Comparison of Chicago area waterways in 1830 and after 1901 when engineers
reversed the flow of the Chicago River.
Exchange of aquatic species between Lake Michigan and the Mississippi drainage basin has probably occurred sporadically for thousands of years. The entire Laurentian Great Lakes Basin was blanketed in the Wisconsin Glaciation more than 14,000 years ago. Following the retreat of the glaciers, vast bodies of water were available for colonization by aquatic species. What we now call the native fish fauna of the Great Lakes Basin originated in large part from the northward dispersal of species from the Mississippi River basin. During the last 14,000 years, Lake Michigan has been periodically connected to, and then isolated from, the Mississippi drainage basin. The last period of natural connection between the two drainage basins ended approximately 6,000 years ago. From then to the early 1900s, the aquatic communities of Lake Michigan and the Mississippi basin developed in relative isolation from each other (although seasonal flooding may have occasionally created small, temporary links between the two systems).
By the early 1900s, engineers had succeeded in reversing the flow of the Chicago River and completed the construction of the Chicago Sanitary and Ship Canal. The two basins were once again connected by a permanent waterway, allowing large volumes of water to flow continuously from Lake Michigan into the Mississippi drainage basin, and greatly facilitating the exchange of species. This new era of species exchange is of concern to biologists for several reasons. The initial era (about 14,000 years ago) involved species moving from an established basin (Mississippi) into a newly created aquatic system (Lake Michigan) that lacked an established aquatic community. Subsequent eras of species exchange involved species that had at least evolved on the same continent. At present, however, the frequency and ease of modern transoceanic shipping and the intentional transport of organisms for the aquaculture and aquarium industries have resulted in the invasion of organisms that evolved on different continents and in highly varied aquatic communities.
Comparison of Chicago area waterways in 1830 and after 1901 when engineers
reversed the flow of the Chicago River.
Transfer and establishment of nonindigenous aquatic species are likely to be highly disruptive. Nonindigenous species may be freed from predators and diseases encountered in their native range and exhibit explosive population growth following establishment in a new area. Conversely, they may harbor and introduce new diseases and parasites that native species have not evolved with and have no defenses against. Once an invading species becomes established in either basin, it is not isolated in that basin. Rather, it can rapidly disperse to the other before control and eradication measures can be developed and implemented. Scientists at INHS field stations along Lake Michigan and the Illinois and Mississippi rivers are actively involved in studying the impacts and transfer rates of invading species within and between the Great Lakes and Mississippi drainage basins.
The zebra mussel (Dreissena polymorpha) is a good example of an invading species that became established in Lake Michigan and quickly dispersed into the Illinois and Mississippi rivers. Zebra mussels became established in the Great Lakes in the late 1980s. In 1993, these mussels reached an average abundance of 60,000 / m2 in the lower Illinois River and were becoming established in the Mississippi and Ohio rivers. Unlike the relatively stable populations in the Great Lakes, zebra mussel populations in the Illinois River exhibit recurrent boom and bust cycles. Periodic high mortality in the Illinois River is presumably due to unfavorable environmental conditions, such as high temperature, high turbidity, or low dissolved oxygen, which persist during low flow periods in the summer months.
Once an invading species successfully moves from one basin to another, the continued success of that species in the new basin may be dependent upon continual dispersal across the interbasin connection. Zebra mussel larvae spawned in a riverine system are quickly carried downstream by river currents. Establishment and maintenance of zebra mussel populations in the main channel of a river are dependent upon the production of new larvae by upriver populations. Estimates of larval growth and drifting rates in the Illinois River indicate that most zebra mussel larvae travel a minimum of 190 miles before settlement. Settlement in the upper 70% of the river is therefore dependent upon larvae produced by populations upriver of the Illinois River headwaters. In this system, the Lake Michigan population may provide a stable, upriver source of larvae. Without the interbasin connection and dispersal of larvae, zebra mussel populations in the river would presumably begin to die off in a downriver direction as their sources of new recruits were eliminated.
INHS researcher Scott Whitney studies the effects of zebra mussels
on
native unionids in the Illinois River.
Other invading species that are now found in both basins include the white perch (Morone americana) and the rudd (Scardinius erythrophthalmus). Many other invading species may be poised for interbasin dispersal in the near future. The round goby (Neogobius melanostomus), tubenose goby (Proter-orhinus marmoratus), and the ruffe (Gymnocephalus cernuus) are three fish species that have become established in the Great Lakes within the past several years. They are expected to have significant, detrimental impacts on several native fish species and may soon appear in the Illinois and Mississippi rivers. The grass carp (Ctenopharyngodon idella) and bighead carp (Hypophthal-michthys nobilis) have recently established breeding populations in the Mississippi and Illinois rivers and may soon disperse into the Great Lakes. The striped mullet (Mugil cephalus) is occasionally collected from the Mississippi drainage basin. An invading zooplankter (Daphnia lumholtzi), recently established in Illinois, was found last year in the Illinois River just 100 miles downstream of Chicago. Daphnia lumholtzi adults and ephippia (resting eggs) could easily be transported from the Illinois River into Lake Michigan via bilge water and live wells of recreational watercraft. Interbasin dispersal does not necessarily mean that an invading species will establish viable populations in both basins. While some species may adapt to environmental conditions in both Lake Michigan and the Mississippi drainage basin, others may not. Unfortunately, the relative ease of interbasin dispersal means that those invading organisms that are capable of surviving in both environments may colonize both basins before adequate studies of ecological impacts and control techniques can be conducted. The Lake Michigan-Mississippi interbasin connection is primarily man-made and highly engineered. It may be possible in a system such as this to construct dispersal barriers which would reduce or eliminate the interbasin dispersal of some invading species. Survey scientists are now planning to conduct studies to document the extent of interbasin dispersal and the feasibility of constructing dispersal barriers.
Jim A. Stoeckel, Richard E. Sparks, K.D. Blodgett, Scott D. Whitney, and Paul T. Raibley, Center for Aquatic Ecology
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