Lake Superior, the largest freshwater lake in the world by surface area, spans approximately 31,700 square miles (82,100 km²) and holds the third-largest volume of freshwater globally. With an average depth of 483 feet (147 meters) and a maximum depth reaching 1,333 feet (406 meters), it is known for its cold temperatures. It is characterized as oligotrophic, meaning it has a low nutrient content, which contributes to its clear waters. Despite these conditions, Lake Superior supports a diverse array of life.
Native Fish Species
Lake Superior provides a habitat for a variety of native fish species. The Lake Trout (Salvelinus namaycush) is a prominent native predator, historically important in the lake’s food web. These fish are found in cooler, deeper waters where they hunt smaller fish. Their populations faced challenges in the past but have seen recovery efforts.
Various whitefish species (Coregonus spp.), including Lake Whitefish (Coregonus clupeaformis) and Lake Herring (Coregonus artedi, also known as cisco), are integral to the lake’s native fish community. Lake Whitefish are bottom feeders, consuming invertebrates, and are a commercial species adapted to both shallow and deeper waters. Lake Herring, a pelagic species, primarily feed on zooplankton and serve as a food source for larger predatory fish like lake trout.
Invertebrates and Other Aquatic Animals
Invertebrates form the foundational layers of Lake Superior’s food web. Crustaceans are important, with species like Mysis diluviana, a small, shrimp-like opossum shrimp, inhabiting the deeper, colder parts of the lake. These mysids are a food source for many fish species, transferring energy to larger predators. Various copepods and amphipods also contribute to the zooplankton community, grazing on microscopic life and serving as prey for fish.
Native mollusks, including several species of mussels and snails, filter water and process organic matter, contributing to nutrient cycling. Aquatic insects, such as mayfly nymphs and caddisfly larvae, are found in shallower areas and serve as food for fish. These invertebrates facilitate nutrient flow and energy transfer throughout the lake’s ecosystem, acting as decomposers, filter feeders, and food sources for higher trophic levels.
Microscopic Life and Aquatic Plants
The base of Lake Superior’s aquatic food web is primarily phytoplankton. These microscopic algae, including diatoms, green algae, and blue-green algae, are primary producers that convert sunlight into energy through photosynthesis. Zooplankton graze on phytoplankton, transferring this energy to larger organisms.
Bacteria and archaea also thrive in Lake Superior, playing roles in nutrient cycling by decomposing organic matter and recycling essential elements. Extensive aquatic plant beds (macrophytes) are limited across Lake Superior, but some plant life can be found. These plants primarily grow in shallower, warmer bays and nearshore areas where light and warmer temperatures are sufficient for growth.
Non-Native and Invasive Species
Lake Superior’s ecosystem has been affected by the introduction of non-native and invasive species, which have established populations and altered native ecological dynamics. The Sea Lamprey (Petromyzon marinus), a parasitic fish, entered the Great Lakes through shipping canals and impacted native fish populations, particularly Lake Trout, by preying on them. Control efforts have been implemented to manage their numbers.
Zebra mussels (Dreissena polymorpha) and quagga mussels (Dreissena rostriformis bugensis) are invasive invertebrates that arrived via ballast water. These filter feeders consume large amounts of phytoplankton, competing with native species for food resources and altering the lake’s food web and water clarity. Lake Superior’s cold and oligotrophic conditions have limited their proliferation compared to other Great Lakes, though their presence is still notable. The Round Goby (Neogobius melanostomus), another invasive fish, competes with native bottom-dwelling fish for food and habitat, and can also consume fish eggs, impacting native fish communities. These species represent ongoing challenges to the lake’s native biodiversity and ecological balance.