Lake Superior

Category: Inland Aquatic Biomes.

Geographic Location: North America.

Summary: The largest freshwater lake on Earth by surface area and the third-largest by volume, this is an ecosystem altered by overfishing, invasive species, and industrial activities.

Lake Superior exceeds all other lakes in the world by surface area, has the third-greatest volume of water for a freshwater lake, and contains the deepest point in the United States. While its formation during the last glaciation was geologically recent, on its shores lie some of the most ancient rocks on Earth. An inland ecological system that has been known by many names and has seen human activities come and go, Lake Superior has undergone troubling changes in water quality and aquatic species.

Geography, Hydrology, Climate

With a surface area of 31,820 square miles (82,410 square kilometers), Lake Superior is the largest expanse of freshwater in the world. Containing 2,900 cubic miles (12,100 cubic kilometers) of water, it is the third-largest lake by volume, and contains more water than all of the other Great Lakes combined. In surface size, it spreads 350 miles (563 kilometers) in length and 160 miles (257 kilometers) in width, and is bounded by 2,726 miles (4,387 kilometers) of shoreline in Minnesota, Wisconsin, and Michigan in the United States, as well as the Canadian province of Ontario along most of its northern waters.

So large is this lake that the summer sun sets about 35 minutes later on its western shore than its eastern shore. Its maximum depth of 733 feet (223 meters) below sea level represents the deepest point of the contiguous continental United States. Its surface averages 600 feet (183 meters) above sea level.

Like all of the Great Lakes, Lake Superior formed when massive glaciers hollowed out its basin during the Laurentian, or Wisconsin, glaciation 12,000–7,000 years ago. When the climate warmed and the glaciers receded, their meltwater filled the basins. As the westernmost lake in the chain, Superior accepts water from more than 200 rivers. Water leaves the lake via the St. Mary’s River to Lake Huron.

Rock on the northern shore of Lake Superior is some of the oldest found on Earth. In particular, the north shore granites formed from magma in the Precambrian Period (between 4.5 billion and 540 million years ago).

Due to its large size, Lake Superior affects regional weather in several ways. Its waters hold the heat of summer and the cold of winter longer than the surrounding land, moderating temperatures as seasons change. Overall, the climate around Lake Superior is more maritime than inland temperate, and resembles Nova Scotia more than Ontario or the surrounding states.

The most pronounced impact is lake effect snow, when westerly winds pick up moisture as they blow over the lake; upon reaching the colder eastern shore, heavy snow falls. Lake effect snows extend 20 to 30 miles inland, primarily on the Ontario shore southeast of the town of Marathon, and from the transboundary city of Sault Ste. Marie to the Wisconsin-Michigan border, that is, nearly the entire south shore. Average annual snowfall in Michigan’s Keweenaw Peninsula exceeds 200 inches in places.

The average annual water temperature is about 40 degrees F (4 C). The lake seldom freezes over completely in winter; the last time was recorded in 1979. Recent metrics indicate several changes in the lake. Summer surface water temperature has increased by 4.5 degrees F (2.5 degrees C) since 1979, whereas summer temperature on the surrounding land has increased 2.7 degrees F (1.5 degrees C) during the same period. While the lake water seems to be building up heat faster than the land, it may also be evaporating faster. In the summer of 2007, monthly historic surface-level lows were set: August at 0.67 feet (0.20 meters) below norm, and September at 0.58 feet (0.18 meters) below the norm, which is 601 feet (183 meters).

The situation turned around beginning in 2014, when cooler temperatures, more ice cover, and higher precipitation caused an increase in lake levels. By 2019, Lake Superior had reached record high levels, topping out at over 603 feet (184 meters).

Biota

Lake Superior is an oligotrophic lake, meaning that it has a low nutrient budget because the surrounding soils release few nutrients. Its deep, cold waters are less productive than those of the other Great Lakes; still, it is home to a variety of species of fish and other organisms. The food web is based on the primary producers, the phytoplankton, here mainly green algae, blue-green algae, diatoms, and flagellates. These are consumed by a variety of filter-feeding animals, including zooplankton, shrimp, snails, and clams, which in turn are consumed by fish.

Over 80 species of fish have been recorded in the lake. These include key native species such as muskellunge (Esox masquinongy), pumpkinseed (Lepomis gibbosus), bloater (Coregonus hoyi), northern pike (Esox lucius), lake sturgeon (Acipenser fulvescens), longnose sucker (Catostomus catostomus), brook trout (Salvelinus fontinalis), and burbot (Lota lota).

Introduced fish in Lake Superior include freshwater drum (Aplodinotus grunniens), rainbow trout (Oncorhynchus mykiss), Atlantic salmon (Salmosalar), rainbow trout (Oncorhynchus mykiss), and brown trout (Salmotrutta)—as well as some particularly damaging invasive species, such as sea lamprey (Petromyzon marinus), round goby (Neogobius melanostomu), white perch (Morone americana), ruffe (Gymnocephalus cernuus), and various Asian carp species, such as the common carp, Cyprinus carpio.

This rich fish community forms the basis of a historically important commercial fishery, based especially on lake whitefish (Coregonus clupeaformis), yellow perch (Perca flavescens), rainbow smelt (Osmerus mordax), and cisco or lake herring (Coregonus artedi). The first records, from 1879, of commercial lake whitefish indicate 2,356 thousand pounds (1,069 kilograms) of production. Catches have fluctuated over the years, but remain high, with 2,481 thousand pounds (1,125 kilograms) reported from 2006, for example. In 1941, commercial catches of lake herring peaked at almost 41,000 pounds (18,597 kilograms).

Fish populations have declined due to overfishing and parasitism by the nonnative sea lamprey, which attaches its mouth to other fishes and scrapes away the flesh with a sharp tongue and teeth. Fish victims often die from fluid loss and infection after several attacks.

Threatened plants in the Lake Superior biome include pitcher’s thistle (Cirsium pitcheri), which grows in the sandy beach and dune habitat at various points along the shore; Fassett’s locoweed (Oxytropis campestris var. chartacea), found between seasonal water-level highs and lows; American hart’s-tongue fern (Asplenium scolopendrium var. americana), a fern that prefers to grow upon stone surfaces; dwarf lake iris (Iris lacustris), a denizen of gravel, moist sand, and tree-shaded areas; and Houghton’s goldenrod (Solidago hoghtonii), which prefers moist environs such as limestone crevices and the swales between dunes.

Human Interaction

Unlike the other Great Lakes, Lake Superior does not have large cities or a dense population along its shore. The world’s farthest-inland port, Duluth (population 86,265 in 2010), sits on the far western point of the lake. Thunder Bay, Ontario, is the largest city on Lake Superior, with a population of approximately 125,000. Because of this, Lake Superior has historically been the least polluted of the Great Lakes, but human activity still poses problems for the lake.

The Lake Superior region is rich in minerals and wildlife, and many of today’s towns and cities grew from mining settlements or fur trading posts. Iron, copper, silver, gold, nickel, and uranium are all mined in the area and transported via the lake to global markets. Iron, in particular, has been a major enterprise here. Between 1875 and the 1950s, iron mining dominated industry on much of the north shore of Lake Superior, and contended with grain shipments for top commodity shipped on its waters. By 1896, one harbor was shipping more than 2.2 million tons (2 million metric tons) of iron annually. This led to the expansion of the fleet of lake freighters that ply Great Lakes waters, the St. Lawrence River, and thus the Atlantic Ocean. Although mining techniques and locations have changed, it still comprises a significant portion of the industry and shipping traffic in the Lake Superior region—and contributes to pollution. In particular, the mining industry has introduced mercury into the water. This mercury is taken in by plankton and then carried up the food chain to fish that are caught and eaten by humans, which can be a health risk. While the amount of mercury released into the lake was highest in the late 1800s and early 1900s and has been lower since, large amounts of mercury made their way into the lake's sediment at that time.

The French, and later other Europeans, entered the fur trade here in the 17th century. The eventual depletion of widely abundant populations of fur-bearing animals caused many settlers to leave the shores of Lake Superior; some companies re-established farther west. Lake Superior today attracts significant tourism, often taking the form of hunting and fishing participants. Isle Royale, Minnesota, for example, which itself contains several lakes, boasts moose and wolf populations that bespeak its relatively intact habitat. Isle Royale is a U.S. National Park; Madeline Island and the Apostle Islands in Wisconsin; and Grand Island, a National Recreation Area in Michigan, are other favorite destinations of boaters.

To combat pollution, the United States and Canada in 1991 formed the Binational Program to Restore and Protect the Lake Superior Basin, a body committed to administering Lake Superior as a demonstration area to prohibit any point source discharge of any toxic, persistent substance. Zero discharge procedures and compliance are monitored by three groups: the Lake Superior Task Force, made up of senior managers from government and environmental bodies; the Lake Superior Work Group, providing technical direction and policy guidance; and the Lake Superior Binational Forum, where representatives from the public and industry regularly air the issues and examine solutions.

The Binational Program has since taken up related issues that affect the biome, such as a comprehensive attack on the deleterious impacts of invasive plant and animal species in Lake Superior. The group has initiatives for monitoring and prevention, ship water ballast testing, mitigation and restoration measures for shoreline habitats, and outreach to such entities as permitting agencies, fishing outfitters and marinas, nurseries, and garden centers.

Bibliography

“Lake Superior Climatology.” Great Lakes Integrated Sciences and Assessments, 2022, glisa.umich.edu/sustained-assessment/superior-climatology/. Accessed 31 Aug. 2022.

Mills, Allison. "The Legacy of Mercury in Lake Superior." Phys.org, 18 Mar. 2016, phys.org/news/2016-03-legacy-mercury-lake-superior.html. Accessed 27 July 2018.

Nissen, Jack, and Karen Hopper Usher. "The State of the Great Lakes: What to Expect from Climate Change." Great Lakes Echo, 24 May 2017, greatlakesecho.org/2017/05/24/the-state-of-the-great-lakes-what-to-expect-from-climate-change/. Accessed 27 July 2018.

Ostlie, Wayne. Great Lakes Ecological Assessment—Threatened and Endangered Plants. Minneapolis, MN: The Nature Conservancy, 1990.

Risjord, Norman K. Shining Big-Sea Waters: The Story of Lake Superior. St. Paul: Minnesota Historical Society Press, 2008.

Thomas, Amy, Sue Greenwood, Roger Eberhardt, Elizabeth LaPlante, Nancy Stadler-Salt, et al. Lake Superior Aquatic Invasive Species Complete Prevention Plan. Chicago: Lake Superior Binational Program, 2009.