Getting Up Close and Personal:
Macro and Micro View of Lake Superior
by Marianne Brokaw
Right,
a giant funnel the KITES project uses to collect samples. KITES is gathering
enormous amounts of data on how Lake Superior functions. Below,
left Images demonstrating the changes in surface temperatures in
Lake Superior. Below,
right a map showing the various points of KITES research. It’s cold, deep, and
isolated--and famous for not giving up its dead. But after three years
of field work in a lake that some would liken to a “big bathtub of
ice water,” scientists at Michigan Tech are persistently coaxing
Lake Superior to give up some of her long-held secrets.
The five-year $4.1 million
KITES project (Keweenaw Interdisciplinary Transport Experiment in Superior)
began in 1998, with major field study during the summers of 1999 and 2000.
Team members gathered voluminous information about Lake Superior from
the sky, the surface, the water depths, and the bottom of the lake. Six
scientists from Michigan Tech, and a number from seven other institutions,
are part of KITES.
The work has not been easy.
The reasons Lake Superior is not well-studied become obvious to anyone
out on a boat gathering data, says Martin Auer (civil and environmental
engineering). “It’s probably one of the most dangerous and difficult
systems to work in because of its great size, its tumultuous weather,
and its great cold.” In fact, Lake Superior “has many traits
of an ocean with definable boundaries,” explains Sarah Green (chemistry),
director of the KITES effort.
To obtain a macro understanding
of lake--currents, production, pollution--scientists must take a micro
view of its contents--bacteria, chemicals, algae, plankton, and larval
fish. They relate that to water temperature, current direction and speed,
and runoff into the lake. Materials and organisms collected have been
sorted, sliced, diced, and even x-rayed. And then there’s the sediment,
the library of the lake where information is waiting to be discovered.
The team is in the midst of
analyzing mountains of data, even while members continue to gather more
information. Judith Budd (geological engineering and sciences) merrily
notes that she already has “280 gigabytes of data on-line and we
are still processing it.” As the data is consolidated, the KITES
team will meet as a group to start creating model systems of the circulation
of the lake, nutrient cycles, and ecology.
They already have confirmed
some hypotheses and found some surprises.
The Macro View
Green points out the Keweenaw
Current is indeed “of fundamental importance to the lake’s unique
circulation.” The current can move 10 percent of the entire volume
of the lake past Eagle Harbor within a mile of the shore every year—about
700 billion gallons, as much water as the Mississippi River delivers to
the Gulf of Mexico.
Such an enormous transport
mechanism, says Green, “acts like a funnel. It can broadcast larval
fish, zooplankton eggs, and other organisms. It has a scouring and depositing
action.”
While people have studied the
current for more than 100 years, satellite data provided the KITES team
with images that actually show the path of the current and how it changes
during the year. The speed and direction is affected by the wind, the
shape of the bottom of the lake, the changing temperatures of the lake,
and the Keweenaw Peninsula jutting into the lake.
The vertical front where the
warm and colder waters meet is the “thermal bar,” defined as
the place where the water temperature is 4 degrees C (39 degrees F). “At
this temperature, water is heavier than at any other temperature, warmer
or cooler,” explains Green.
Because the current’s
warmer water is less dense and moves faster, it can contain and direct
sediment flows, affecting the distribution of nutrients and other organisms.
The direction of sediment transport
is crucial to anyone tracking the movement of stamp sands (black sand
resulting from the stamping plants that processed copper ore on the Keweenaw
Peninsula in years past that can be toxic to plankton), the dilution of
pollutants discharged into the lake, or the growth of larval fish who
need warmer nutrient-rich water.
Green says, “when we look
at sediments off the tip of the peninsula near Eagle Harbor, we find particles
loaded with copper that came from the Freda-Redridge shore, more than
60 miles to the southwest.”
Budd, who deciphers satellite
data to get estimates of temperature and surface chlorophyll concentrations,
says, “we’re developing technology for lakes. We are the first
and only user of the satellite data in the Great Lakes--a niche for us.”
Budd and her graduate students correlate their information with ship-based
instruments that gather similar data from the surface.
From the sky we move to the
water.
The Micro View
On board a research boat, team
members collect samples in huge funnels suspended about 30 meters below
the surface. “They fill up with muck,” explains Noel Urban (civil
and environmental engineering). “Most of it is sediment that was
resuspended during storm activity, plus the algae that have grown and
died, plus river sediments.”
Reading the muck is a critical,
focused, sometimes tedious, business. But examining the contents of sediment
has led to some interesting conclusions about Lake Superior.
The first is that the amount
of phosphorus in the lake limits plankton growth. “The plankton only
grow until the phosphorous limits further growth,” Green said. “We
are finding uniform phosphorous levels in the lake. Even though it may
be washing in, it’s being consumed by plankton down to a certain
level.” If you added more phosphorous, you’d stimulate plankton
production.
The scientists have also found
that a band of algae forms at a depth of 30 meters in the summer. Called
the deep chlorophyll maximum (DCM), the band is five-to-seven meters thick.
This phenomenon, relatively rare in fresh water, may be a settling of
organic matter that accumulates as it hits the cooler denser water below.
Zooplankton, a primary fish
food, gather at the DCM to feed on the algae. According to Martin Auer,
“feeding by zooplankton generates a lot of dissolved organic carbon
and so bacteria also love to live in that layer.” Disturbances to
the DCM could change the feeding cycle of organisms in the lake.
Another interesting discovery
is that Lake Superior is a net generator of carbon dioxide, unlike oceans
which absorb it. Phytoplankton absorb carbon dioxide and zooplankton expire
it. But Superior doesn’t have enough plant life to absorb it all.
Global warming concerns make the effect of lakes and oceans on carbon
dioxide an important consideration.
Future Ramifications
Charles Kerfoot (biological
sciences), who studies the remains of phytoplankton found in the sediment,
says the near-shore environment has increased its productivity by seven
to twelve fold. The reason seems to be increased concentrations of phosphate.
Because of the thermal barrier, “the phosphorous tends to be constrained
near the shore region.”
Further, a lot of the species
that are characteristic of the near-shore zone were never in the lake
originally. Sediment cores show a type of zooplankton named Daphnia has
had successions of three species replacing each other within the boundary
of a hundred years.
Kerfoot says that “the
ones that are there right now are not the same that were there 125 years
ago and are not the same that were there 30 to 40 years ago. A rapid replacement
of species suggests progressive eutrophication.”
The increased productivity
near-shore could mean a shift away from cold-water fish species like herring,
trout, or steelhead; and warm-water species like perch or walleye could
start to dominate.
Urban points out: “The
more you stimulate plankton production, the more fish you can catch--up
to the point where the pollution dirties the lake, then it is bad for
cabins, resorts, and recreation in the water.”
A Voyage of Discovery
These kinds of changes take
decades to occur, but tracking them is important fundamental science.
“We’re taking a system that’s been poorly studied, and
we’re trying to find out how it works,” explains Kerfoot. “Then
we’ll know how to ask the right questions when problems do come up.”
One hundred forty years ago,
Louis Agassiz, a Swiss scientist who worked at Harvard, wanted to explore
the great unknown Lake Superior. He discovered fish that had never been
seen; he was one of the pioneer explorers of the biology of Lake Superior.
Kerfoot leans forward and explains
earnestly, “we’re kind of doing the same thing. Only Agassiz
was limited to a fish net and a hand glass. Now we have sophisticated
analytical tools, radioisotopes, special microscopes, all sorts of things.
“We’re continuing
that voyage of discovery that Agassiz started many many years ago. Some
day people are going to build on the work we do. And when there are problems
out there in Lake Superior, they’ll know how to fix them, because
we were there first and learned how it worked.”
Above, Sarah Green, a chemistry professor at Michigan Tech and
director of the five-year KITES project.
The
underlying theme of KITES is transport of materials within the lake, from
“resting eggs” to larval fish, from “stamp sand” to
algae. Three major regions of data collection include the mouth of the
Ontonagon River, the area along Eagle Harbor, and off the point of the
Keweenaw Peninsula.
Unlike
the perception that Lake Superior has two temperatures, cold and really
cold, the KITES team found that the lake temperature varies widely during
the year and throughout the lake. Coastal waters warm up and cool off
faster than the deep central waters.
Cold
as it is, the lake has been slowly warming. Warmer water can support more
life. For example, there is six to eight times more plankton in Lake Michigan
than in Lake Superior. The coastal waters of Superior show what scientists
call “progressive eutrophication,” meaning these waters are
becoming more productive--more phosphorous, more algae, more bacteria--and
showing more change toward growth of warmer water species.