Diatoms—microscopic algae with glasslike shells of silica—are the major forms of algae, although green and blue-green algae are abundant during the summer in Lakes Erie, Ontario, and Michigan. Copepods and cladocerans, microscopic crustaceans, are important in the animal forms of plankton. Most abundant during the spring months in the upper lakes, plankton reaches two peaks of abundance—spring and fall—in the lower lakes and in the more productive waters of the upper lakes.

The organisms living on the bottom in shallow waters are the same kinds of snails, clams, worms, mayflies, and caddisflies found in most small lakes. The deep waters, however, are the realm of some organisms that are found only in the deep, cold lakes of the northern latitudes. These include the delicate opossum shrimp, the deepwater scud (a crustacean), two types of copepods, and the deepwater sculpin (a spiny, large-headed fish). During the 1980s two nonindigenous creatures—the spiny water flea (a crustacean) and the zebra mussel (a mollusk)—gained access to the lakes and established large populations. Both have threatened the food web of native species, and the zebra mussel has become a nuisance by clogging water-intake pipes, covering spawning reefs, and fouling boat hulls.

The fish community of the lakes and tributaries includes representatives of most families of North American fishes. Lake trout, whitefish, salmon, and lake herring have always been important in the lakes, while perch, walleye, bass, and catfish are abundant in the shallow, warmer waters. The fish population has changed drastically over the past century. By 1880 the damming and pollution of tributaries had caused the elimination of Atlantic salmon and the restriction of whitefish. At the same time, lake sturgeon were purposely overfished, and carp were introduced. Smelt entered Lake Michigan in 1927 and in a short time had spread throughout the lakes; however, since the early 1980s their populations have dwindled dramatically. The predatory sea lamprey migrated into the upper lakes and established spawning populations in the 1930s, causing the collapse of the lake trout populations in Lakes Huron and Michigan by the early 1950s. Other large predators also were drastically reduced in numbers by the sea lamprey. Consequently, when the alewife migrated into the upper lakes between 1931 and 1954, it met little competition and predation and soon became the most abundant species. Alewives are of little commercial value, however, and have caused costly sanitary operations when, periodically, millions of them die and are washed up to rot on the beaches. A major international program to control the sea lamprey reduced its population substantially; and during the 1960s lake trout were reintroduced when it appeared that the sea lamprey was under control. Coho and chinook salmon were introduced both to provide a sports fishery and to control the alewife. Among the accidently introduced species, the small, perchlike ruffe preys on the eggs of native fishes, while the bottom-dwelling round goby competes for food and habitat with indigenous species.

Herring and ring-billed gulls and terns are the most common birds; small islands are important nesting grounds for them. The lakes are important as wintering areas for ducks such as the scaup and the old-squaw duck, and a diversity of shorebirds and songbirds migrate through the region during the spring and fall. Various places along the shoreline, such as Point Pelée (Ontario) in Lake Erie and Whitefish Point (Michigan) in Lake Superior, are favoured locations for bird-watching.

Resources

Although most of the virgin pine forests in the basin were felled by 1910, timber remains important and is supported by both federal and state governments. About half of the land in the counties bordering the lakes in the United States is devoted to farming, while nearly one-third of that in the bordering Canadian provinces is under agriculture. The major crops raised are corn (maize), soybeans, wheat, animal fodder, and fruits and vegetables; pork, beef, and dairy products are produced as well.

The ranges around Lake Superior—such as the Mesabi in Minnesota and the Marquette in Michigan—are a major source of iron ore for the United States. Peak production occurred in 1953, when almost 100 million net tons were produced. The large deposits of rich ores have since been depleted, but low-grade taconite ores are now efficiently processed into iron-ore pellets. Lake Superior’s Keweenaw Peninsula was once a major source of copper; sources outside of the lakes are now relatively more important. Sand dunes along the eastern shore of Lake Michigan are valued for their beauty and as a source of clean sand for industry.

Of major importance is the water supply that the lakes provide for bordering industries and municipalities. Hydroelectric generating stations exist on the St. Marys, Niagara, and St. Lawrence rivers. Numerous coal- and nuclear-powered generating plants around the lakes utilize lake water for cooling.

Industry and transportation

The industry of the lakes region is highly diversified. The large steel mills in Illinois, Indiana, Ohio, Michigan, and Ontario and the automobile industry centred in the Detroit area produce a large proportion of the continent’s steel and motor vehicles. Nonetheless, many more workers are employed in the service industry.

Most of the total shipping tonnage includes iron ore, coal, grain, and limestone for lake ports, but some tonnage is shipped overseas through the St. Lawrence Seaway. The Welland Canal allows passage around Niagara Falls from Lake Ontario into Lake Erie, and the channels and locks in the St. Marys River at Sault Ste. Marie have made Lake Superior accessible to ships up to 800 feet in length.