The zebra mussel, a native of the Caspian Sea, was brought to the Great Lakes through ballast water. The bivalve was first detected in 1988 in Lake St. Clair, and in just over a decade, it has spread to the Mississippi River and connected waters, as well as isolated inland lakes and ponds. The zebra mussels have very few predators, which combined with the ease with which its planktonic larvae can be transported, zebra mussels have proved difficult to contain.
The zebra mussel congregates in dense colonies
up to acres in size, sticking to any hard surface available in the
water. Densities of the mussels can exceed 30,000 mussels per square
meter in some areas (20). The zebra mussel
colonies take over lakebed habitat, crowding out native mussels and
other benthic
organisms. Haag et al have shown that zebra mussel are having a devastating
effect on native bivalves, causing increased mortality and reduced
fitness in the native species (21).
Zebra mussels are very efficient filter-feeders, removing plankton and particulate matter from the water column. Although this may seem beneficial to recreational swimmers and boaters, the increase in water clarity indicates a significant decline in the food and nutrients available to native species. Zebra mussels are thought to be a leading factor in the drastic decline of Diporeia populations by removing its food supply from the water column. In turn, this has cascade effects on organisms in upper trophic levels who depend on Diporeia for food.
