zebra mussellsZebra mussels are a huge pest accidentally brought into the Great Lakes, possibly on the hulls or in the ballast water of vessels from Europe, which has been plagued by zebra mussels for centuries. A temperate, freshwater species, zebra mussels have spread to many other lakes and rivers in the US and Canada. Prodigious eaters, they filter out nearly all the phytoplankton (and small zooplankton) in the 15-40 micrometer size. By removing most of the food for microscopic zooplankton and filter feeders, which in turn support larval and juvenile fishes and other animals, zebra mussels can effectively starve the native populations of infested lakes and rivers. Lakes that were full of phytoplankton before zebra mussel infestation are devoid of the algae afterwards.

Zebra mussels were first discovered in 1988 in Lake St. Clair, a small lake between Lake Erie and Lake Huron. Their free-swimming, nearly invisible larvae, called veligers, may have been discharged from the ballast water of one or more transoceanic ships in 1985 or 1986. Since then, they have spread almost to New England, and states are working feverishly to educate boaters so the mussels don't spread farther.

Within two to three weeks, the veligers "settle" and attach by strong byssus threads to hard surfaces such as rock, wood, glass, rubber, fiberglass, metal, gravel, and native mussels. They grow best in areas of free-flowing water where they can filter out large quantities of plankton. Colonies of zebra mussels may accumulate and clog water-intake pipes and screens of drinking water facilities, industrial facilities, power generating plants, golf course irrigation pipes, cooling systems of boat engines, and boat hulls. Zebra mussels grow rapidly to about thumbnail size, mature within a year, and reproduce prolifically. An adult female can produce 30,000-100,000 eggs each year!

Zebra mussels (Dreissena polymorpha) are native to eastern Europe, having existed there long before industrialization. There, people building factories and other facilities just take them into account and design ways to work around them, For example, a designer may build facilities with two sets of pipes so one set can be closed off when it becomes fouled. The clean one takes over while the other is de-musseled. In western Europe, where zebra mussels have been around for 200 years, diving ducks have made them an important part of their diet.

Besides clogging pipes and devouring most of the available microscopic food supply, zebra mussels may present a health hazard by increasing human and wildlife exposure to organic pollutants such as PCBs and PAHs. Studies have shown that zebra mussels can accumulate the pollutants in their tissues in concentrations 300,000 times greater than in the environment. They deposit these pollutants as pseudofeces, loose pellets of mucous mixed with particulate matter that they filter from the water. Scavenging animals that eat the pseudofeces may pass these pollutants up the food chain. When the zebra mussels coat bathing beaches, the sharp-edged mussels cut the tender feet of swimmers.

Trace the invasion of zebra mussels in the U.S., and predict their next attack!

  1. Discuss the history of zebra mussels and their impact on humans and the food chain.
  2. Trace by satellite the route by which zebra mussels came into the Great Lakes on the hulls of commercial vessels.
  3. The Zebra Mussel Range Map is prepared by the New York Sea Grant Program. It is updated about every 3-4 months to report new sightings. Using the map from May, 1995, connect the round dots and bold lines which indicate the locations of zebra mussel sightings.

Shade in the infested areas.

How far South have zebra mussels spread? How far North? East? West?

Have they reached New England? What infested body of water is closest to Maine?

Once they are introduced into a body of water on boat hulls or in ballast water, how can they spread? (by floating downstream, free-swimming veligers, by attaching to something else and being carried)

  1. Find the small lake called Lake St. Clair between Lake Erie and Lake Huron, where zebra mussels were first sighted in the US.

Using an atlas to determine scale, calculate how far zebra mussels have spread in a decade.

  1. The spread of the zebra mussels may be limited by water temperature. Zebra mussels don't do well where average summer temperatures are above 81°F (27°C) and where average summer temperatures are below 54°F(12°C). Females spawn when water temperatures get above 54°F.

Using an infrared satellite image showing water temperatures in late summer (at their peak), predict which areas might manage to evade zebra mussel infestation.

Tracking phytoplankton decline

Lake Erie was one of the first places infested by zebra mussels. Adult zebra mussels were discovered there in 1988, so they may have been present since 1986. Examine CZCS images of the Great Lakes to determine the amount of phytoplankton in 1985. Examine a SeaWIFS image of the area today and discuss the change over the last ten years.

Trying to control zebra mussels

Find out more about the techniques that are being used to get rid of these animals. Chlorine has worked effectively to kill zebra mussels, but it also harms everything else, so alternatives more benign to the ecosystem are being explored. See the bibliography for places to contact for more information.

Duck migration on the Great Lakes may be changing as a result of the increase in food supply. How could you research whether or not this is happening?

Mussel observations

For obvious reasons, it is illegal to transport zebra mussels over state lines, so you can't study them directly (except their empty shells), but you can examine blue mussels which have a similar physiology. Blue mussels are not the pests their cousins are.

Examine live blue mussels to get an idea of what zebra mussels look like. Zebra mussels are smaller, and as their name suggests, variegated in color. Discuss why you can not buy live zebra mussels in Maine.

Aliens in the environment

According to a 1993 study, at least 4,500 non-indigenous species have set up residence in the United States. Fifteen percent of these cause severe harm, ranging from the displacment of native species to the destruction of habitat to an estimated $97 billion economic impact in market losses and pest control costs between 1906 and 1991.

Research other creatures (plants and animals) that have been introduced accidentally into New England habitats and how they have affected the environment (for example, feather grass Phragmites, the gypsy moth, a seaweed known as "deadman's fingers" Codium fragile tomentoides, the common periwinkle Littorina littorea, etc.)

Discuss means by which this problem could be averted or reduced (including more stringent national laws, better regulatory coordination among states, and environmental education about the potential dangers of importing non-native species).