The large, commercially harvested marine shrimp are not present in Lake Michigan. However, the lake is home to several types of small, native crustaceans that function ecologically as shrimp, sometimes called “micro-shrimp.” These organisms, while physically small, play a significant role in the Great Lakes food web and are a focus of ecological study. Their role as a connector between the bottom of the food chain and larger fish is undeniable.
Defining the Crustaceans of Lake Michigan
Lake Michigan contains two principal native crustaceans often grouped into the “shrimp-like” category: the opossum shrimp and the deepwater amphipod.
The opossum shrimp, Mysis diluviana, is a true mysid found throughout the deep, cold areas of the lake. This species reaches a length of about one to two centimeters and is characterized by its translucent appearance and stalked eyes. Unlike most crustaceans, the female Mysis carries its young in a brood pouch.
The deepwater amphipod, Diporeia species, is not a true shrimp but belongs to a closely related group of crustaceans. Diporeia is a small, rice-grain-sized organism that once dominated the benthic community in the deeper, offshore waters. It is notable for its high lipid content, making it an energy-rich food source for fish. Both Mysis and Diporeia are glacial relicts, descendants of marine organisms trapped in the Great Lakes basin after the last ice age.
The Role of Micro-Shrimp in the Ecosystem
These tiny crustaceans perform a dual function in the lake’s ecosystem, acting as both detritivores and an energy transfer mechanism.
Diporeia: The Detritivore
Diporeia is primarily a detritivore, feeding on organic material and algae that sink to the lake bottom. By consuming this settling material, Diporeia helps recycle nutrients and keeps the lakebed clean.
Mysis: The Omnivore and Link
Mysis diluviana is an omnivore, feeding on both phytoplankton and zooplankton in the water column. The opossum shrimp exhibits diel vertical migration, staying near the lake bottom during the day to avoid predators and migrating upward at night to feed. This movement makes Mysis a biological link, transferring energy from the deeper benthic zone to the upper pelagic waters.
The accumulated energy in both species, especially the high lipid reserves of Diporeia, makes them a primary food source for many important fish. Fish species like lake whitefish, sculpin, and juvenile lake trout rely heavily on these crustaceans for growth and survival. This efficient energy transfer from primary producers to these fish is a fundamental process in the offshore food web structure of Lake Michigan.
Population Dynamics and Environmental Stressors
The crustacean populations of Lake Michigan have undergone significant changes, particularly the Diporeia species. Since the late 1980s and early 1990s, the Diporeia population has experienced a widespread decline across large areas of the lake. In some deep-water areas, densities have fallen by over 90%, with the species disappearing entirely from depths shallower than 90 meters.
This decline coincides directly with the establishment and spread of invasive dreissenid mussels, specifically the zebra and quagga mussels. The mussels are highly efficient filter feeders that consume the same settling algae and diatoms Diporeia relies on for food. The extensive filtering activity of the mussels starves the benthic community by intercepting food material before it reaches the lake bottom.
While competition for food is the leading hypothesis, the exact mechanism for the Diporeia decline remains complex. Some studies suggest the mussels’ waste products, called pseudofeces, may also play a role. The loss of Diporeia has had cascading effects on the fish populations that relied upon it, altering the energy flow of the deepwater ecosystem. Mysis diluviana populations have also shown signs of decline since the early 2000s, a trend linked to the overall reduction in the lake’s productivity following the invasion.