Zebra and quagga mussels are perhaps the best known Great Lakes invaders. Zebra mussels (Dreissena polymorpha) first appeared in Lake St. Clair in 1986. They are native to the Black, Caspian, and Azov Seas. Quagga mussels (Dreissena rostriformis bugensis) were first sited in Lake Erie in 1989. They are native to the Dneiper River drainage of Ukraine. Zebra and quagga mussels have colonized all the Great Lakes, although in Lake Superior they are found only in the harbors of Duluth, MN and Thunder Bay, ON. Quagga mussels are better adapted to living in colder temperatures and colonizing soft substrates than zebra mussels, thus are able to inhabit deeper areas of the Great Lakes. Quagga mussels are now the dominant invasive mussel species in the Great Lakes.
Impacts of zebra and quagga mussels
Colonization of the Great Lakes by zebra and quagga mussels has profoundly impacted the Great Lakes ecosystem.
Physical habitat alteration
- Quagga mussels in particular are adapted to colonizing soft substrates, where they alter habitat complexity and displace native species
Elimination of native mussel species
- Zebra and quagga mussels adhere to the shells of native mussel species, reducing their ability to move, feed, and breed, eventually killing them.
- Dreissenid mussels also out-compete native mussels and other filter-feeding organisms for food.
Changes to food webs
- The introduction and expansion of invasive mussels is thought to have contributed to declines of Diporeia, a native, shrimp-like organism that was once a primary food source for several Great Lakes fish species.
- Over a period of 15 years, Diporeia densities in Lake Michigan declined from 5,200 per square meter on average to 82 animals per square meter. This decline coincides with the dramatic expansion of quagga mussel populations in the lake. Similar changes have been observed in eastern Lake Erie and in Lake Ontario.
- Lake whitefish, a commercially valuable Great Lakes fish species that depends heavily on Diporeia as a food source, are now less robust (and lower in economic value) in Lake Michigan, most likely due to the decline of its primary food.
Changes to nutrient dynamics in the nearshore
- Zebra and quagga mussels are efficient filter-feeders. By feeding on phytoplankton and tiny zooplankton, they divert energy and nutrients that would be available to large zooplankton and their predators from the water column to the lake bottom.
- As more biomass is removed from the water column, water clarity increases, promoting the growth of bottom-dwelling nuisance algae like Cladophora, large mats of which wash up on shore in rotting clumps after it dies.
- Zebra and quagga mussels appear to promote the growth of the bacteria responsible for type E botulism and to concentrate the botulism toxin. More than 52,000 waterbird deaths were attributed to Type E botulism from 2002 to 2006 on the Great Lakes.
Altered contaminant cycling
- Zebra and quagga mussels transfer contaminants from water column and sediments into their tissues.
- Fish and waterbirds feeding on dreissenid mussels can bioaccumulate these contaminants, which can affect their survival and reproductive success.
- Water intakes: Zebra and quagga mussels attach to hard substrates, including water intake structures, causing the structures to become clogged. This greatly increases maintenance costs for water treatment and power generation facilities.
- Fishery impacts (commercial and recreational): Food web changes driven by zebra and quagga mussels have reduced the amount and quality of food available to Great Lakes sport fishes.
- Recreation and tourism: Losses to tourism industry from nuisance algal blooms that close beaches, limit recreational uses
Mapping invasive mussels as a Great Lakes stressor
We obtained data on zebra and quagga mussel distributions from research scientists throughout the Great Lakes region. Data from all sources were in numbers per square meter at point locations. To obtain estimates of density over the Lakes as a continuous surface, we combined the densities of zebra and quagga mussels at each sample point and used the ArcGIS kriging function for each lake.
Spatial distribution of zebra and quagga mussels as a stressor in the Laurentian Great Lakes (Inset: Western Lake Ontario).