The Great Lakes, a vast interconnected system of freshwater lakes, represent a unique ecosystem. These five lakes—Superior, Michigan, Huron, Erie, and Ontario—collectively hold approximately one-fifth of the world’s surface freshwater. Spanning over 94,000 square miles, the system was formed around 14,000 years ago as glaciers retreated from North America. The Great Lakes provide essential resources for surrounding communities, including drinking water and recreational opportunities.
Fish Species
The Great Lakes support a diverse array of fish, with 139 native species. These include forage fish like sculpins, gizzard shad, shiners, and ciscoes, which form the base of the food web. These smaller fish sustain larger predators such as lake trout, walleye, large and smallmouth bass, and brook trout.
Lake whitefish and yellow perch are also significant, supporting both recreational and commercial fishing. Sport fishing is popular, with species like Chinook, Coho, and Atlantic salmon, along with rainbow trout (steelhead), providing angling opportunities. Muskellunge, a large predatory fish, also inhabits these waters.
Unique native species, such as the lake sturgeon, are bottom-feeding fish. Lake sturgeon use river habitats for spawning, which improves stream habitats for other aquatic organisms.
Wildlife Beyond Fish
Beyond fish, the Great Lakes region is home to diverse animal life. Over 300 bird species are found here, many using the lakes for breeding, feeding, and resting during migration. Common birds include waterfowl, various gulls, and raptors like bald eagles and northern harriers.
Other notable species include common loons, double-crested cormorants, common terns, bobolinks, least bitterns, and sandhill cranes. The endangered Kirtland’s warbler also breeds in the region.
Various mammals also inhabit areas in and around the Great Lakes, including larger animals such as gray wolves, moose, and black bears, particularly in the northern reaches. Smaller mammals like beavers, otters, Canada lynx, coyotes, foxes, elk, and white-tailed deer are also present.
Amphibians, with over 30 species in the basin, rely on both aquatic and terrestrial environments. Frogs such as the northern leopard frog, bullfrog, chorus frog, green frog, and American toad, along with salamanders like mudpuppies and eastern newts, are found in the wetlands. Their permeable skin makes them sensitive indicators of wetland health. Reptiles, including the Eastern Gartersnake, Northern Watersnake, and Eastern Massasauga Rattlesnake, also interact with the diverse habitats surrounding the lakes.
Invertebrates and Aquatic Plants
The less visible inhabitants of the Great Lakes, invertebrates and aquatic plants, play important roles in the ecosystem. Diverse invertebrates, such as native freshwater mussels, crayfish, and various insect larvae, contribute to the food web. These organisms often serve as food for fish.
Aquatic plants, including submerged, emergent, and floating vegetation, provide essential habitat and food sources for numerous species. These plants also contribute oxygen to the water, supporting aquatic life. Submerged macrophytes grow in the lake depths, while forms of algae like Cladophora can be found attached to the bottom. Their presence and distribution are influenced by factors such as water clarity and nutrient availability.
The Challenge of Invasive Species
Invasive species pose a significant threat to the native biodiversity of the Great Lakes. These non-native organisms disrupt food webs, alter habitats, and outcompete native species. Over 180 aquatic species have been introduced into the Great Lakes; a fraction have become invasive.
Zebra and quagga mussels, originating from Eastern Europe, arrived in the Great Lakes via ship ballast water in the 1980s. These mussels are efficient filter feeders, consuming large quantities of phytoplankton, which reduces food for native filter feeders and other organisms. Their filtering activity also increases water clarity, allowing sunlight to penetrate deeper and promote the growth of nuisance algae like Cladophora on the lake bottom. These mussels can clog water intake pipes, attach to boat hulls, and injure people with their sharp shells. Quagga mussels, with their ability to colonize deeper waters, have largely replaced zebra mussels in many areas and have been linked to avian botulism outbreaks.
The sea lamprey, a parasitic fish native to the Atlantic Ocean, gained access to the Great Lakes through canal systems, such as the Welland Canal, in the early 20th century. Sea lampreys attach to fish with a suction-cup mouth and rasp through their skin to feed on blood and body fluids, often killing their hosts. Before control efforts, a single sea lamprey could destroy up to 40 pounds of fish, leading to declines in lake trout and other commercially valuable fish. Control programs using lampricides, barriers, and traps have reduced sea lamprey populations by 90-95%, helping to restore fish stocks.
The round goby, an invasive species, arrived from Eurasia via ballast water and is an aggressive bottom-dweller found in all five Great Lakes. Round gobies compete with native fish for food and consume the eggs of sport fish like lake trout, lake sturgeon, walleye, and smallmouth bass. They can survive in poor water quality and have been linked to botulism outbreaks by consuming zebra mussels. Despite their negative impacts, some native predators, including smallmouth bass, burbot, and Lake Erie water snakes, have begun to prey on round gobies.