Lake's 'dead zones' perplex scientists,
but tiny invader may be partly to blame
Chicago Tribune and The Baltimore Sun
Lake Erie, once so polluted and putrid that it was irreverently
called "the place where fish go to die," is now often cited
as a model for ecosystem recovery.
But on the 30th anniversary of the Clean Water Act,
legislation inspired in part by Lake Erie's near-death
in the 1960s, scientists say parts of the Great Lake are
dying once again.
Mysterious dead zones, or areas without oxygen, have
returned to about half of the lake's central basin. Yellow
perch and prized walleye populations declined in the 1990s.
Meanwhile, avian botulism, also present in the 1960s,
has killed thousands of water birds, including common
loons and ring-billed gulls.
Decades ago, the problem was chemical pollution, primarily
phosphorus from sewage, detergent and fertilizer. Today,
despite controls, the phosphorus is back and scientists
suspect the culprit is biological pollution, due in part
to changes wrought by invasive species such as the zebra
The dead zones in Lake Erie are a bit counterintuitive
- they occur because there is too much life. When too
many nutrients like phosphorus are in the water, algae
grow maniacally. Algae blooms are followed by a die-off,
and as the material decays, it consumes oxygen like a
forest fire. No fish, plants or insects can live in oxygen-free
Located in depths below 40 feet, the dead zones are
ominously spreading east in the middle of the lake from
the Lake Erie Islands toward Erie, Pa., during the summer
Lake Erie's central basin has been running out of oxygen
more rapidly than ever, partly because of its flat geography
and average depth of 60 to 80 feet.
The fish that can leave the area do. Everything else
The Clean Water Act, which limited phosphates and supplied
more than $80 billion to upgrade sewage-treatment plants,
largely solved the problem of chemical pollution.
But in the mid-1990s, the U.S. EPA's Great Lakes National
Program office discovered that phosphorus levels and dead
zones had both increased.
To figure out why, a team of researchers led by Gerry
Matisoff, chairman of the department of geological sciences
at Case Western Reserve University, and Jan Ciborowski
of the University of Windsor began a two-year, $2 million
study in June.
Their working theory is that zebra mussels and the related
quagga mussels, which have a maddeningly efficient ability
to filter debris and extract zooplankton, use phosphorus
in a different way from other species.
"We think (invasives) might have interrupted the natural
process of carbon and phosphorus cycling (in Lake Erie)
and how the ecosystem interacts with their chemistry,"
said Matisoff, who spent most of the summer on the EPA's
research vessel, the Lake Guardian.
Tiny but prolific, zebra mussels quickly can take over
a body of water, clogging power-plant intake pipes, stealing
food and oxygen from other species and even suffocating
native mussels that they attach themselves to and eventually
In the Mississippi, they have pushed a species of native
mussel closer to extinction. In Lake St. Clair on the
Michigan-Ontario border, they might be poisoning a kind
of duck known to eat them.
Native to the Caspian Sea, zebra mussels were detected
in North America in 1988, apparently having made their
way over in the ballast tanks of oceangoing freighters.
Within a few years, the species had established itself
throughout the Great Lakes and the Mississippi River and
its tributaries, and now is found in waters from Minnesota
Zebra mussels have been found in the upper reaches of
the Chesapeake Bay watershed, in parts of the Susquehanna
River in New York, increasing fears that they could pose
a threat to municipal water systems and power facilities
in Maryland such as the Conowingo Dam.
Once a colony begins, it can grow rapidly - one female
zebra mussel can lay more than 1 million eggs in a single
spawning season. Power plants around the world have spent
millions of dollars unclogging pipes where zebra mussels
Unlike native freshwater species, zebra mussels have
Velcro-like threads that enable them to attach themselves
to seemingly anything hard in a river - rocks, boats,
pipes, mussels. This ability has allowed zebra mussels
to spread far and wide, hitching barnacle-like onto boats
and moving greater distances than they could on their
In Lake Erie, as mussels vacuum organic matter out of
the water, they expel the phosphorus their food contained
and it sinks to the bottom. This helps the algae grow,
and the algae drain the oxygen from the deep water. As
material decomposes, it consumes oxygen and is not replenished
until fall, when temperature changes cause the lake water
But the lake's variability from year to year is a major
problem for scientists trying to understand the processes
at play. Round gobies, a newer invasive species, are now
eating zebra mussels, and no one is sure what impact that
will have. In addition, underreported phosphorus levels
and climatic change could be exacerbating the situation.
Phosphorus loads are supposed to remain below 11,000
metric tons per year to keep Lake Erie healthy, but scientists,
thinking Lake Erie was rebounding, relaxed their monitoring
efforts in the 1990s as funding dwindled.
David Dolan, a professor of natural and applied sciences
at the University of Wisconsin Green Bay, determined that
phosphorus levels spiked in 1997 at 17,000 metric tons
but are now below the target level.
The phosphorus increases are probably related to the
amount of rainfall the basin receives and how much fertilizer
makes its way from sewers and farms to streams and into
lakes, said Dolan, an environmental statistics expert.
Climate change might be another factor. When the lake
was recovering, water levels were rising, but since 1997
the water level has dropped by 3 to 4 feet, reducing the
oxygen reservoir, said Jeffrey Reutter, director of the
Ohio Sea Grant Program. With less water, it's more difficult
to keep the phosphorus level down.
Should Lake Erie grow worse, the effects on the region
and its billion-dollar fishery could be huge.
The lake also supplies drinking water to 11 million
people. When blooms of algae die and sink to the bottom,
they release chemicals that smell foul, taste musty and
can be detected in tap water.
"We know Cleveland is the poster child of what goes
wrong when you don't pay attention to the lakes and rivers,"
Mayor Jane Campbell told Great Lakes commissioners at
their annual meeting this week, as she called for help
with a billion-dollar sewer-improvement project.
"Now we're trying to turn it around to be the poster
child for the solution."