Lake Michigan, the third-largest of the Great Lakes, represents a vast and complex freshwater ecosystem. It is the only Great Lake located entirely within the United States, stretching over 22,400 square miles with depths reaching 923 feet. This immense basin provides a diversity of habitats, from shallow coastal zones to the cold, dark waters of the deep offshore region. The creatures that inhabit Lake Michigan, from microscopic organisms to large predatory fish, are part of a delicately balanced food web shaped by both natural processes and human intervention.
The Foundation: Plankton and Benthic Invertebrates
The base of Lake Michigan’s food web consists of microscopic organisms, collectively known as plankton, which float in the water column. Phytoplankton, the primary producers, are tiny plants like diatoms and green algae that convert sunlight into energy. These organisms are then consumed by zooplankton, which include small crustaceans like copepods and water fleas.
Benthic invertebrates, organisms living on or in the lake bottom, are also a fundamental part of the ecosystem. Historically, the native amphipod Diporeia was a crucial food source for many bottom-feeding fish, but its population has declined severely. This decline coincides with the establishment of invasive mussels, which are now the dominant invertebrates in the lake.
The introduction of invasive Zebra and Quagga mussels has fundamentally restructured the ecosystem. These filter-feeding mollusks remove massive amounts of phytoplankton from the water column, dramatically reducing the food available for native zooplankton and Diporeia. Quagga mussels, in particular, carpet the lake floor and can survive in deeper, colder waters than their Zebra mussel cousins.
By filtering the water, the mussels increase water clarity, which allows sunlight to penetrate deeper and promote the growth of nuisance algae on the lake bottom. They also alter nutrient cycling by sequestering organic material and excreting phosphorus and nitrogen, shifting energy from the open water to the lake floor.
The Main Aquatic Predators: Fish Species
The fish community in Lake Michigan is a mixture of native species, intentionally stocked non-natives, and accidentally introduced invasive species. The native deep-water predator is the Lake Trout (Salvelinus namaycush), which was nearly eliminated by overfishing and the invasive Sea Lamprey in the mid-20th century. Conservation and stocking efforts, along with Sea Lamprey control, have been necessary to support its resurgence.
Other important native fish include the Yellow Perch, a popular sport fish that schools in near-shore waters at depths less than 30 feet, and the Cisco, also known as lake herring. The Lake Whitefish (Coregonus clupeaformis) is a highly valued commercial species that has adapted its diet in recent decades to include invasive mussels.
Starting in the 1960s, Pacific Salmon species were intentionally introduced to create a sport fishery and to control another invasive fish, the Alewife (Alosa pseudoharengus). Chinook Salmon and Coho Salmon are now major components of the recreational fishing industry, feeding almost exclusively on Alewife. Steelhead, a lake-run form of Rainbow Trout, and Brown Trout are also stocked to sustain the popular trout and salmon sport fishery.
The most detrimental invasive species is the Sea Lamprey (Petromyzon marinus), a parasitic jawless fish that attaches to larger fish to consume their blood and bodily fluids. Its introduction led to a collapse of the native Lake Trout population before widespread control efforts were implemented. Another significant invader is the Round Goby (Neogobius melanostomus), a small bottom-dweller that has become a food source for some predatory fish, notably Lake Trout and Steelhead, because the goby feeds on the abundant Quagga mussels.
Shoreline and Surface Dwellers
The waters and immediate shoreline of Lake Michigan support a variety of animals that are not strictly aquatic, but rely on the lake’s resources for survival. The lake is a significant stopover point for millions of migratory birds and provides year-round habitat for various waterfowl. Ducks, gulls, and diving birds such as Common Loons use the lake for feeding and nesting.
The near-shore environment is also home to semi-aquatic mammals. River Otters, Mink, and Beavers are found in the coastal marshlands and river mouths, relying on the lake’s fish and plant life for their diet. These mammals are often seen utilizing the natural vegetation and fallen logs along undeveloped shorelines.
Amphibians and reptiles also inhabit the shallow waters and adjacent habitats. Various turtle species, including Snapping Turtles, are common in the near-shore zones, basking on logs and laying eggs in the sandy shorelines. Frogs and certain non-venomous water snakes thrive in the quiet coves and wetlands.