Geotimes
Web Extra Friday,
November 21, 2003
Mercury transitions in the
Everglades
Mercury levels in fish in the Everglades have dropped substantially over the past
decade, which may be directly related to the control and eventual cessation of
mercury discharges by local power plants and incinerators, according to a
study released this month by the Florida Department of Environmental Protection
(DEP).
The findings have led some researchers and organizations to call for immediate
cessation of mercury-producing activities elsewhere, in order to create almost-immediate
local benefits. However, the connection between fish recovery and the closure
of the plants is not definite, and other sources of mercury may potentially offset
the benefits of local controls.
This mercury
problem haunts the Everglades and its restoration. Birds and fish bioaccumulate
the toxic metal in the form of methylmercury, building it up in their bodies with
everything they consume, and then adults bequeath it to their offspring, which
might suffer birth defects or other disorders. A tiny amount of mercury —
less then a part per trillion — in the background signal of an ecosystem
is more than enough to have toxic effects (see Geotimes,
August 2003).
Aerial view of the Everglades National Park, where birds and fish have been plagued
by high levels of mercury. Photograph courtesy of the National Park Service.
Although mercury is present naturally, the highest impact comes from the anthropogenic
release of the element in three different forms, each with different paths. Of
mercury ejected into the atmosphere, the most active and damaging is the oxidized
form, Hg II, which more readily converts to methylmercury once it enters an ecosystem.
The unique environment of the Everglades seems to facilitate that conversion —
for example, researchers have linked increased sulfur additions from sugarcane
crops to the creation of methylmercury — by flushing in new sources of mercury
daily, in water and air.
The Florida report shows an almost one-to-one correlation between mercury in the
system and in largemouth bass, a top predator fish. Tom Atkeson, the program administrator
and a mercury specialist with the Florida DEP, presented some of the reports results
to policy-makers and concerned citizens this month, in a Capitol Hill briefing
sponsored by the Environmental and Energy Study Institute. He said that in 1995
and 1996, the majority of mercury deposition tended to be from local sources.
During the late 1980s and early 1990s, Florida's power plants and incinerators
cut down their mercury emissions, dropping to almost zero by 1999. Today, almost
10 years after those first efforts, the decrease in fish mercury loads has halved,
according to DEP monitoring. "The system is responding more quickly than
previously thought," Atkeson says.
Robert Mason, of the University of Maryland's Center for Environmental Science
and Chesapeake Biological Laboratory, says that "response time in Everglades
is much more rapid than it would be anywhere else." The system response time,
he says, is due to the combination of warmer temperatures and higher biological
productivity, along with a high ratio of sediment surface area to water volume
because it is so shallow.
Though the decrease in mercury in fish may be attributable to shutting down incinerators
and other local sources in the early 1990s, global sources today provide much
more than half the dose of mercury to the Everglades — more likely 80 to
85 percent of the load, says William Landing, a chemical oceanographer at Florida
State University, Tallahassee. "Despite dramatic reductions in the last 15
years, that hasn't changed deposition in 10 years," he says. "The bottom
line is that in southern Florida, the dominant source is long-range transport"
of mercury releases elsewhere on the planet, according to recently collected atmospheric
data.
The Florida DEP has not yet settled on an atmospheric model for mercury deposition.
However, says David Krabbenhoft, of the U.S. Geological Survey in Middleton, Wisc.,
the aquatic cycling model from the Florida report "is an excellent model,
put together right," for a system that is very difficult to understand. "The
fact that the model matched the decline amazingly well basically makes me a believer,"
Krabbenhoft says. "But I have some hesitations," he continues, most
notably because of a window of data that is lacking between the first deposition
decreases (when power plants and other sources first started cutting emissions
in the 1980s) and later decreases in fish, from measurements beginning in the
mid-1990s.
Ideally, Krabbenhoft says, monitoring in 1985 would have enabled the DEP scientists
to quantify the response to the decrease in load. "We don't have that —
instead, we have sediment cores and time series in fish and birds" from which
to draw conclusions, he says. He also notes how difficult it may be to extrapolate
from Florida, which can be considered an end member in atmospheric and environmental
conditions in relation to the rest of the United States. "South Florida is
number one in most cases for both concentration and deposition rates," he
adds. "We don't know why that is."
Atkeson also says that there are a number of "peculiar things" about
Florida that cannot translate to other regions, from incinerators sitting next
to wetlands to the panhandle's unique atmospheric circulation patterns. Nevertheless,
Atkeson says, his take-home message is that yes, there is an overarching mercury
cycle that effects the Everglades, "but under that umbrella, there's considerable
room to make a difference."
Naomi Lubick
Link:
"In
Search of the Mercury Solution," Geotimes, August 2003
Mercury
Program, the Florida Department of Environmental Protection
AGI's Government Affairs' summary
of a recent House Science Committee hearing and background
on
mercury
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