The zebra mussel - Dreissena polymorpha

The zebra mussel provides a particularly scary example of some of the effects Lodge mentions. It is a native of southern Russia. It was introduced into the Great Lakes, apparently in 1985 or 1986 via water ballast from a foreign ship. By 1992 it had been spread throughout the Great Lakes and the St. Lawrence, the Mississippi from St. Paul almost to the Louisiana border, the Illinois River, the Ohio River, the Tennessee River, the Arkansas River, the Susquehana River, and the Hudson River (Figure 4). Authorities in Connecticut presume that it is only a matter of time before it reaches the Housatonic and the Connecticut Rivers. It has been found in Twin Lakes in northwestern Connecticut. Individual mussels are usually 25-35mm in length, occasionally 50mm.⁵

Figure 4: Introduction and early spread of the zebra mussel in North America. (a) Distribution as of 21 November 1990. (b) Distribution as of 15 October 1992. (From [8]).

There are two reasons why the spread of the zebra mussel is of particular concern.

1. It is one of the most notorious biofoulers in the world. By the year 2000, the cost of industrial, utility, and municipal water-use reductions plus the impact of the zebra mussel on navigation, boating, and sport fishing could reach $5 billion in the Great Lakes alone.
   • Veliger densities in the Saratove Reservoir have reached 550,000 per cubic meter. In other reservoirs consistent densities of 300,000-400,000 per cubic meter have been reported, but a density of 2 million per cubic meter is not uncommon.

   • The Chernobyl intake had a density of 1-2 million adults per square meter; the total biomass in the cooling pond was 1.69 million kg; the greatest density was 19 kg/m$^2$.

   • The Detroit Edison power plant has densities of up to 750,000 animals per square meter.

   • The opening of water intake tubes is sometimes reduced to as little as 20-30% of its original diameter.

2. It is a very efficient filterfeeder. The zebra mussels in Lake St. Clair, for example, can completely filter all of its water in 24 days, assuming a density of 10,000/m$^2$. A more realistic density is 50,000/m$^2$, leading to an expected filtering time of only 10 days. This may have a positive effect on polluted (or eutrophic) aquatic ecosystems, but it may also pose problems for many planktivores and dramatically alter the structure of aquatic communities.
   • It removes a large amount of particulate matter from water, promoting a change from a relatively homogeneous environment of turbid water and silty sand substrata to an environment of clearer water with patches of macrophytes. In Lake Erie, Secchi disk transparencies almost doubled and chlorophyll a declined 54% between 1988 and 1989.

   • A scenario for Lake St. Claire⁶
     • Biodeposition of most nutrients in the water on the lake floor

     • Decline in primary production

     • Increased development of the benthic community

     • Reduction in biomass and production of zooplankton and fish

   • It removes contaminants from lake water, but it also concentrates them on lake floors and shorelines.

   • Direct threat to other mussels. Unionids with as many as 15,000 zebra mussels on their shell have been found. Often unable to open and close their valves fully.