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.
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;
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.
Upper Mississippi River Lock and Dam
Source: St. Paul District Corps of Engineers
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.
Lock and Dam 19 on the Upper Mississippi
River at Keokuk, Iowa
Source: Keokuk Office of Tourism
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
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
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
and aquatic plants that may disrupt water
flow over mussel beds, inhibit feeding, and reduce the supply
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
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
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.
Zebra mussels pose
the most immediate threat to
freshwater mussels in the Upper
Mississippi River System!
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
transported to the United States in the ballast water of transatlantic
ships navigating the Great Lakes.
Cross section of ships
showing ballast tanks and ballast water cycle
Single zebra mussel showing byssal
threads used for attachment.
Source: Ohio Sea Grant
Zebra mussels invaded Lake Michigan,
the Illinois River by a man-made
connection at Chicago, and were
first found in the Upper Mississippi River in 1991. They are
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
develop as planktonic organisms (veligers) drifting in the current
until they become large enough to attach to the
They produce thousands of veligers and reproduce several times
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 attached to a barge on
the Upper Mississippi River, Minneapolis-St. Paul, Minnesota,
and the hull of a houseboat in La Crosse, Wisconsin.
Zebra mussels also attach by the hundreds to the shells of native
freshwater mussels. Zebra mussels compete with native mussels
food and may interfere with successful reproduction. Severe
freshwater mussel mortality is not likely to occur until zebra
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.
Zebra Mussels (Dreissena polymorpha)
Upper Mississippi River, Pool 10
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.
Quagga mussel - (Dreissena
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.