Deep beneath the surface of Lake Superior lies a world that remains difficult to explore. At its deepest point, the lake plunges more than 1,300 feet into an underwater basin known as Superior Maximus, where the water is near-freezing, sunlight cannot reach, and pressure is many times greater than at the surface.
These conditions make the lake bottom an unusual freshwater habitat. Yet the darkness and cold do not mean that the deepest parts are lifeless. A small but specialized community of fish and other aquatic organisms has adapted to life in the deeper waters and along the lake floor.
Much of this environment remains poorly explored, and researchers continue to investigate both the biology of the deep lake and the material preserved within its cold freshwater. Shipwrecks, submerged timber, sediments, and other remains add another dimension to a lake floor that holds both living communities and evidence of the past.
So while Lake Superior may appear quiet and empty from the surface, its deepest waters contain a specialized ecosystem that is far more interesting than its dark and seemingly barren appearance suggests.
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Deepwater Sculpin
Lake Superior’s deepwater sculpin is one of the fish most closely associated with its deep, cold bottom waters. This small, bottom-dwelling fish has no swim bladder, an adaptation that allows it to live under the high pressure found at considerable depths. It feeds mainly on small aquatic invertebrates living on or near the lake floor.
Deepwater sculpin are commonly found hundreds of feet below the surface, but they have been recorded at depths of more than 1,300 feet, including the deepest parts of Lake Superior. Their large eyes are adapted to the very limited light available in deep water, although the deepest habitat is essentially dark.
The species is also an important part of the deepwater food web. Deepwater sculpin provide prey for larger fish, including siscowet lake trout and burbot, linking the small organisms of the lake bottom with some of Lake Superior’s major deepwater predators.
Siscowet Lake Trout
The siscowet lake trout is a deepwater form of lake trout found only in Lake Superior. It is particularly well adapted to the cold, dark waters of the lake’s deeper areas and is one of the important predators in its deepwater food web.
Siscowets are known for their unusually high body-fat content, which can reach about 70% of their body weight. This adaptation helps them survive in the cold, deep waters where food can be limited. They are most abundant at depths of about 300 to 600 feet, although they can occur much deeper, including depths of more than 1,000 feet.
Their large eyes are another adaptation to deepwater conditions, where very little light penetrates. During the day, siscowets can remain in deep water near the lake bottom, where they hunt other fish adapted to the same environment.
Siscowets feed on deepwater fish such as deepwater sculpin, kiyi, and other coregonines, making them an important predator in Lake Superior’s deepwater food web. Their feeding habits connect smaller bottom-dwelling fish with larger predators and help make them an important part of the ecosystem in the deeper parts of the lake.
Kiyi
The kiyi (Coregonus kiyi) is a small, slender freshwater whitefish that lives in the deep waters of the Great Lakes. Today, it is largely restricted to Lake Superior, where it is associated with the cold, deep parts of the lake.
Kiyi are generally found at depths of about 260 to 650 feet, although their distribution can vary with season and location. They are rarely encountered in shallow water and spend much of their lives in deeper parts of the lake, where the water remains cold and light is limited.
The species has adaptations that help it function in this low-light environment. Its eyes are particularly suited to the blue-shifted light that penetrates into deeper water, allowing the kiyi to detect prey under conditions where sunlight is extremely limited.
Kiyi are also an important part of the deepwater food web. They feed on small aquatic organisms and in turn provide food for larger fish, including siscowet lake trout. Their presence in Lake Superior helps connect the smaller organisms living in deep water with the larger predators that hunt there.
Opossum Shrimp
The opossum shrimp (Mysis diluviana) is a small native crustacean found in the cold, deep waters of Lake Superior. Despite its name, it is not a true shrimp but a mysid, a group of small shrimp-like crustaceans that form an important part of the lake’s food web.
Opossum shrimp spend much of the daylight hours in deeper water near the lake bottom, where they feed on small organisms and organic material. At night, they can move upward into the water column to feed, a daily movement that helps connect organisms living near the bottom with fish higher in the water. Opossum shrimp are also an important food source for lake trout and other deepwater fish.
The name “opossum shrimp” comes from the way females carry their young in a brood pouch beneath the body, similar to the pouch of a marsupial. This is one of the most distinctive features of these tiny crustaceans.
Because they are small but abundant enough to serve as prey for larger fish, opossum shrimp are an important part of the food web in Lake Superior’s deeper waters.
Burbot
Burbot is a cold-water fish that spends much of its time near the lake bottom, making it another important part of Lake Superior’s deepwater ecosystem. It can tolerate the cold conditions found in deeper water and feeds on a variety of aquatic organisms, including smaller fish and invertebrates.
Burbot also interacts with some of the specialized species found in the deeper parts of the lake. It can feed on deepwater sculpin and other small fish, making it part of the same food web that connects bottom-dwelling organisms with larger predators.
Although burbot can live in deep water, it is not a specialist of the extreme depths of Superior Maximus. Instead, it is associated with the lake’s broader cold, deep, near-bottom habitat.
Other Bottom-Dwelling Animals
Fish are not the only animals found near the bottom of Lake Superior. The lake floor also supports a variety of small aquatic invertebrates that live on or within the sediments and provide food for larger animals.
Amphipods, including Diporeia, are important bottom-dwelling crustaceans found in Lake Superior. These small, shrimp-like animals live among bottom sediments and organic material, where they feed on organic matter and other small particles. They can also become an important food source for fish that forage near the lake floor.
The bottom community also includes mollusks, aquatic insect larvae, worms, and other small invertebrates. Some live directly on the sediment, while others burrow into it or attach themselves to rocks and other surfaces. Their abundance and distribution vary with depth, bottom type, and available food.
Together, these small organisms form an important part of the food base for fish living in Lake Superior’s deeper waters.
What Else Is Found on the Bottom of Lake Superior?
The bottom of Lake Superior contains much more than living animals. Its floor includes bedrock, glacial sediments, sand, silt, and clay, along with underwater ridges, shelves, and other landforms created by ancient glaciers. In deeper areas, layers of sediment have accumulated over thousands of years.
The lake floor also contains numerous shipwrecks, many of which have been preserved unusually well by Lake Superior’s cold freshwater. One of the best-known is the SS Edmund Fitzgerald, which sank in 1975 in about 530 feet of water. Other wrecks are scattered across the lake, providing a record of the vessels that once traveled its waters.
Submerged timber and other historical material can also remain preserved in the cold freshwater environment. Much of the deep lake floor remains difficult to explore, so researchers continue to use remotely operated and autonomous underwater vehicles to study its geology, biology, and historic remains.
These features help explain why the bottom of Lake Superior is not simply a dark, empty surface. It is a combination of living communities, geological formations, and preserved remains from the lake’s long history.