Great Lakes ecosystem

The Great Lakes are five bodies of water spanning the eastern United States and the southern Canadian border. Together, Lake Superior, Lake Michigan, Lake Huron, Lake Erie, and Lake Ontario comprise the largest group of lakes on Earth's surface, covering over 94,000 square miles (244,000 square kilometers), bordered by approximately 10,9000 miles (17,549 kilometers) of shoreline, and containing an estimated 35,000 islands. This one continuous drainage basin spans 750 miles (1,200 kilometers) and contains one-fifth of the world's surface freshwater, totaling 6 quadrillion gallons of fresh water. Their combined surface area is larger than the combined surface area of the US states of New York, New Jersey, Connecticut, Rhode Island, Massachusetts, Vermont, and New Hampshire. The Great Lakes are framed by the states of Minnesota, Wisconsin, Illinois, Indiana, Michigan, Ohio, New York, and Pennsylvania, and the provinces of Quebec and Ontario. The United States has more shoreline on the Great Lakes than it has on the Atlantic Ocean and Gulf of Mexico combined. Michigan's 2,232 miles (3,592 kilometers) of shoreline is second only to Alaska's. The longest suspension bridge in the world, the Mackinac Bridge, spans Lake Michigan and Lake Huron across the Straits of Mackinac. Finished in 1957, the bridge is 5 miles (8 kilometers) long and rises 552 feet (168 meters).

In addition to their notable size and volume, several other features make the Great Lake ecosystem unique among other lake ecosystems. The largest island in an inland body of water, Manitoulin Island, is in Lake Huron. The island itself has over one hundred fresh water lakes, the largest of which is Lake Manitou, which is the world's largest freshwater lake located on a freshwater island.

The Great Lakes formed when the Laurentide, or Wisconsin, glaciation began to retreat 14,000 years ago. As the edge of the nearly 2-miles-thick (3.29 kilometers), moving ice sheet melted, the giant basins carved by the glacier filled with water, forming huge Lake Agassiz, which held more water than all lakes on Earth today combined. The repeated drainage events of Lake Agassiz formed the first versions of the Great Lakes, which were initially much larger than they are today, and flowed east to west. The lakes continued to change shape and size as the climate shifted and the land recovered from the weight of the glaciers, a process known as isostatic rebound. Around 4,000 years ago, the Great Lakes took on their present configuration. They flow from west to east, starting at Lake Superior's southeastern corner through the St. Mary's River to Lakes Michigan and Huron, then down the St. Clair and Detroit Rivers to Lake Erie, then flowing north through the Niagara River, over Niagara Falls, and into Lake Ontario, and finally into the St. Lawrence River, joining the Atlantic Ocean over 2,000 miles (3,219 kilometers) from its origins.

Climate, Water Levels, and Lake Stratification

Great Lakes weather is affected by three prominent air systems. There is a very dry and cold arctic system that comes from the north; another dry, but warm Pacific system that comes from the west; and finally, a warm, wet tropical system from the south and the Gulf of Mexico. Because of their large size and volume, the Great Lakes produce “lake effects” on regional weather. For example, the eastern shores and nearby land in Michigan, Ohio, Pennsylvania, New York, and Ontario can receive especially heavy snowfall. This happens when westerly winds blow across ice-free water in the middle of the lakes in the winter, picking up warmth and moisture. When these moisture-laden winds reach the colder landmasses on the eastern shores, heavy snowfall can occur. Another “lake effect” is the moderation of seasonal temperatures in a band around each lake. The latent summer heat in the water causes later autumn frosts, and the cold water keeps summer temperatures cooler lakeside than they are inland.

Like all lakes, the Great Lakes are dynamic bodies of water that undergo complex processes and have a variety of subsystems that change seasonally and on longer cycles. One such cycle is the stratification or layering of water in the lakes because of changes in water density caused by seasonal temperature changes. Colder water is denser, so it stays deep in the lake, forming a layer called the hypolimnion during the summer months when the sun warms the surface water, forming a layer called the epilimnion. A middle layer, called the thermocline, is an area of rapid temperature change between the cold and warm waters. Scientists reported in 2015 that the summer stratification for Lake Superior is earlier each year by approximately one half day each year, which in turn makes the fall mixing of warm and cooler waters later. Scientists from the American Society of Limnology and Oceanography noted in 2008 that the summer stratification season of Lake Superior increased from 145 days to 170 days during the previous one hundred years.

The warm waters of the epilimnion are the most productive, growing algae that form the base of the food chain. When temperatures fall in autumn, surface waters cool, becoming denser and eventually sinking, causing a mixing of layers, called turnover. In winter, surface waters are often colder than those below. The layering and turnover of water annually are important for water quality. Turnover is the main way in which oxygen-poor water in the deeper areas of the lakes can be mixed with surface water containing more dissolved oxygen. This prevents anoxia, or complete oxygen depletion, of the lower levels of most of the lakes. However, the process of stratification during the summer also tends to restrict dilution of pollutants from effluents and land runoff.

Water levels also go through cycles in the Great Lakes. Seasonal fluctuations of about 12 to 18 inches (30 to 46 centimeters) result from regular changes in water amounts in the lakes. In the fall and early winter, when the air above the lakes is cold and dry and the lakes are relatively warm, evaporation from the lakes is greatest. This can be seen when fog or mist forms and appears to float over the lakes. Consequently, water levels decline to their seasonal lows. As the snow melts in the spring, runoff to the lakes increases. Evaporation from the lakes is also least in the spring and summer, when the air above the lakes is warm and moist and the lakes are cold. With more water entering the lakes than leaving, water levels rise to their peak in the summer.

Changes in water levels can also be short-term. For example, a strong sustained wind from one direction can push the water level up at one end of a lake, which makes the level drop by a corresponding amount at the opposite end. These events are most common on Lake Erie because of its east–west orientation in an area of prevailing westerly winds and its generally shallow western end. Long-term fluctuations occur over periods of consecutive years. Continuous wet and cold years will cause water levels to rise. Likewise, consecutive warm and dry years will cause water levels to decline. Over the last century, the range from extreme high to extreme low water levels has been nearly 4 feet (1.2 meters) for Lake Superior, and between 6 and 7 feet (1.8 and 2.1 meters) for the other Great Lakes.

Aquatic Life

It is estimated that there were as many as 139 species of fish indigenous to the Great Lakes. Those inhabiting the nearshore areas included smallmouth (Micropterus dolomieu) and largemouth bass (Micropterus salmoides), muskellunge (Esox masquinongy), northern pike (Esox lucius), and channel catfish (Ictalurus punctatus). In the open water were lake herring (Coregonus artedi), lake whitefish (Coregonus clupeaformis), walleye (Sander vitreus, formerly Stizostedion vitreum), sauger (Sander Canadensis), freshwater drum (Aplodinotus grunniens), lake trout (Salvelinus namaycush) and white bass (Morone chrysops), blue pike (Sander vitreus glaucus), and Atlantic salmon (Salmo salar). The last two species are now considered extinct from the Great Lakes. Because of their different characteristics, the lakes varied in their species composition.

There are at least 34 known non-native species of fish in the Great Lakes that were introduced to the lakes through the discharge of ballast water from cargo ships, intentional introduction (such as three species of Pacific salmon), migration around barriers in canals, and escaping from aquaculture facilities. Several species of non-native fish are considered invasive and harmful to the aquatic environment.

Warm, shallow Lake Erie is the most productive, while deep Lake Superior is the least productive. Currently, hatchery-reared coho salmon (Oncorhynchus kisutch) and chinook salmon (Oncorhynchus tshawytscha) are the most plentiful top predators in the open lakes, except in the western portion of Lake Erie, which is dominated by walleye. This diversity of fish formed part of a complex food web, with sunlight, dead organic matter, and bacteria at its base; plankton and small-bodied fish in the middle; and predaceous fish, birds, and humans at the top. Changes in the species composition of the Great Lakes basin in the last 200 years have been the result of human activities. Many native fish species have been lost by overfishing, habitat destruction, contaminants, or the arrival of exotic or nonindigenous species.

Early Inhabitants and Current Population

Because humans are thought to have been in North America as early as 12,000 years ago, they probably experienced some of the changes that the Great Lakes underwent. In particular, their shoreline settlements may have been affected by changes in drainage events. Over time, approximately 120 bands of native peoples have occupied the Great Lakes basin. Descendants of the first settlers were using copper from the south shore of Lake Superior 6,000 years ago. The population in the Great Lakes area is estimated to have been between 60,000 and 117,000 in the sixteenth century,when Europeans began their search for a passage to the Orient through the Great Lakes. The native people occupied widely scattered villages, fished and hunted, and grew corn, squash, beans, and tobacco. Present-day names for these tribes include the Chippewa, Fox, Huron, Iroquois, Ottawa, Potawatomi, and Sioux. Today, Native American fishing treaties protect the fishing rights of Great Lakes tribes.

The first written record of the Great Lakes is of Lake Huron, called La Mer Douce—the sweet sea—by French explorer Samuel de Champlain in 1615, and Lake Ontario appears shortly thereafter. Accounts of Lake Superior and Lake Michigan appeared in 1622 and 1634, respectively, and of Lake Erie in 1669. After European settlement, major cities grew up rapidly on the shores on the Great Lakes. The largest of these include Chicago, Milwaukee, and Detroit on Lake Michigan; Cleveland on Lake Erie; and Toronto on Lake Ontario. One-tenth of the population of the United States, and one-quarter of the population of Canada, currently live near the Great Lakes and depend on them for drinking water, industry, agriculture, recreation, food, transportation, and tourism.

Commercial Fishery

Fishing has been a historically important industry of the Great Lakes, increasing about 20 percent per year after 1820. Commercially important species include lake trout, salmon, lake whitefish, and yellow perch. The greatest fishing harvests were recorded in 1889 and 1899, at about 147 million pounds (67,000 metric tons). By 1929, tens of thousands of people were generating income from commercial fishing on the Great Lakes, with the first commercial fishing law in Michigan being instituted that same year. Over the course of the next several decades, however, commercial fishing and size of harvests decreased significantly due to overfishing, pollution, habitat degradation, and the introduction of exotic and invasive species. The value of the fishery also decreased when it shifted to smaller, low-value species. Catches of Lake Michigan whitefish declined from over 5,000 tons in 1930 to 25 tons in 1957. Even more dramatic, lake trout catch declined from almost 7,000 tons in 1943 to 3 tons in 1952. Populations of these species and others plummeted after the invasion of the lake by sea lamprey.

The commercial and sports fishery on the Great Lakes is estimated to produce over $4 billion annually and is dependant on a mix of native species such as yellow perch, whitefish, walleye and introduced species such as smelt (Osmerus), alewife (Alosa pseudoharengus), splake, a hybrid between a lake trout and a brook trout (Salvelinus fontinalis). and Pacific salmon (Salmo and Onchorynchus). Commercial fishing of lake trout is allowed only in Lake Superior, where populations appear stable. In addition, researchers estimate that 25 to 40 percent of the salmon and trout populations in Lakes Michigan, Huron, and Ontario are now self-reproducing, citing improved habitat, water quality, and stream conditions. A number of species, including coho and chinook salmon, are raised in hatcheries and stocked in the lakes. The Great Lakes Fishery Commission was established in 1955 by the Canadian/U.S. Convention on Great Lakes Fisheries. The commission coordinates fisheries research, controls the invasive sea lamprey, and facilitates cooperative fishery management among the state, provincial, tribal, and federal management agencies.

Navigation, Shipping, and Canals

Although the Great Lakes were naturally connected to the Atlantic Ocean via the St. Lawrence River, this passage was not available to vessels because of rapids and waterfalls, especially Niagara Falls. In 1959, the St. Lawrence Seaway, a series of canals, locks, and dams, was completed. As part of this system, the 26-mile (42-kilometer) Welland Canal connects Lake Ontario to Lake Erie, allowing ships to bypass Niagara Falls. The locks are not wide enough to allow passage of the widest freighters; much of the shipping moves materials among Great Lakes ports. The primary commodities are iron ore (more than 6 million tons annually), coal (more than 4 million tons annually) and limestone (more than 3 million tons annually). Other cargoes include cement, salt, grain, and sand. The fleet of 300 ships from the last century has been reduced to about 140 in recent years. The largest boats are 1,000 feet (304.8 meters) long.

The Great Lakes are also connected to the Mississippi River via the Chicago Sanitary and Ship Canal. Completed in 1900, the canal is 28 miles (45 kilometers) long, 202 feet (62 meters) wide, and 24 feet (7.3 meters) deep. The canal was constructed to divert Chicago's treated sewage from entering Lake Michigan, the source of the city's drinking water. The canal reversed the flow of water in the Chicago River, which naturally flowed into Lake Michigan. Once the canal was opened, approximately 2,068 million gallons per day from Lake Michigan began to flow into it, pushing Chicago's treated sewage toward the Mississippi River and the Gulf of Mexico. The Great Lakes have a long history of shipwreck, groundings, and collisions. From the 1679 sinking of Le Griffon with its cargo of furs to the 1975 loss of the Edmund Fitzgerald, thousands of ships and thousands of lives have been lost; many of these losses involved vessels in the cargo trade. The Great Lakes Shipwreck Museum estimates that over 6,000 ships and over 30,000 lives have been lost.

Invasive Species

The Great Lakes Fishery Commission estimates that there are thirty-four non-native fish species in the Great lakes, many of which are considered invasive and therefore harmful to the lake ecosystems. The most invasive species are sea lampreys, alewives, and zebra mussels, all of which are a major problem for Great Lakes ecosystems. Many non-native species have entered the lakes via the St. Lawrence Seaway and the Chicago Sanitary and Shipping Canal. One of the most established invasive species is the sea lamprey (Petromyzon marinus), which is native to the Atlantic coast of North America, the Finger Lakes in New York, and Lake Champlain in New York and Vermont. It is thought to have invaded Lake Ontario via the Erie Canal. It spread to Lake Erie by way of the Welland Canal, and to Lakes Huron, Michigan, and Superior soon afterward. Sea lamprey attack other fish by attaching to them with a suction cup–like mouth and scraping away their flesh with sharp teeth and tongue. These fish usually die from blood loss and infection. Sea lamprey decimated populations of lake trout, lake whitefish, chub, and lake herring in the 1930s and 1940s, taking a huge toll on the commercial fishing of the Great Lakes region. Control efforts include applying electric currents, chemical lampricide, and physical barriers.

The alewife (Alosa pseudoharengus) is another invasive species whose population increased greatly when lampreys reduced populations of native fish. Alewives are native to the Atlantic coastal areas, but entered the Great Lakes through the Welland Canal in the 1940s. From time to time, large numbers of alewives die when they are exposed to rapid temperature changes, lack of food, or stress associated with living in freshwater or spawning. Zebra mussels (Dreissena polymorpha) are believed to have come from the Caspian Sea in Europe. They first entered the Great Lakes in the late 1980s in the ballast of ships. Zebra mussels can be up to 2 inches (50 millimeters) long, have striped shells, and can live four to five years. They form dense colonies in sediment and drainage pipes, blocking water flow. They also consume plankton by filter-feeding, competing with fish for food. Annual cost to manage zebra mussel populations exceeds $250 million.

Other invasive species of concern include Asiatic clam (Corbicula fluminea), bighead carp (Aristichthys nobilis), bloody-red shrimp (Hemimysis anomala), European rudd (Scardinius erythrophthalmus), European ruffe (Gymnocephalus cernuus), New Zealand mud snail (Potamopyrgus antipodarum), quagga mussel (Dreissena rostriformis bugensis), round goby (Apollonia melanostomus), rusty crayfish (Orconectes rusticus), silver (or Asian) carp (Hypophthalmichthys molitrix), white perch (Morone americana), and the aquatic plants European frog-bit (Hydrocharis morsus-ranae), hydrilla (Hydrilla verticillata), and Eurasian watermilfoil (Myriophyllum spicatum).

Water Quality, Pollution, and Water Diversions

Great Lakes water and sediments contain more than 800 toxic contaminants. The US Environmental Protection Agency has identified forty-three areas that have water impaired by pollutants such as mercury and polychlorinated biphenyls (PCBs). These invisible chemicals, many of which are no longer discharged, inhabit the sediment of the lake bottoms. Wave action and the bottom-feeding organisms bring them back into the water column and the food chain, including fish. Pollutants enter Great Lakes waters from three main sources: point sources, nonpoint sources, and the atmosphere. Point sources of contaminants, such as heavy metals and human waste, include industrial discharge drainpipes and sewage treatment plants. Nonpoint source pollution, including agricultural pesticides and fertilizers, salts and oils from roads, sediments from construction sites and eroding stream banks, and animal waste from farm pastures, comes from many different sources and is transported to the Great Lakes when rain and snowmelt runoff pick up pollutants and drain them into waterways. Airborne contaminants, such as mercury, phosphorus, and PCBs originate from the burning of fossil fuels and waste and enter the lakes via rain and snow.

In March and April 1993, an outbreak of the protozoan Cryptosporidium occurred in the drinking water system of Milwaukee, Wisconsin. Approximately 403,000 people became sick with stomach cramps, diarrhea, and dehydration, and 104 people died, making this the largest documented disease outbreak caused by a waterborne pathogen in US history. The United States draws over 40 billion gallons of water from the Great Lakes every day, much of it for electricity generation. Some towns, such as Pleasant Prairie, Wisconsin, and Akron, Ohio, are allowed to divert Great Lakes water for public supply. Some large-scale schemes to divert Great Lakes water to, for example, Great Plains states to recharge the Ogallala aquifer, to Wyoming for mining, or to Asia, have been denied. In 2014, a major public health crisis occurred in the Toledo, Ohio, region when toxic algae in Lake Erie forced a shut down of water supplies for over 500,000 people in the area. Unfortunately, Lake Erie has suffered from centuries of human mistreatment, which has resulted in high levels of pollution and increased eutrophication, or natural aging of the lake.

Legislation, Treaties, and Policies

Several treaties, agreements, and laws govern use of water from the Great Lakes. The 1909 Boundary Waters Treaty between the United States and Canada deals with all matters concerning flow and quality of waters passing over the international border. It also established the International Joint Commission to resolve disputes and review any projects that would affect the lakes' flow and uses, including boating and navigation. In the 1972 Great Lakes Water Quality Agreement, the United States and Canada agreed to control pollution, research and monitor lake health, and work toward restoration with an ecosystem approach. The 1985 Great Lakes Charter is a nonbinding agreement between all Great Lake states and provinces to cooperatively manage the lakes. Accordingly, before any new diversion or consumptive use of more than 5 million gallons per day of any Great Lakes water is begun, all parties must agree. The 1986 Water Resources Development Act gives veto power to each of the eight Great Lakes states' governors to block diversion or export of water from the Great Lakes outside the region. This act is reauthorized every two years and was signed into law once again in 2022.

The 2001 Great Lakes Charter Annex between the eight Great Lakes governors and two premiers set the intention to develop new protections to prevent harmful water withdrawals and diversions of Great Lakes water. The most recent agreement, the Great Lakes–St. Lawrence River Basin Compact and Agreement of 2008, seeks to ban the diversion of waters, with some limited exceptions, and to set responsible standards for water use and conservation within the basin. Each of the eight Great Lakes states must develop policies for water conservation, according to the compact. These accords have improved the management of the Great Lakes. In 2009, a record $475 million was authorized by the US government to improve the condition of the Great Lakes for future generations.

In 2012, the United States and Canada signed the updated Great Lakes Water Quality Agreement (GLWQA) that authorizes each country to act on potential and real threats to the water quality of any of the Great Lakes. Updated provisions of the agreement included measures to address invasive species, degradation of habitat, climate change, and threats to human health from toxic chemicals, algae, and ship discharges and spills.

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