Clearing the Air on Mercury

More fish are added to the mercury hot list, but troubles may actually be abating in some areas. The full story for Florida anglers.

If you could get over the shock of seeing cobia and seatrout on the latest fish consumption advisory from the Florida Department of Health, you'd read some good news between the lines. Largemouth bass in the northern two thirds of the Everglades got promoted from "No Consumption" status to "Limited Consumption."

That means state researchers saw declines in tissue concentrations of methylmercury, an especially toxic form of that silvery stuff you used to see in all thermometers.

From a peak in the mid 1990s, in 3-year-old largemouth bass mercury declined by a conservative analysis about 60 percent, maybe as much as 75 percent," said Tom Atkeson, mercury coordinator with the state Department of Environmental Protection.

Atkeson attributes the decline-specifically in the Loxahatchee National Wildlife Refuge and South Florida's Water Conservation Areas 2 and 3-to regional efforts to curtail mercury-laden air pollutants.

For those of you who haven't been reading the gloom-and-doom advisories circulating lately, a little chemistry lesson: Mercury in its elemental form occurs naturally, but when waste incinerators, powerplants, mining operations and other industrial processes disperse it into the atmosphere, it undergoes a chemical transformation into a gaseous, reactive form. Rainfall brings it back to the ground, and sulfate-reducing bacteria convert it into organic methylmercury.

The Everglades happens to be a hot zone for two reasons, Atkeson explained. One, the shallow waters and matted algae foster the growth of this kind of bacteria; and two, prevailing winds blow airborne pollutants out there most every day in the rainy season. A third factor-yet to be fully studied-is how sulfur related to agricultural practices may enhance the conversion of methylmercury.

Links in the aquatic food web give methylmercury a direct route to predatory fish like bass. The toxin "bioaccumulates" over time, meaning older, higher level predators tend to store higher concentrations. The danger to humans is that methylmercury is readily absorbed by the digestive tract-and from there it goes right to the brain and nervous system. It also has an insidious ability to affect infants in utero. If consumed in large enough quantities, methylmercury can inflict crippling, even fatal, damage.

That methylmercury is potentially dangerous to consumers of fish has been known for decades. In the 1950s, hundreds of villagers in coastal Japan died after eating fish and shellfish tainted with mercury pollution. Many others were afflicted with neurological symptoms ranging from numbness in lips and limbs, to loss of control of muscles, to brain damage. Many infants there were born with mental retardation and other birth defects.

Florida health officials began issuing dietary consumption advisories in the late 1980s, after researchers discovered high levels of methylmercury in Everglades bass and some other fish.

But unlike the case in Japan-where a plastics factory had been pouring mercury-laden sludge into a bay for years-in Florida there was no smoking gun (nor was there anything that could even be remotely termed a public health crisis). As it turns out, explained Atkeson, changes in industry and waste management, on a local and regional scale, had by 1990 already reduced the deposition of mercury in the South Florida environment.

"We were in the middle of the movie; we couldn't see the beginning," said Atkeson, a biologist by training, hired by the state in the early '90s to study the mercury problem.

One of the first Florida studies determined that rainfall, not surface runoff, was the primary vehicle carrying mercury to the Everglades. "Fresh mercury loads each year were dominated by rainfall deposition," said Atkeson.