Rich ecosystems in our backyard.
When we think of an estuary, we usually picture a river flowing into a bay and then into the ocean. The Chesapeake Bay, the Mississippi Delta, and Hudson Bay are key parts of North America’s natural and human ecology. The mixture of fresh water and salt water creates a unique ecosystem which welcomes spawning fish, migrating birds and other wildlife. And wetlands act as a sponge to filter pollutants and protect shorelines from erosion and floods.
Not all estuaries are a mix of salt water and freshwater. In the Great Lakes, estuaries abound at all scales. The most popular tourist destination on the North Shore of Lake Superior, Gooseberry Falls, is an estuary where the water slows as it approaches the lake.
Great Lakes estuaries are classified as drowned river valleys or rias. When the most recent of North America’s glaciers melted about 12,000 years ago, the land that had been pressed down by the glacier rebounded and the surface tilted, causing lakeshores to the south to sink, “drowning” river mouths. (The same geologic process created the estuaries on the Atlantic coast.)
In Lake Erie, Old Woman Creek, one of the lake’s few estuaries retaining its natural characteristics, is designated a National Estuarine Research Reserve. Scientists conduct research and have long-term data on water quality, weather, biological communities, habitat, and land-use and land-cover characteristics.
St. Louis River
The largest tributary to Lake Superior on the U.S. side, the St. Louis River, creates an estuary that once teemed with wild rice, birds, and fish, including the ancient sturgeon. Before immigrants arrived, Dakota people lived here, and tradition says the Anishinaabe people came from the East Coast following a prophesy that they should travel until they found “food that grows out of the water,” the wild rice that would sustain them forever.
Early immigrants from Europe were also drawn to the rich estuary. Industries such as sawmills, paper mills, oil refining, and shipping degraded the lower reaches of the river so thoroughly that by the 1970s people couldn’t eat the fish or swim in the water. The river was dead—so polluted that decomposition of nutrients was eating up the oxygen. People avoided the estuary because it was so smelly.
In 1987 the United States and Canada signed the Great Lakes Water Quality Agreement, which committed the two nations to cleaning up historic pollution in 43 locations around the world’s largest source of fresh water. The federal government has invested $3.48 billion in these cleanups under the Great Lakes Restoration Initiative since 2010.
The St. Louis estuary is the second largest such area in the United States. Cleanup projects address pollution in the water, degraded habitat, fish tumors and other deformities, aesthetics, beach closures, and limits on human consumption of fish. Thirty years of effort have resulted in a newly functioning ecosystem and an attractive place for people.
Deanna Erickson is the Interim Reserve Manager at the Lake Superior National Estuarine Research Reserve, based in Superior, Wisconsin. She says although it’s a freshwater system, it behaves a lot like a saltwater estuary. Most importantly, there is a mix of different waters. “In the river, the water is dark—it has a lot of tannins leached from decaying plant material—and it’s generally a lot warmer and more nutrient-rich than Lake Superior,” she says. This mixing creates a valuable overlap of habitats: the warmer, protected water in the river, and the cold, sometimes harsh conditions of the Lake. Many plants and animals can use both places for different life functions, making ideal conditions for a rich diversity of plants and animals to thrive.
That all-important mixing is caused by the flow of water from the river into the lake, of course. But there’s another force at work. It’s the seiche (pronounced ‘saysh”), the periodic rise and fall of water levels in the Lake, caused by pressure changes, including wind and storms. This can actually cause the river to flow backward every few hours. “Stand at Duluth’s lift bridge and look at the water: half the time it’s going upstream, half the time it’s moving downstream,” says Erickson. It’s very similar to a tide, and that movement carries oxygen into the more stagnant back bays of the estuary. “The seiche is helping the river get a breath of fresh air,” she says.
Meanwhile, wetlands in the estuary feed the Lake. The rich, marshy habitat teems with plankton, algae, and macroinvertebrates that feed the fish that live most of their lives in the Lake, moving into the river to spawn. “Recent research suggests all the walleyes in western Lake Superior are spawning in the St. Louis River,” Erickson says.
Remediation of industrial pollution, coupled with enhancements for animal habitat and human enjoyment, have changed the estuary for the better. But threats remain. Shipping in and out of the nation’s inland-most port have brought invasive species. There are now 100 non-native species in Lake Superior. “I worry about the impacts of those invasives,” says Erickson. “Combined with climate change, they might do very well, and at the same time climate change can make it more difficult for native species to do as well as they should.” Increased flooding due to climate change also brings more pollutants and silt into the bay.
H. J. “Bud” Harris has spent his entire professional life trying to figure out how to bring back the rich ecosystems of another Great Lakes estuary in Wisconsin’s Green Bay. Harris is Professor Emeritus of Natural and Applied Sciences at the University of Wisconsin-Green Bay. He marvels at the complexity of this big estuary where the Fox River empties into Lake Michigan.
What makes it so special? “It’s all about water levels,” Harris says. The Bay experiences changing water levels at several scales and time cycles. Long-term fluctuation can change water levels as much as five feet. Based on beach ridges along Lake Michigan and radiocarbon dating of soil core samples, scientists have identified a high-water cycle of 120-200 years, and within that cycle a shorter cycle averaging 32 years. This year, levels have been very high.
Meanwhile, an annual cycle brings high water in midsummer after spring runoff. In addition, there’s the seiche. “In Green Bay it’s an 11-hour period, very regular, almost like a tide,” says Harris. “There’s a tremendous exchange of material in this cycle, inorganic matter going out, organic matter being delivered to the wetlands.”
Of course, different plants are adapted to live in different water levels, and over the years, the system goes through a repeating pattern of plant dominance. When water levels go down dramatically, the substrate emerges; that’s the soil below the water. At that point, Harris says, a new succession of plants and associated bird life begins. “Some plants, such as bur reed (genus Sparganium), produce seeds that can be buried underwater for decades. When the conditions are right, the seeds germinate and begin a predictable progression of plant life. At first you get low-growing plants, like water marigolds (genus Bidens),” he says. Then bulrush (genus Scirpus), will thrive, providing food and nesting material for muskrats, waterfowl, and Forster’s terns, which sometimes build their nests on muskrat lodges. This phase persists for three to four years, and meanwhile another group of plants comes in, robust emergent plants including cattails, that provide homes for Yellow-headed Blackbirds and Least Bitterns. The progression takes about seven years and then repeats itself.
Plants growing below the water are also important. Vallisnaria or eelgrass provides food for diving ducks, and geese consume underwater plants ravenously. Unfortunately, the Fox River and others that flow into the Bay carry lots of floating particles, runoff from the watershed. This blocks light and can interrupt natural development. “We’re now trying to manage the uplands to make the Fox River and others flowing into the Bay less turbid,” says Harris. “We can’t get real restoration until we solve this runoff problem, and that’s going to require major changes in agriculture,” he says.
The long, narrow shape of Green Bay produces an interesting set of currents. The long spit of land jutting into Lake Michigan is part of the Niagara Escarpment, the arc of bedrock that curves from Niagara Falls through the Great Lakes to emerge as the Door Peninsula. Warm water from the Fox River flows north into the Lake hugging the eastern edge of the bay. Colder water from the main part of the Lake flows south along the west shore. “You’ve got a counter-clockwise movement of water here, and it forms several large gyres, circular currents about a third of the way up the bay,” Harris explains. Nutrient pollution from the river tends to drop here among these currents, feeding algae. As the algae decompose they consume oxygen, creating a dead zone in this part of the bay.
The pollution historically came from sawmills, paper mills, and other industry that settlers built. As the population grew, human waste was dumped directly into the river. At the worst point there was literally no oxygen in the lower Fox River. “We’d get mats floating up from the bottom because of the anaerobic conditions,” Harris recalls. The bacterial action created methane which pushed rotting materials to the surface. “We used to say, ‘The river burped.’”
The city built a sewage treatment plant in the 1930s, and Green Bay was also the site of massive cleanup efforts as part of the Great Lakes Water Quality Agreement.
Millions of cubic yards of sediment have been dredged from the river, to remove polychlorinated biphenyls, PCBs, and other pollutants, mostly from paper mills. Now scientists monitor the water, observing dramatic decreases in pollutant concentrations. Harris notes that erosion will continue to release some of the pollutants, and “It’s my guess we’ve got another 20 years before it’s safe to eat some of the fish.”
Water quality has improved so much that researchers have re-introduced burrowing mayflies (genus Hexagenia), a key part of the food chain, which was extirpated from the bay. Harris calls it the “canary in the coal mine,” and is encouraged to see the success of the effort.
The important thing to understand about Green Bay, and about all natural systems, Harris says, is that they are constantly changing. “People see a place at one point in time and think that’s how it is, but the system is dynamic, which is not only natural, but necessary,” he says.
Estuaries worldwide are among the most productive and life-supporting environments on the planet. Knowing this can help us protect and enjoy them more deeply.