrets, which display a variety of ill effects from mercury poisoning, experiments by Frederick show. At the average levels seen in 1994, young egrets would have fewer red blood cells, weigh less and would be less interested in hunting, the experiments show. Because chicks excrete a lot of mercury in their feathers while they are growing but not when they become adults, these problems tend to hit birds just as they become adult -- which also is when they face the most stressful and dangerous part of their lives learning to hunt and avoid predators. "We believe mercury poisoning definitely results in an increase in mortality, although we don't know how much of an increase there is," Frederick said.

The declines may be a result of decreased mercury use, researchers and state officials said. Mercury in products and industrial processes has been declining since the late 1980s, when battery manufacturers and others began eliminating the toxic metal, said Tom Atkeson, the DEP's mercury coordinator. Early this decade, the DEP became the first agency in the United States to require mercury-emissions limits on incinerators, he said. Because of the time needed to upgrade industrial facilities, those controls have become prevalent in the last two to three years.

"The data are only just now coming in, but the numbers show that we should see another big step down in incinerator mercury emissions as a result of the requirement of advanced pollution controls," Atkeson said.
-end-
Writer: Aaron Hoover, ahoover@ufl.edu
Sources: Peter Frederick, (352) 846-0565, pcf@gnv.ifas.ufl.edu
Tom Atkeson, (850) 921-0884, thomas.atkeson@dep.state.fl.us

University of Florida

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