Superior Confidence

From curlicue currents to the graveyard where algae go to die, Michigan Tech scientists are teasing new knowledge loose from the world's largest lake.

Superior, they say, never gives up her dead. This greatest of the Great Lakes is also loath to part with her secrets. Three years after the conclusion of the most comprehensive study ever undertaken of Lake Superior, every discovery has led to new questions.

The five-year, $4.1-million KITES project (Keweenaw Interdisciplinary Transport Experiment in Superior) began in 1998, with Michigan Tech leading eight institutions in numerous studies in and around the Keweenaw Eddy. This river in a lake pokes along the western shore of the Keweenaw Peninsula at a mere one mile per hour. But in one year, it moves as much water as the Mississippi River pours into the Gulf of Mexico over the same duration.

The scientists looked at the current up close, in boats, and from far away, using satellite images. And they found some surprises.

"We saw some very interesting features on the satellite, including the Keweenaw Eddy," said Sarah Green, chair of the Department of Chemistry and KITES principal investigator. "It's a curlicue of chlorophyll that forms off the tip of the Keweenaw Peninsula, usually in late summer."

It's made of algae and other assorted plant material, and, from miles above the Earth, the eddy looks bright green. "The current shoots around the tip of the Keweenaw and then spirals into a curlicue," Green explains. Exactly what it is and why it forms bears further investigating, she says.

Nancy Auer, an associate professor of biological sciences, focused her KITES study on Diporeia, tiny, exquisite invertebrates that are part of the foundation of Lake Superior's food chain. These shrimp-like creatures are disappearing in the other Great Lakes, and nobody knows why.

If you're going to study something, you have to find out where they live, and over the course of the study, Auer discovered that Diporeia like to hang out on the floor of the lake, anywhere from fifty to one hundred meters below the water's surface.

"In any lake, everybody eats everybody," Auer says. "And Diporeia are a key element in the fish food web." Whitefish eat Diporeia, as do forage fish such as sculpin, which in turn are eaten by lake trout.

"If human-induced things are changing Diporeia, that will change Lake Superior's fish population as well," Auer predicted. "A lot of people up this way depend on fish for their livelihood, and not just fishermen. Think of all the tourists who come here for a whitefish dinner."

Through KITES, Auer and two dozen students gathered data on Diporeia for several years: a solid baseline from which to detect changes that could rattle the entire Lake Superior ecosystem. They still don't know why Diporeia are dying in the other Great Lakes, but, if it starts happening here, they'll be able to measure it-and perhaps do something about it.

Marty Auer, a professor of civil and environmental engineering (and Nancy's husband), studied something even smaller than Diporeia: the phytoplankton that they eat. He discovered that, like people, phytoplankton can be found in different parts of Lake Superior, depending on the season.

In spring, they grow in the warm, shallow waters close to shore, spreading out gradually as surface water heats up across the lake. But, by the end of summer, a dense layer of algae forms about thirty to forty meters beneath the surface, the boundary of the lake's cold, bottom layer and its warm (by comparison), upper layer.

This Deep Chlorophyll Maximum, or DCM, is found in oceans and in several clear lakes, but scientists are not in agreement about how or why it forms. Auer doubts that it occurs because algae prefer to set up housekeeping in cold, dark places. The reality, he thinks, is more grim: unlike fish, algae can't swim. Thus, they are doomed to sink down to the colder, denser water, struggle for life in miserable conditions, and finally die.

"They become trapped there," he says. "It's like an elephants' graveyard."

Biological Sciences Professor W. Charles Kerfoot found that the small animals known as zooplankton do have a choice in where they live.

"Intriguing data on zooplankton distributions showed how they hide from hungry fish," he says. "During the day, when they are more easily spotted, they hang out in the debris zone." Located just above the DCM, the shadowy debris zone is a resting place for organic flotsam and jetsam settling from the upper waters. Zooplankton can find food there while they blend in with floating particles, all but invisible to fish.

"Then at night, they migrate toward the surface where the tastiest phytoplankton are," Kerfoot notes.

For Green, Lake Superior's biggest puzzle is the Case of the Extra Carbon. Simply stated, plants inhale carbon from the atmosphere (in the form of carbon dioxide) during photosynthesis, animals and bacteria exhale it, and some of it sinks to the bottom of the lake in the form of dead organic material. These processes should be in equilibrium in a given ecosystem. However, analysis by Noel Urban, associate professor of civil and environmental engineering, shows that, in Superior, they may not be.

"It looks like there is more carbon going out than coming in, and we don't understand that," Green says. "We can't balance it."

It could be because the researchers have missed something, she says. Or their estimates were wrong. Or the carbon cycle fluctuates, and KITES didn't last long enough to detect it.

Or maybe the lake really is sending out way more carbon than it's taking in, which begs a very strange question: could this big, clean, crystalline lake be contributing to global warming through the greenhouse effect?

Indeed, Green wonders aloud if the carbon cycle has somehow been thrown out of balance by climate change. Nobody knows. But it's another good question to ask the lake.

       © 2006, Michigan Tech Magazine

       Michigan Tech Magazine | Fall 2006 | http://www.mtu.edu/