Lake Superior is the largest freshwater lake in the world by surface area, and the deepest and coldest of the Great Lakes. Its deepest layers maintain an average temperature of approximately 4°C (39°F). The water’s purity and low nutrient content classify it as an ultra-oligotrophic lake, meaning its productivity is naturally limited compared to warmer, shallower systems. This combination of depth, low temperature, and clarity creates a specialized environment that supports a cold-adapted community of life.
The Deepwater Fish Community
The offshore waters of Lake Superior remain perpetually cold and are home to native fish species highly specialized for this deep, dark environment. The apex predator is the Lake Trout (Salvelinus namaycush), which has evolved into distinct ecotypes, including the fatter Siscowet and leaner forms. Siscowet Lake Trout thrive in the deep, cold water by accumulating a high-fat content, which helps them store energy and maintain buoyancy in the high-pressure environment.
Ciscoes, a collection of deepwater chubs, form the primary forage base for the trout. The Kiyi (Coregonus kiyi) inhabits depths of 80 to over 200 meters, adapted to the dim, blue-shifted light. Lake Whitefish (Coregonus clupeaformis), another native salmonid, also utilize these cold, deep areas, feeding on benthic organisms found along the lake bottom.
Nearshore and Shoreline Fauna
The narrow nearshore strip and surrounding shoreline support a variety of semi-aquatic vertebrates that rely on the lake’s perimeter. North American River Otters (Lontra canadensis) are common along rocky coasts and river mouths, utilizing webbed feet and a thick, water-repellent coat to hunt for fish and crayfish in the cold water. These playful mammals establish dens along the water’s edge, often featuring an underwater entrance.
The shoreline is also a haven for birdlife, providing nesting sites and feeding grounds for species like the Common Loon and the Herring Gull. Bald Eagles and Osprey frequently hunt the near-surface waters for fish. Large mammals such as Beaver, whose lodges are constructed along tributary streams, and Moose, which occasionally wade into the shallows to feed on aquatic vegetation, demonstrate a reliance on the lake-edge ecosystem.
The Hidden World: Invertebrates and Plankton
The foundation of the lake’s food web rests upon microscopic life and small invertebrates. Phytoplankton, the primary producers, form the base of the pelagic food chain, converting sunlight into energy in the upper water column. These are consumed by zooplankton, which in turn are a food source for many small fish and crustaceans.
The deepwater crustacean Mysis diluviana, commonly known as the opossum shrimp, performs a daily vertical migration. These organisms reside near the lake bottom during the day to avoid visual predators, then ascending hundreds of meters at night to feed on plankton. This movement transfers nutrients from the bottom (benthic) habitat to the open water (pelagic) food web, where they serve as a nutritious food source for deepwater fish.
Non-Native Species and Ecosystem Changes
The Lake Superior ecosystem faces ongoing disruption from species introduced from outside the basin, which compete with or prey upon native populations. The parasitic Sea Lamprey (Petromyzon marinus) entered the lake through shipping canals and became a major threat to native fish, particularly Lake Trout, by latching onto them and feeding on their blood. Control efforts focus on the larval stage, using the selective lampricide TFM in spawning tributaries and constructing low-head barriers to block adult migration.
Another significant invader is the Spiny Water Flea (Bythotrephes longimanus), a carnivorous zooplankton introduced via ballast water from transoceanic vessels. This tiny crustacean preys voraciously on native zooplankton, which are a primary food source for juvenile native fish. The Spiny Water Flea’s long, barbed tail spine makes it difficult for small fish to consume, effectively removing a key food source and leading to slower growth rates in native fish. Introduced Pacific salmon, such as Coho and Chinook, have also been intentionally stocked, adding to the predatory pressure on the lake’s forage fish community.