The Great Lakes basin, encompassing lakes Superior, Michigan, Huron, Erie, and Ontario, represents the largest surface freshwater system on Earth. This vast, interconnected system holds approximately one-fifth of the world’s surface fresh water and supports a rich array of life. The region is home to over 160 fish species that have adapted to its varying depths and temperatures.
Deepwater and Open Water Aquatic Life
The deep, cold waters of the Great Lakes support a specialized community of organisms. Lake Trout, the native apex predator, thrive in these cold, oxygen-rich environments, with forms such as the Siscowet Lake Trout inhabiting depths greater than 250 feet in Lake Superior. Lake Whitefish, a commercially significant native species, are benthic coolwater fish that feed primarily on bottom-dwelling invertebrates.
Smaller, foundational life forms drive the aquatic food web in these offshore areas. Deepwater sculpins are small, bottom-dwelling fish that prey on smaller crustaceans. Historically, the deep-water amphipod Diporeia was the dominant benthic organism and a high-energy food source for many native fish.
The water column is dominated by plankton (phytoplankton and zooplankton). Zooplankton are a primary food source for many larval and small adult fish, forming the base of the pelagic food chain. However, Diporeia populations have declined drastically in all but Lake Superior, and zooplankton communities have shifted, indicating profound changes in the aquatic ecosystem.
Wildlife of the Shoreline and Littoral Zone
The shoreline and nearshore littoral zones are dynamic interfaces where aquatic and terrestrial life mingle. Semi-aquatic mammals are prominent in these coastal wetlands and marshes, relying on both land and water for survival. North American Beavers act as ecosystem engineers, using woody materials to build dams and lodges that create diverse wetland habitats.
Muskrats are smaller rodents that construct lodges from cattails and other vegetation; their feeding habits help maintain open water channels in dense marshes. River Otters hunt fish and crayfish in the sheltered bays and tributaries, representing a successful population recovery after decades of decline.
Amphibians and reptiles (herpetofauna) also depend on the coastal wetlands and sandy shorelines. Long-lived species like the Blanding’s Turtle, recognizable by its bright yellow throat, use sandy upland areas to lay their eggs. Snapping Turtles and Painted Turtles are often monitored as bioindicators because their long lifespans allow them to accumulate and reflect environmental contaminants.
Avian Residents and Migrants
The Great Lakes region serves as a major corridor along the Mississippi Flyway, making it a stopover and breeding ground for billions of birds. The large water bodies influence migration, with many nocturnal migrants following the shorelines to minimize the energy expenditure of flying over open water. This concentrates birds along the coast.
Gulls and other piscivorous birds are year-round fixtures, with the Ring-billed Gull forming one of the world’s largest colonies on Lake Ontario. The Double-crested Cormorant population saw a resurgence following the ban on toxic chemicals like DDT, and they are now common while diving for fish.
Raptors also dominate the Great Lakes skies, preying on the abundant fish resources. Ospreys are specialized fish-eaters, using their unique footpads and reversible toes to snatch fish after a dive. Bald Eagles are more opportunistic feeders that consume waterfowl and carrion, often stealing fish from Ospreys, a behavior known as kleptoparasitism.
Non-Native Species Altering the Ecosystem
The Great Lakes ecosystem has been fundamentally reshaped by the introduction of over 180 non-native aquatic species, primarily arriving via the ballast water of transoceanic ships. Among the most impactful are the Zebra and Quagga mussels, which are voracious filter feeders that consume massive amounts of plankton. This filtering action diverts energy to the lake bottom, leading to a decline in native invertebrates like the amphipod Diporeia, and resulting in clearer water that allows for more benthic algae growth.
The mussels also create a pathway for toxins, as they bioaccumulate contaminants that are then passed up the food chain. The Round Goby, an aggressive bottom-dwelling fish, consumes the mussels and transfers those concentrated toxins to predators like Walleye and certain water birds. Round Gobies also compete aggressively with native fish for food and habitat while preying on the eggs and fry of species such as Smallmouth Bass.
The Sea Lamprey, a jawless parasitic fish that entered the lakes through shipping canals, caused the collapse of native predator populations in the mid-20th century. Using a circular suction mouth and sharp teeth, a single lamprey can kill up to 40 pounds of fish, devastating populations of Lake Trout and Lake Whitefish. Extensive control programs have reduced the lamprey population by approximately 90%, allowing for the partial recovery of native fish stocks.