Great Lakes: Resource at Risk For
centuries the Great Lakes have been treated callously. These five
magnificent
lakes--Superior, Michigan, Huron, Erie, and
Ontario--located along the eastern half of the
Canadian-U.S. border have served as a virtual sewer
catching waste from industry, agriculture, commercial shipping,
and households. Their natural barriers to other water systems
have been breached, exposing indigenous ecosystems to
aggressive invaders. They’ve been used as a highway for
colossal ships that require deepened and broadened channels to
crisscross the lakes, and that import exotic species along with
their intended cargo. At times it could seem that this
long-suffering water system will see no end of indignities. But
recent renewed focus on the unique and tremendous value of this
resource by governments and communities surrounding the lakes
may turn the tide of neglect and abuse.
According to the U.S. Environmental
Protection Agency (EPA), the Great Lakes contain 21% of the
Earth’s and about 84% of United States’ surface
freshwater. That’s about 22,000 cubic kilometers of water
spread over 94,250 square miles. Each year the lakes provide
more than 6.7 million cubic meters of water to municipalities
and quadruple that to industry. They support a
commercial fishery worth about $13 million as of 2002,
according to the U.S. Geological Survey (USGS), and a
sport-fishing industry of nearly $1.3 billion as of 2001,
according to the U.S. Fish and Wildlife Service. Today about
25% of Canadians and 10% of Americans--a total of more
than 33 million people--live in the Great Lakes watershed.
“The whole industrial expansion that
took place during the ‘robber baron’ era [of the
late nineteenth century] expanded along the shores of the Great
Lakes,” says Deborah Swackhamer, a University of
Minnesota professor of environmental chemistry. Soon ships were
carrying iron ore, coal, and limestone from mines and quarries
to steel mills and later steel to factories and products to
markets. In addition to serving as a transportation system, the
lakes provided a place to discharge manufacturing by-products.
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Natural treasures. The Great Lakes (seen here
from the vantage point of the Witch Tree at Lake Superior Grand
Portage in Minnesota) are a priceless natural resource, but
decades of abuse and pollution are taking their toll on these
once pristine bodies of water.
image: Jeff Gunderson/Minnesota Sea Grant |
Unlike a sewer, however, whatever enters
this lake system stays awhile. On average, less than 1% of the
five lakes’ water turns over each year, which means that
many pollutants stay in place. They settle in sediments, adhere
to other surfaces, become suspended in water, and bioaccumulate
in organisms. Similarly, with the exception of migratory birds,
most wildlife in the basin spend their entire life cycle in or
near the lakes.
As a result of all these stressors, the
lakes now house fish that are dangerous to eat, water that can
be unsafe to drink, anoxic “dead zones”--areas
in which virtually no plants or animals can survive--that
appear each summer like clockwork, and an ever-growing
population of unwanted species from other parts of the world.
The Great Catch-alls?
According to the
EPA, 362 contaminants have been identified in the Great Lakes
system, only about a
third of which have been evaluated for their effects on
wildlife and human health. Two decades ago the International
Joint Commission (IJC)--an organization that was formed by
the Boundary Waters Treaty of 1909 to prevent and mediate
boundary water disputes between Canada and the United
States--identified 11 of these as “critical
pollutants” that required immediate attention. The list
includes polychlorinated biphenyls (PCBs), DDT, dieldrin,
toxaphene, mirex, mercury, benzo[a]pyrene, hexachlorobenzene, furans, dioxins, and
alkylated lead.
All of these substances bioaccumulate in
organisms and persist in the environment. Joseph Makarewicz,
a
professor of environmental science and biology at The State
University of New York at Brockport, explains that many of
these chemicals are attracted to fats and repelled by water. “They readily move into tissue,” he says,
“but basically it’s through phytoplankton,
zooplankton, forage fish . . . into the various salmons and
lake trout.” So the amount of contaminant in the water
may be very small, even difficult to measure, but once it gets
taken up by phytoplankton, it then biomagnifies--or
becomes more concentrated--as it moves up through the food
chain.
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Industrial waste bins. Industries along the lakes including
paper companies, power plants, chemical manufacturing, and
myriad others have routinely polluted the waters and ecosystems.
images: Minnesota Sea Grant; U.S. EPA |
Over the past 20 years, as the American
and Canadian governments have attempted to compel industries
to
better control or completely discontinue use of these
substances, their levels in the environment have dropped. “We can demonstrate that DDT did definitely increase over
the course of the fifties and sixties, peaked in the seventies,
and then began to decline again,” Swackhamer says.
“We can show the same for PCBs.” In response to the
presence of these chemicals, over the years a smorgasbord of
local, state, provincial, and federal public-awareness
campaigns have been launched to alert citizens to the hazards
of exposure to these chemicals.
The campaigns have met with mixed results.
At-risk populations--those who consume large amounts of
fish--are
still being exposed, says Heraline E. Hicks, a senior
environmental health scientist and manager of the Great Lakes
Program at the Agency for Toxic Substances and Disease
Registry. That’s because contamination with certain
chemicals, such as PCBs, is ubiquitous in certain kinds of
Great Lakes fish.
“People who are eating fish out of
the Great Lakes as a major nutritional source are really
getting hit hard,” says Keri Hornbuckle, a University of
Iowa associate professor of environmental engineering. The
problem, she says, is complex. Some groups of people, including
low-income African Americans, Native Americans, and
non-English speakers such as some Hmong immigrants,
depend on the contaminated fish for their subsistence and can
be hard to reach with or resistant to cautionary messages.
A suite of studies
by different research groups suggest the health impacts may be
profound. “We
have seen changes in the sex ratio of children who were born to
parents who were exposed to PCBs,” says Hicks. In a
January 2002 Journal of
Occupational and Environmental Medicine study of Lake Michigan fish eaters and their
children, men with blood PCB levels of greater than 6 parts per
billion were more likely to father male children than female
children. The ratio of boys to girls in this population was
about 154 boys for every 100 girls, whereas the normal human
sex ratio is 103 boys to every 100 girls, says Hicks.
Interestingly, a study in the 12 March 2003 issue of Environmental Health also
found sex ratio effects, except that maternal exposure appeared
to result in more girls.
Still another study, published in the
February 1999 issue of Environmental
Research showed that couples in
which the man had a high body burden of PCBs due to his pattern
of fish consumption took longer to conceive. And research
published in the December 1997 American
Journal of Epidemiology hinted
at still more potential reproductive effects: women who ate
PCB-contaminated freshwater fish experienced a shortened
menstrual cycle.
image: Chris Reuther/EHP |
Since the U.S. EPA banned production and
most uses of PCBs almost three decades ago, levels of the
chemicals in lake water have declined steadily. Over the years
levels of PCBs--whose primary use was in nonflammable oil
for such devices as switches and electrical transformers and
capacitors, as well as a lubricant--have also dropped
significantly in some species. According to the EPA, typical
levels of PCBs in Great Lakes lake trout during the 1970s were
22 milligrams per kilogram. By the 1980s, they had dropped to
about 6 milligrams per kilogram, just slightly above current
levels.
Nonetheless, PCBs continue to cycle
through the environment. They are mixed into sediments on the
lakes’ floors, where they can be released by such
disturbances as dredging or strong storms. Land sources
continue to pollute as well, Hornbuckle says. The lightest of
the 120 types of PCB compounds that have been found in the
Great Lakes appear to remain active in the environment the
longest.
And where do these long-banned compounds
come from today? Hornbuckle has tracked them to industrial
sites, many abandoned, surrounding the Great Lakes. Soils that
were contaminated in the 1950s and 1960s in cities such as
Chicago and Milwaukee appear to be an ongoing source of PCBs,
she says. The chemicals volatilize on warm days, then enter the
water system when blown over the lakes or the rivers and
streams that feed them. Such atmospheric deposition is now the
leading way PCBs are introduced into the lakes, Hornbuckle
says. Plus, there are still PCBs remaining in service in older
transformers and capacitors.
Mercury’s effects on human and
environmental health are similar to those of PCBs. Today, says
Michael Murray, a staff scientist with the National Wildlife
Federation, much--if not most--of the mercury
entering the Great Lakes comes via atmospheric deposition.
Major sources of mercury releases to the air include coal-fired
power plants, incineration of mercury-containing devices,
mercury cell chloralkali plants, and industrial boilers.
From the 1940s through 1960s the lakes
were bombarded directly with mercury-laden industrial waste. “Dow
Chemical released about four hundred tons of mercury [into Lake
Huron] from two chloralkali plants that they
had,” says Michael Gilbertson, who recently retired as
a biologist and secretary of the Workgroup on Ecosystem Health
with the IJC. The result, he says, has been an outbreak among
a
series of communities on the Canadian side of the Great Lakes
of suspected mercury-related poisoning. Symptoms observed have
included tremors, deafness, and blindness.
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Failing the fish, the fishermen, and the
folk alike. Pollutants are accumulating in Great Lakes fish, making
many of them unsafe to eat. This hazard threatens the livelihoods
of many and the health of even more. (Above: trap net whitefish
in Lake Saganaga Bounding Waters Canoe Area Wilderness, Minnesota;
left: early fall fishing on the Oswego River, Oswego, New
York.)
images: Left to right: Pat MacNeill/New
York Sea Grant Extension; Jeff Gunderson/Minnesota Sea Grant
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In an unpublished dissertation, Gilbertson
wrote that a review of Canadian hospitalization records reveals
that in some communities near the lakes four times as many
males are being admitted with cerebral palsy as in inland
areas. Gilbertson says methylmercury is the only substance
known to be associated with cerebral palsy, and males are more
susceptible than females to neurological damage in utero from
exposure to methylmercury.
More recently developed pollutants also
threaten Great Lakes fauna--possibly including humans.
Some that are appearing in the environment in increasing
quantities are the polybrominated diphenyl ethers (PBDEs),
which were introduced in the early 1970s as flame retardants in
consumer products. “No one paid any attention to these
compounds until the last couple of years,” says David
Carpenter, a professor of environmental health and toxicology
at The State University of New York at Albany’s Institute
for Health and the Environment, “and now we’re just
finding them everywhere.”
Although PBDEs are mixed with plastic
polymers during manufacturing, they don’t bind chemically
with the plastic. As a result, they leach readily from products
that contain them. Like PCBs, PBDEs concentrate in fats. That
and their resistance to degradation combine to create a
persistent chemical that bioaccumulates. “Their
manufacture increased exponentially over the last decade or so,
and we’re seeing them in fish and in sediments with that
same increase,” Swackhamer says. “We can go back to
the sediments since we didn’t measure them in the
environment until a few years ago; we can actually use the
sediments to go back and demonstrate the fact that
they’ve increased exponentially all through the
mid-eighties and into the nineties.”
PBDEs have now been found in fish from all
five of the lakes. According to a University of Wisconsin study
published in the 1 March 2001 issue of Environmental Science and Technology, Lake Michigan salmon (which are an introduced
species managed for sport fisheries and sustained through
stocking programs) contain PBDEs at levels above 100 parts per
billion, one of the world’s highest concentrations for
salmon in open waters. The authors also found PBDEs in some
forage fish, such as alewife and smelt. Levels of PBDEs also
appear to be increasing in human tissue, according to a review
published in the May 2000 issue of EHP. Although the human health impacts of PBDEs
aren’t well understood, these chemicals have been shown
in animal studies to have effects similar to those of PCBs,
including effects on brain development, learning, memory,
thyroid levels, and reproduction.
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Source:
U.S. EPA
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The sources of PBDEs are ubiquitous. “PBDEs are used everywhere--in furniture, carpeting,
computers, and vehicles,” says Sergei Chernyak, a
research scientist in the University of Michigan School of
Public Health. To gauge the contribution of domestic sources of
PBDEs to the environment, Chernyak’s research team
compared PBDE levels inside and outside homes. The levels
inside a typical house were 70 times the levels just outside
the building. “Although this was a pilot study, it seems
that houses have strong sources of these contaminants, which is
a public health concern,” Chernyak says.
With such a diffuse source, stemming the
flow of PBDEs to the Great Lakes poses a much more difficult
problem than that of PCBs, whose original sources could be
traced to large industrial sites, Chernyak says. “The
source to the lake came not from water nor from the shore but
probably from [the air],” he says.
Such pollutants can devastate wildlife in
subtle and hard-to-detect ways, says Philip Cook, a research
chemist for the U.S. EPA. For example, research by Cook and
colleagues published in the 1 September 2003 issue of Environmental Science and Technology indicates that the introduction of dioxins
and certain related PCB compounds to Lake Ontario in the 1950s
spelled the end of lake trout, the lake’s top predator.
“This was a finding that would contradict prevailing
opinions in the Great Lakes, specifically that lake trout,
which had declined to extirpation--in other words the
population was gone by 1960--was due to other ecological
factors, not chemical toxicity,” Cook says. Exotic
species, overfishing, loss of habitat, and water quality
reductions had all been blamed, he says, but “we think
there is evidence to show that at least up until very recently
this toxicity was a factor that impaired any success in trying
to reintroduce the species.”
Sewage poses still
another threat to the lakes. According to the IJC’s September
2004 Twelfth Biennial Report on Great Lakes Water
Quality, each year rainstorms send
trillions of gallons of untreated human sewage into the lakes.
In 2001, Lake Michigan alone absorbed 52 billion gallons of
sewage and partially treated wastewater.
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Teeming at the water’s edge. Ever-encroaching cities such as Chicago, Illinois,
draw on Great Lakes resources while dispensing sewage
and refuse into the waters at alarming rates--and
with escalating consequences.
images: Digital Vision; inset: Lake Michigan Federation
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With the sewage come nitrates, which can
cause gastric problems and methemoglobinemia if they find their
way into drinking water in excessive amounts. It also brings
any number of pathogens, including the Cryptosporidium parasite
and Escherichia coli, which can cause severe, and sometimes deadly,
intestinal diseases.
Today most raw sewage contributions to the
Great Lakes are inadvertent, and they most often come from
overflowing sewer systems during storms. A smaller, but still
significant, source of sewage is the thousands of privately
owned septic systems on the farms, vacation homes, and other
rural dwellings that ring the lakes, says Kathleen Halvorsen,
an associate professor of natural resource policy at Michigan
Technological University. Some 10-20% of these systems
are failing, she says. Still, she adds, it’s the big
cities that contribute the most untreated sewage.
The Great Melting Pots?
Almost as diverse as the pollutants that
have found their ways into the Great Lakes are the plants and
animals that now call the lakes home. The Great Lakes connect
to the Atlantic through the Saint Lawrence River, which flows
out of Lake Ontario. But until the early 1800s Niagara Falls,
at the west end of Lake Ontario, prevented water and the
animals that live in it from reaching the rest of the Great
Lakes system. In the 1800s, however, two waterways--the
Erie Canal and the Welland Canal--opened these lakes to
the Atlantic Ocean. These new channels provided a foot in the
door for invasive species, some traveling under their own
power, others hitching rides aboard the ships that began plying
the waters.
Since the early 1900s, 170 or so nonnative
species have found their way into the Great Lakes, says Edward
Mills, a fish ecology professor at the Cornell Biological Field
Station. Some, such as the zebra mussel, stowed away on
oceangoing ships. Some, such as the rusty crayfish, escaped
their fate as bait for anglers. Some, such as the banded
mystery snail, were liberated from home aquariums. Some, such
as garden loosestrife, were cultivated. Some, such as the
orange-spotted sunfish, swam in through man-made canals. And
some, such as the rainbow trout, were introduced intentionally
as game fish.
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Ships of fate? A ship arrives
in Duluth ship canal in Minnesota (above), just one of
thousands that ply the Great Lakes every year. Along with
the myriad goods in their hulls, their ballasts carry invasive
species such as zebra mussels (right), which can have disastrous
effects on infrastructure and native ecosystems.
images, top to bottom: Jerry Bielicki/U.S. Army Corps of Engineers; Center
for Great Lakes and Aquatic Sciences
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These newcomers can devastate their new
homes. By 1947, for example, the sea lamprey--an eel-like
fish that sucks bodily fluids from other fish--had invaded
all of the lakes. The lamprey--which probably swam up the
Hudson--has contributed to the collapse of whitefish and
lake trout fisheries. A more recent and possibly more damaging
invader, the zebra mussel, arrived in the ballast water of a
transoceanic vessel. Since its first Great Lakes sighting in
1988, this fingernail-sized mollusk has found its way into
seemingly every nook and cranny of the lake system.
Zebra mussels and their fellow invaders,
quagga mussels, are the only mollusks that can attach
themselves firmly to solid objects. The solid objects they
select for their colonies include cooling pipes at power
plants, boat hulls, propellers, and docks, which--at
mussel densities of up to 70,000 per square meter--quickly
become clogged and fouled. They’re also radically
altering the ecological balance of the lakes, Makarewicz says.
The mussels are such effective filter feeders that they strip
water of the various plankton that indigenous creatures eat.
In
the process their fatty tissues accumulate concentrations of
PCBs, methylmercury, and other contaminants at about 10 times
the density of native mussels, giving the fish and waterfowl
that feed on them an extra shot of biomagnified poison.
In not much more than a decade, Makarewicz
says, the zebra and quagga mussels have undone years of work
and at least $5 billion spent on Lake Erie alone to upgrade
water quality by reducing phosphates from fertilizers,
detergents, and industrial discharges. “There was major
improvement in all of the basins of Erie,” he says.
“The walleye populations that [fisheries] began
stocking--which is a sport fish--came back in big
numbers and things looked very good. And then the zebra mussels
changed that.” Although phosphates are largely under
control, he says, invasive mussles have re-degraded the quality
of the water by overclarifying it.
Zebra mussels, quagga
mussels, and a fish known as the round goby have been tagged
as the most likely
culprits in recent type E botulism outbreaks in Lake Erie. “The real mechanism has not been clearly distinguished
yet,” Mills says, “but it does look like it’s
some relationship between the mussels--mostly quagga
mussels--and gobies.” The highly efficient mussels
clarify the water so much that sunlight can reach normally
shaded aquatic plants and promote their growth. When the
unwonted proliferation of growth dies, its decay consumes
oxygen, providing ideal conditions for the Clostridium botulinum
bacteria to grow. The mussels also may concentrate C. botulinum,
which is biomagnified when the mussels become meals for round
goby. “We know gobies feed on mussels,” Mills says.
“The gobies then get the Clostridium bacteria toxin. They die and then whatever
eats them--it could be fish or birds feeding on them as
they roll in on shore--then gets the toxin and
dies.”
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A sticky situation. The
sea lamprey (right), a nonnative species that probably
swam into the Great Lakes from the Hudson River, attaches
to lake trout and whitefish (above). Sea lampreys have
invaded all the Great Lakes and wiped out fisheries.
images, top to bottom: U.S. Fish and Wildlife Service; Great Lakes
Sea Grant Network Exotic Species Graphics Library
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Similarly, says R.
Peter Richards, a water quality hydrologist at Ohio’s Heidelberg
College, zebra mussels may be the cause of recent increases in
blooms of the
toxic algae Microcystis. “It is at least a nuisance algae, if not
a potential health problem,” he says. “The zebra
mussels don’t like to eat Microcystis, so they suck all of the particles out of the
water and they spit the Microcystis back out. At the same time they digest all the
other stuff and spit it out as pseudofeces, which releases
nutrients that feed the Microcystis. Zebra mussels are killing off the competition
and fertilizing the Microcystis.”
In spite of a 1993 rule that calls for
oceangoing ships to flush their ballast tanks before entering
the Great Lakes system, the influx of invasive species has
actually increased. One reason may lie in the loophole that
ships that are fully loaded with cargo, and so in theory are
free of ballast water, aren’t required to flush their
tanks. But even “empty” ballast tanks can contain
up to 10 tons of sediment and trapped water, and species can
be
released into the lakes as the ships unload and reload cargo.
Another reason may be the types of species
that are coming in. Spiny water flea spores have been
identified in ballast-tank residue, and it’s possible
this is how this and other alien species have found their way
into the lakes. In inhospital conditions, mating spiny water
fleas produce “resting eggs” that are first carried
in the females’ brood pouches and later released to sink
to lake sediments, where they lie dormant until the water warms
in spring or summer. The creature’s sharp spines make it
impossible for all but the largest predators to eat them.
Meanwhile, the spiny water flea itself competes aggressively
for the plankton that so many native species depend on for
food.
Flushing ballast, even if made more
effective, won’t control invaders that travel to the
lakes through other routes, such as the Asian carp, which is
knocking on the lake system’s southernmost door. In an
attempt to stop the voracious fish, an electrified barrier has
been strung across the Chicago River, which flows out of Lake
Michigan. So far Asian carp--a term used for multiple
species of carp used in Southern aquaculture--have not
been found in the Great Lakes. But they are less than 50 miles
away on the Illinois River, where in some stretches they have
become the dominant species.
Asian carp, which include bighead carp and
silver carp, are thought to have escaped from U.S. fish farms
during the Mississippi River floods more than 10 years ago.
They are used for pet food because they quickly grow quite
large. Bighead carp can weigh more than 100 pounds and because
they can eat as much as 40 pounds of plankton per day, some
ecologists warn that they could equal or even outdo zebra
mussels in terms of how efficiently and quickly they ravage the
ecosystems they invade.
In threatening native
species, these invaders also reduce biodiversity, says Dora Passino-Reader,
a USGS fishery research biologist. Zebra mussels, for example,
reproduce so quickly, are so hardy, and (for a mussel) are so
aggressive--as many as 10,000 have been found affixed to
the shell of one native mussel--that they suppress native
mussels’ movement, feeding, and reproductive behavior. To
take another example, several species of cisco, which were
already depressed by the mid twentieth century by overfishing,
were then dealt the final blow by three exotic
species--the parasitic sea lamprey, a predator, and the
alewife and rainbow smelt, competitors. “Some of these
species have disappeared,” says Passino-Reader.
“[Ciscoes are] not located in the Great Lakes at all
anymore. They aren’t able to live in the Great Lakes
after the invasion of the alewife. The competition and
predators have changed so much that these species have actually
been lost.” Alewife are also high in the enzyme
thiaminase, she says, which breaks down vitamin B1 (thiamine)
and contributes to mortality in the offspring of the
alewife’s predators.
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Tiny hitchhikers. Spiny
water fleas (above in a gill net; close up at right) are
one of many new invasive species that arrive as dormant
eggs in ship ballast and later hatch in the lakes.
images, top to bottom: Center for Great Lakes and Aquatic Sciences;
Great Lakes Sea Grant Network Exotic Species Graphics Library
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Although programs to eliminate sources of
known pollutants--notably PCBs, DDT, and
mercury--have largely stemmed the flow of these substances
into the lakes, foreign species have proven more stubborn,
Mills says. “When these species get in, it’s pretty
much irreversible,” he says. “From the chemical
side of things, once you control a nasty chemical you have some
hope that the system will respond, and usually it responds
quite quickly. But on the biological side of things, very
rarely does an invasive species go extinct. The system has to
change very dramatically for that to happen.”
Invasive species are not the only cause
of the Great Lakes’ biodiversity woes, however. According to
Lucinda Johnson, associate director of the University of
Minnesota’s Center for Water and the Environment, the
Great Lakes system’s coastal marshes, sand dunes, rocky
shorelines, prairies, savannas, and forests--and the
creatures that live in them--are being displaced by human
development. For example, coastal marshes, which house an
estimated 30% of Great Lakes species, are also a frequent
victim of development itself as well as the fallout from
development: toxic runoff rich in road salt and sediment. As
the large cities that ring the Great Lakes continue to grow,
sprawl fallout such as subdivisions, roads, golf courses, and
vacation homes poses an ever-increasing threat to Great Lakes
ecosystems.
The Great Warm-up?
Recent weather trends over the Great Lakes
have included decreased precipitation, higher-than-usual air
temperatures, and less ice cover in winter. As a result of
these influences, water levels in Lake Michigan and Lake Huron
dropped faster from 1998 to 2002 than during any other recorded
period, according to research from the National Oceanic and
Atmospheric Administration (NOAA) published in the August 2004 Bulletin of the American Meteorological Society. In recent years Lake Superior has been at its
lowest level since 1926, Lake Erie hasn’t been as low
since 1966, and Lakes Michigan and Huron are at their lowest
levels since 1965.
Some environmentalists have pointed to
these alarmingly low water levels as evidence of global
warming. But, says Douglas Wilcox, branch chief for coastal and
wetland ecology at the USGS Great Lakes Science Center, the
current low levels are still within the realm of low lake
levels on the upper lakes that have been seen through recorded
and even prerecorded history. Wilcox says the long-term data
suggest we could be entering a longer-term warming cycle with
a naturally resulting long-term low lake level cycle. “On
the other hand,” he says, “if you throw in
anthropogenic warming on top of that natural cycle, maybe the
low lake level that will occur seventy-five or a hundred years
from now--when the low cycle occurs again--may be
much lower than it would have been naturally if it hadn’t
been for human impacts.”
Low lake levels are necessary, Wilcox
explains, for natural restoration of lake wetland ecosystems.
During low water levels, sediments are exposed allowing other
lake plants to germinate and grow. The resulting wetlands
provide habitat for species such as frogs and wading birds. But
for people who depend on the lakes for their livelihood, lower
water means lower profits. In 2000, for example, lake carriers,
which move such cargo as ore, coal, and grain within the lake
system, had to reduce their loads by 5-8%, according to a
2004 NOAA brochure titled “Water Levels of the Great
Lakes.” Sustained low water periods invariably bring with
them political pressure to deepen shipping channels and
harbors, says Emily Green, the Sierra Club’s Great Lakes
program director. But dredging in these lakes can mean
releasing pollutants that have settled into lake bed sediments.
Hornbuckle says the Great Lakes’
infamous violent storms can resuspend PCBs and DDT compounds
that have settled into sediments. And with climate change,
Johnson says, have come more of the violent storms, and storms
that are more violent than before. “There has been a
doubling over the last century in the number and intensity of
storm events, and those [trends] are expected to hold into the
next century,” she says. “The prediction is that
there will be an additional doubling of intense storm
events.” The effects of these powerful storms, she says,
will be magnified by increasing human development--with
its destruction of wetlands and addition of impervious
surfaces, such as roads and sidewalks--in the lakes’ watershed.
This, she says, is a recipe for increased lakeshore erosion,
water turbidity, and resuspension of sediments.
R. Michael McKay, a biology professor at
Bowling Green State University, says plummeting water levels
would also worsen another ongoing problem: dead zones. Lake
Erie, for example, has periodically suffered bouts of anoxia.
Naturally occurring ridges, or “sills,” divide the
lake into three basins. Many summers nearly all of the
dissolved oxygen in the central basin is consumed, dooming
those creatures that cannot escape to the west or east.
“Because of thermal stratification in summer there is no
way of getting new oxygen down into the bottom water,” McKay
says.
A Great Future?
A number of programs are under way to
address problems in the Great Lakes [for in-depth coverage of
these programs, see “The
Great Lakes: Awash in Policy,” p. A174 this issue]. Perhaps some stressors on
the Great Lakes can be mitigated, but others are here for the
long haul, Cook says. “Once you pollute the Great Lakes
it takes a long, long time for these very persistent chemicals
to clear from the system so that levels can return to
presumably noneffect levels,” he says.
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