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Never-ending Motion: Lake Superior's Wetlands

Old photograph of Duluth's harbor area as it looked it 1869. Prior to the dredging of the shipping canal, the area was peatland

The Duluth Harbor area is it looked in 1869, replete with peat. Paul B. Gaylord, courtesty of the Northeast Minnesota Historical Center.


The harbor area as it looks today. The peatland has been completely replaced by deep, open water.

The harbor area as it looks today.

Similar to salt marshes fringing oceanic coasts, the wetlands of Lake Superior are crucibles of life. They cover far less than one percent of Lake Superior's surface but they are vastly more important than their apparent size, generating much of the energy fueling the system's food web.

"Coastal wetlands in Lake Superior are critical habitat for fish as spawning areas, as nursery areas, and as feeding areas," said Janet Keough, acting director of the U.S. Environmental Protection Agency's Mid-continent Research Division. "Over 50 species of fish are closely associated with coastal wetlands and water flow determines what lies at the base of their food chain."

Keough made these points during two well-received talks hosted by Minnesota Sea Grant in Duluth and Grand Portage, MN. She described that lagoon and barrier, riverine, and embayments are the most common forms of about a half-dozen wetland types on Lake Superior. Minnesota's coast sports some alder slough embayments, but the topography is so steep that wetlands mainly form where streams slow and mesh with the brisk water of the lake (riverine). Both the more unusual delta and dune-generated wetlands occur along the south shores of Lake Superior. Open shoreline wetlands persist only by surviving battering waves and scouring ice; they are uncommon around Lake Superior.

In this wet and hungry world, the food chain is influenced by relatively calm water, active water, or some combination thereof. Plankton float freely and abundantly in slow-moving water, but the algae living on the surface of plants (epiphytic algae) becomes more plentiful fodder as the flow of water increases.

Although free-floating plankton and epiphytic algae have quite a bit in common, they incorporate different forms of carbon and nitrogen into their cells. Using a technique that contrasts these different elements, researchers can identify the life stage and diet of a fish through its carbon and nitrogen "fingerprint." When tracking the veiled inner workings of coastal habitats, carbon and nitrogen fingerprints reveal who (or what) was for dinner. For instance, before it's swallowed by a walleye, a hungry minnow might have gulped free-floating plankton or grazed algae from the stems of bulrushes for most of its life. Knowing this information illuminates the life of the fish, and allows researchers to understand aspects of the environment in which it swam.

Water Action!

About every 12 hours and 25 minutes it's "high tide" on Minnesota's coast. With a subtle tug, the moon pulls Lake Superior skyward by less than several centimeters, a measure so modest it's easily masked by waves and swamped by seiches (seiches are defined on the back page of this newsletter). Still, such physical forces over such a large volume of water make Superior's coastal areas neither like the tide-washed shores of an ocean nor the wave-lapped shores of an inland lake.

"It's almost like the Great Lakes have a third type of coastline," said Keough. "They're not oceanic, but they're certainly subjected to more frequent and greater changes in water level than smaller lakes."

Seiches, those surges of water reverberating across the basin, are timed in hours and measured in multiple inches. Overlaying these daily fluctuations, seasonal weather freezes, floods, then dries Superior's shallows in an annual rhythm. Beyond this, multi-year periods of high and low water create, destroy, and alter wetlands in kaleidoscopic patterns.

Seiches rock the lake during the entire ice-free season flushing wetlands with water as well as oxygen. The cold, nutrient-impoverished water of Lake Superior is typically saturated with oxygen. Seiche activity can influence the composition of the fish community simply by delivering oxygen to backwaters that can become low enough in oxygen to discourage fish.

The vegetation and plant diversity of coastal wetlands thrive on the fluctuations. Fed by snow, rain, rivers, and groundwater, Lake Superior swells to its September/October peak, about a foot above a March/April trough. Lake Superior water levels peak months after inland lake levels begin a descent into the growing season.

Peat the Disappearing Act

Marshes, with their hydrophilic (water-loving) sedges, rushes and reeds, dominate Great Lakes wetlands but Lake Superior retains something that the other Great Lakes have virtually lost peatland. Partly due to less human activity, peatlands, replete with sedges and sphagnum, still occur in many wetlands hugging Lake Superior's coast.

"Peatlands, sometimes called fens, were once very common along the Great Lakes, now they're almost extinct in the lower Great Lakes," said Keough. "We changed the coasts by adding nutrients, changing the water flow, and through shoreline development so peatland couldn't persist. My feeling is that most Lake Superior wetlands had peat in their backwaters; unique floating islands with orchids and other rare native plants.

"Lake Superior has some of the best, most pristine wetlands of all the Great Lakes," said Keough. "These wetlands and their associated plant and animal life are influenced by our actions but they are sustained by nature's unfettered hydro-power."

This power complies with the dictums of wind, seasonal cycles, and global patterns. Its forces are evident in Lake Superior's dynamic wetlands, which follow neither oceanic nor inland paradigms.

Her presentation was part of the "Superior Science for You!" speaker series made possible with a grant funded under the Coastal Zone Management Act by NOAA's Office of Ocean and Coastal Resource Management in conjunction with Minnesota's Lake Superior Coastal Program.


By Sharon Moen
November 2003

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