The accidental introduction of this species is believed to have been made in 1985 or 1986 when transoceanic ships discharged ballast

water with microscopic zebra mussel larvae into Lake St. Clair at the southern end of Lake Erie, Michigan. Since that time, zebra mussels have spread rapidly throughout the Great Lakes and waterways of nearby states and is pushing its range westward.

The rapid dispersal of the zebra mussel is attributed in part to its high reproductive rate and its planktonic larval stage that, unlike that of native freshwater mollusks, does not require a host fish for development and maturation. Zebra mussels easily adapt to new bodies of water, irrespective of local fish, and reproduce rapidly wherever oxygen and particulate food are available, calcium levels are adequate, and currents are not too swift. They are spread by flowing waters that carry the larvae downstream and by commercial and recreational vessels. Zebra mussels have the ability to attach themselves to hard surfaces, including boat hulls and boating equipment.

Where zebra mussels concentrate, they can block intake pipes to industrial and water treatment plants and to recreational boats, spoil beaches and marinas, crowd out native species, and alter aquatic ecosystems. Zebra mussels can be lethal to native species by preventing them from feeding, growing, moving, respiring, and reproducing. Because native mussels have an important role in nutrient cycling and sediment mixing, the accelerated decline of freshwater mussels could seriously affect the ecology of affected river systems.

Zebra mussels are capable of extracting large amounts of phytoplankton from the water column for food. A single adult can filter about 1 L (1.07 qt) of water per day. A consequence of this is the removal of phytoplankton from the water column, which would otherwise be available as food to native mussels, fishes, and other aquatic organisms. This competition for phytoplankton, the base of the food chain, may cause long-term harm to aquatic systems.

For more information on zebra mussels, visit these Web sites:
U.S. Geological Survey
University of Wisconsin Sea Grant Institute
Earth Systems Education

Eurasian watermilfoil (Myriophyllum spicatum) is a plant with feathery underwater foliage. It originated in Europe and Asia and was introduced into North America many years ago. It was once commonly sold as an aquarium plant.

The introduction of milfoil can drastically alter a waterbody's ecology. Milfoil forms very dense mats of vegetation on the surface of the water. These mats interfere with recreational activities such as swimming,

fishing, water skiing, and boating. The sheer mass of plants can cause flooding, and the stagnant mats can create good habitat for mosquitoes. Milfoil mats can rob oxygen from the water by preventing the wind from mixing the oxygenated surface waters to deeper water. The dense mats of vegetation can also increase the sedimentation rate by trapping sediments. Milfoil also starts spring growth sooner than native aquatic plants and can shade out these beneficial plants. When milfoil invades new territory, typically the species diversity of aquatic plants declines. While some species of waterfowl will eat milfoil, it is not considered to be a good food source.

Milfoil can infest an entire lake within two years of introduction to the system. Milfoil reproduces extremely rapidly through the formation of plant fragments. In the late summer and fall, the plants become brittle and naturally break apart. These fragments float to other areas, sink, and start new plants. Milfoil also grows from fragments created by boaters or other disturbances during any time of year. A new plant can start from a tiny piece of a milfoil plant. This is why milfoil can so easily be transported from lake to lake on boat trailers or fishing gear. Once established in its new home, water currents may carry milfoil fragments and start new colonies within the same waterbody. Once milfoil becomes well-established within a waterbody, it is difficult or impossible to remove.

For more information on Eurasian water-milfoil, visit these Web sites:
U.S. Geological Survey
University of Minnesota Sea Grant Institute