Zebra mussels (Dreissena polymorpha) are suspected to be the cause of erratic water flows on the Huron River downstream from the Argo Dam. A team of divers from Solomon Diving in Monroe, Michigan, are on site Tuesday morning to go under the icy waters of Argo Pond and inspect the intake pipes that lead to the pond level sensor system that controls the dam. A failure of this control system last week led to over four days of erratic river levels ("Argo Dam control system fails, causing Huron River to rise and fall quickly," AnnArbor.com, January 26, 2010).

The divers aren't in the water yet, so it's time to look at the zebra mussels—where they are now, how they got here, what they do and how to manage them when they arrive.

Zebra mussel map

Both zebra mussels (in red) and quagga mussels (in purple) are in widespread distribution in Michigan. 

Courtesy of the US Geological Survey, Nonindigenous Aquatic Species Database

Where are zebra mussels from?

The zebra mussel was first described in the lakes of southeast Russia, and their natural distribution also includes the Black and Caspian Seas. They have a long history as an invasive species, and were successfully established in Great Britain (1824), The Netherlands (1827), The Czech Republic (1893), Sweden (1920), Italy (1973), the Great Lakes in the USA (1988) and California (2008). Long-distance transport to the United State and Canada was helped by the opening of the St. Lawrence Seaway, which allowed vessels from Europe to dump their ballast water into the Great Lakes.

A related species, the quagga mussel (Dreissena rostriformis bugensis), has followed a similar path. Where the two species are found, the quagga mussel tends to outcompete the zebra mussel and it grows in greater densities.

What do zebra mussels do to dams?

There's nothing that a zebra mussel loves more than to attach itself to the inside of a pipe carrying fresh water. The mussels are filter feeders, drinking up to a quart of water per day and slurping up plankton from the water. When they die, the next round of zebra mussels attach themselves to the layer of shells below them, building up a thick crust of debris. The layers build up until the pipe is largely or completely blocked.

Dams are controlled by monitoring water levels on the impoundment above them. If the intake valve to the monitoring well is blocked, changes in water level will not be registered right away, and water level controls will be erratic. In areas where zebra mussels are plentiful, regular inspections and maintenance are required to keep intake valves clear.

Diving for zebra mussels

It's not as glamorous as deep sea diving for pearls. Divers go underwater and inspect pipes and intake valves for clogs, and break through the clogs as needed to let water run free.

The City of Ann Arbor has contracted with Solomon Diving from Monroe, Michigan, for the Argo Dam dive this morning. They have been in the business of helping utilities control zebra mussel populations since the 1980s.

In the winter, of course, dive operations are much slower and colder; in general, you'd expect that a routine dam maintenance schedule would do this work in more temperate circumstances.

For further reading

• Nonindigenous invasive species: zebra mussel, USGS. This site has maps of zebra and quagga mussel distribution nationwide.
• Frequently asked questions about the zebra mussel, USGS Southeast Ecological Science Center.
• Zebra Mussel Watch, University of Wisconsin Sea Grant
• Zebra Mussels, USGS Great Lakes Science Center
• Great Lakes: "Amazing change", Michigan Today, July 2009. On the changes to the ecosystem of Lake Michigan resulting from invasive mussel species.

Edward Vielmetti writes about the Huron River for AnnArbor.com.