Freshwater
Mussels of the Upper Mississippi River System
Current Threats
Dramatic declines in numbers and diversity of mussels in many rivers, lakes, and streams across the eastern United States has been attributed to a variety of factors. These include degradation of their habitat by dams and impoundment, channelization and dredging, pollution, sedimentation, fish kills that eliminate potential host fish, and introduction of non-native species. |
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Dams There are 29 locks and dams on the Upper Mississippi River between St. Louis, Missouri, and Minneapolis-St. Paul, Minnesota. The locks and dams were constructed by the U.S. Army Corps of Engineers to provide a 9-foot channel for commercial navigation. The dams do not control floods; during high water, the dam gates are raised out of the water. Below St. Louis, the river is deep enough so that locks and dams are not needed for commercial navigation. In general, dams transform rivers into lakes and impoundments. The resulting changes in water depth, water currents, temperature, and restructured fish and algal communities can negatively affect freshwater mussels. Additionally, because dams slow current velocity, they allow suspended sediments to drop out of the water column and settle onto the river bottom, which can literally bury mussel beds. These habitat changes are usually not favorable to most mussel species that inhabit free-flowing rivers.
Because some dams are effective barriers to fish and mussel migration, they can isolate upstream communities from those downstream. The classic example of this on the Mississippi River concerns the creation of Lock and Dam 19 at Keokuk, Iowa. This dam blocked the upstream migration of skipjack herring, the only known host of the ebonyshell and elephantear mussels. Consequently, these two species have been nearly eradicated above Lock and Dam 19. River biologists have found that the movements of other fish species have also been restricted by dam construction, possibly affecting upstream distribution and survival of juvenile mussels in the Upper Mississippi River System.
Chemicals and Nutrients Chemicals are found everywhere in our homes and the environment. Used unwisely, some chemicals can contaminate the aquatic environment and kill mussels, fish, and other organisms. They can also be of concern to mussels when they bind with suspended sediments that drop to the river or lake bottom where mussels live. If this occurs over a long time, contaminants may accumulate in the tissues of mussels as they continually filter water for food.
Recent research has shown that mussels are extremely sensitive to certain pollutants such as ammonia and heavy metals, compared to other aquatic species. Fertilizers are applied to lawns, crops, and other areas within the watershed of the Upper Mississippi River System. Additionally, the excess nutrients from the fertilizers promotes the rapid growth of algae and aquatic plants that may disrupt water flow over mussel beds, inhibit feeding, and reduce the supply of oxygen. Sedimentation The combination of intensive land use, wetland drainage, and stream channelization resulted in high rates of soil loss after World War II in agricultural areas. Soil washed into streams and larger rivers as fine silts and clay. In many areas, siltation occurred at such high rates that backwaters and side channels were filled with fine sediment.
Mussels are affected by a variety of factors related to sedimentation. The most obvious is direct burial of mussels by sediment. Mussel beds located near tributary inflows and in slow flowing water where silt settles out can often be covered deep enough to suffocate the population. One experiment found that as little as one-quarter of an inch of silt covering the substrate caused death in about 90% of the mussel species examined. A more chronic condition is the habitat alteration that occurs during sedimentation. When sedimentation occurs on gravel beds, silt fills the small spaces between gravel and rocks where the mussels reside. Some species are able to survive in the modified habitat, but many less-tolerant species perish. Exotic Species
Zebra Mussels Zebra mussels (Dreissena polymorpha) have nearly eliminated most native freshwater mussels in some portions of the Great Lakes. Zebra mussels are native to Europe and Asia and were transported to the United States in the ballast water of transatlantic ships navigating the Great Lakes.
Zebra mussels invaded Lake Michigan, entered the Illinois River by a man-made connection at Chicago, and were first found in the Upper Mississippi River in 1991. They are currently found throughout the Upper Mississippi River System, many of its tributaries, and inland lakes.
Zebra mussels attach to hard surfaces with byssal threads that secrete a strong glue-like substance. They do not require a fish host; they develop as planktonic organisms (veligers) drifting in the current until they become large enough to attach to the bottom or objects. They produce thousands of veligers and reproduce several times a year. Because zebra mussels readily attach to tow boats, barges, and recreational crafts, they can be easily transported to upstream portions of the Upper Mississippi River.
Zebra mussels also attach by the hundreds to the shells of native freshwater mussels. Zebra mussels compete with native mussels for food and may interfere with successful reproduction. Severe freshwater mussel mortality is not likely to occur until zebra mussel infestation intensity reaches about 100 zebra mussels per freshwater mussel. In the river bottom, zebra mussels have been reported at densities of over 67,000 per square meter. In the video, you can see zebra mussels carperting the bottom of the river and attached to native mussels.
Other Invasive Species The Quagga mussel, (Dreissena bugensis), is very similar to the zebra mussel and has the potential to be a significant threat to native mussels. It also came from Europe by ocean vessels and became established in the Great Lakes in the 1980s. It can live in deeper water, and grow and reproduce at lower water temperatures than zebra mussels, which may help it establish a wider range. Over the past few years, quagga mussels now outnumber zebra mussels in the Great Lakes. For some unknown reason, it has not yet infested the Upper Mississippi River System.
The Golden mussel, (Limnoperna fortunei), is another invasive species that attaches to native mussels similar to zebra mussels. It has not been found in the United States yet, but has invaded South America from the discharge of ballast water from ocean vessels. Like zebra mussels, it disperses quickly throughout watersheds it invades and adversely effects native mussels and industrial water systems, generating facilities and water treatment plants. Another threat to native mussels of the UMRS is the Black carp, (Mylopharyngodon piceus). This species entered the United States in the 1970’s in shipments of imported Grass carp, (Ctenopharyngodon idella), a species used to control aquatic vegetation in hatchery ponds. Black carp feed on native mussels and snails and have been found in Illinois, Missouri and Louisiana. In particular, in 2004 one specimen was collected near Lock and Dam 24 on the Upper Mississippi River. |
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