Zebra Mussel (Dreissena polymorpha)
Zebra mussels (Dreissena polymorpha) were introduced to North America most likely from the ballast water of ships that traveled to the Great Lakes from the region surrounding the Caspian Sea. Since their discovery in Lake St. Clair (A small lake between Lake Huron and Lake Erie) in the mid 1980s they spread quickly to all of the Great Lakes, and many other waterways including the Hudson River in 1991 and the Upper Susquehanna River Watershed around 2001. (Horvath & Alfred 2005) Populations grew extremely fast and because they are filter-feeders, mussels quickly had an impact on the aquatic ecosystems they invaded. An individual zebra mussel can filter up to a gallon of water per day while feeding on algae. After zebra mussels were introduced to Lake Erie, visibility in the lake went from 6 feet to 30 feet. In the Hudson River, post introduction water filtering rates increased from 3 percent to 100 percent a day. (Blankenship 2009)
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Filtering the water may sound like a good thing, but the problem with zebra mussels is they are too effective at what they do. When Zebra Mussels invade a waterway, they cover every available hard surface and can consume practically everything in sight. After introduction into the Hudson River, the phytoplankton biomass decreased by 80-90 percent – this led to a dramatic effect on biodiversity, nearly wiping out native mussels. (Blankenship 2009) As mussels filter the water, they are also using up large amounts of dissolved oxygen, and lower amounts of D.O. can lead to problems with all aquatic life as mussels consume it faster than it can be replaced. In addition to the ecological impact, Zebra Mussels have also had a significant economic impact as they attach themselves to water intake pipes and become a real problem for industrial and municipal water plants. The U.S. Army Corps of Engineers has estimated that Zebra Mussels caused more than five billion dollars worth of damage from 1993 to 1999.
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Since the discovery of these aquatic invaders, the biggest worry in Pennsylvania was that Zebra Mussels would find their way into the Susquehanna River and from there ultimately invade the Chesapeake Bay. Both waterways are already threatened by a number of environmental stressors, and the addition of an invasive species as devastating as Zebra Mussels could push them over the edge. In November of 2009, this worry became a reality, as mussels were confirmed at the Conowingo Dam, which is on the Susquehanna River along the PA/Maryland border. This was the first time Zebra Mussels had been found in the lower Susquehanna, and combined with their discovery in Muddy Run (a tributary of the Susquehanna), it confirms their ability to spread rapidly into new ecosystems. (Blankenship 2009) Zebra Mussels expand their range in a very unique manner. They will first invade a lake or other body of slow moving water, then as their numbers increase they will spill out into rivers below the lake and “carpet the bottom for several hundred yards”. (Blankenship 2009) In fast moving water they will float with the current until they find another lake or slow moving section where they can settle and reproduce. The multiple hydroelectric dams found on the Susquehanna River would allow for this method of expansion to be especially successful.
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In an effort to better understand the rate at which Zebra Mussel populations grow, it is important to examine data from regions where the species is already well established. The activity below asks you to graph the population growth of Zebra Mussels in the Mississippi River Watershed in an effort to characterize the growth as exponential or linear. This distinction is important when assesing the effect that the spread of zebra mussel populations could have on local ecosystems. While the data is not specific to Pennsylvania waterways, the trends that you will find can be applied to the growth of mussel populations in areas such as Lake Erie and other aquatic environments in the Northeast U.S.
Lesson #2: Zebra Mussel Population Growth
zebra_mussel_exponential_growth.doc | |
File Size: | 200 kb |
File Type: | doc |