Lake Superior, the world’s largest freshwater lake by surface area, holds a vast deepwater environment. With a maximum depth of 1,333 feet (406 meters) and an average of 483 feet (147 meters), its depths create a unique habitat. Below 660 feet (200 meters), the water temperature remains a constant 39°F (4°C), resulting in a cold, dark, and high-pressure setting. These conditions support a diverse array of life adapted to this aquatic world.
Fish of the Deep Waters
The deep waters of Lake Superior are home to several specialized fish species, including lake trout, ciscoes, and deepwater sculpins. Lake trout (Salvelinus namaycush), the primary predator in Lake Superior’s deep regions, exist in different forms. The siscowet lake trout, a deepwater type, thrives at depths typically between 300 and 600 feet, and has been found at over 1,000 feet. Siscowet are characterized by their high fat content, which aids in buoyancy and energy storage in the cold, deep environment.
Ciscoes, also known as chubs, inhabit these cold, deep waters. The kiyi (Coregonus kiyi), a deepwater cisco endemic to the Great Lakes, now primarily persists in Lake Superior, where it is common. Kiyi are often found at depths greater than 260 feet. The bloater (Coregonus hoyi) is another native deepwater cisco that feeds on zooplankton and other organisms near the lake bottom.
The shortjaw cisco (Coregonus zenithicus), once a dominant deepwater cisco in Lake Superior, has seen declining populations but is still present. These fish typically inhabit depths ranging from 180 to 374 feet. Deepwater sculpins (Myoxocephalus thompsonii) are also abundant in Lake Superior’s deep areas, residing on the lakebed. They are found in waters deeper than 65 feet (20 meters), feeding on aquatic invertebrates and serving as a food source for lake trout.
Bottom-Dwelling Invertebrates
The lakebed of Lake Superior supports a variety of invertebrate life, forming part of the deepwater food web. The amphipod Diporeia has historically been a significant component of the benthic food web in the Great Lakes. Unlike in other Great Lakes where its populations have declined due to invasive mussels, Diporeia populations in Lake Superior have remained stable.
Mysis diluviana, commonly known as the opossum shrimp, is another important invertebrate. This native mysid crustacean plays a role in the offshore food webs of Lake Superior, serving as a food source for many fish. Mysis diluviana is an omnivorous feeder, consuming zooplankton, phytoplankton, suspended organic detritus, or benthic organic deposits.
Aquatic worms, specifically oligochaetes, are prevalent inhabitants of Lake Superior’s bottom. These segmented worms are typically found in bottom muck or debris, where they feed on detritus and algae. Lake Superior is known to host at least 63 species of oligochaetes. Some species can tolerate very low dissolved oxygen levels.
The Microbial Ecosystem
Beneath the visible layers of fish and invertebrates, a vast microbial ecosystem thrives at the bottom of Lake Superior. This microscopic world primarily consists of bacteria, archaea, and fungi. These microorganisms play an indispensable role in the lake’s overall health and functioning.
These inhabitants are responsible for critical processes such as decomposition and nutrient cycling. They break down organic matter, releasing nutrients back into the water column. This recycling of materials supports the deepwater ecosystem. These microbial communities form the foundational base of the food chain, converting resources into forms that can sustain other life.
Adapting to Lake Superior’s Depths
Life in Lake Superior’s depths presents challenges, including extreme cold, perpetual darkness, immense pressure, and limited food resources. Organisms inhabiting this environment have evolved remarkable physiological and behavioral adaptations to survive. Many deepwater species exhibit slow metabolic rates, which helps them conserve energy in a food-scarce and cold environment.
Specialized sensory organs are common among these deepwater residents. Kiyi have evolved acute eyesight, giving them an advantage in the shadowy depths where light is minimal. Deepwater sculpins possess sensitive vision, which may help them detect predators like siscowet lake trout. The high lipid content of siscowet lake trout serves as both an energy reserve and aids in buoyancy control, allowing them to navigate varying depths.
Reproductive strategies are tailored to the deepwater environment. Lake trout, for example, lay their eggs in rocky spawning grounds, with newly hatched young later moving to deeper waters. These adaptations enable the diverse array of life at the bottom of Lake Superior to persist in this challenging freshwater habitat.