Invasive planktonic species face unique challenges in maintaining viable populations in river systems following initial establishment. Species with sessile adult and planktonic larval stages produce young that are carried downriver by currents, preventing them from settling out and replenishing the original benthic adult population. Species that are planktonic their entire lives produce young that drift downriver along with the adult parent population, neither stage being able to swim upstream against prevailing currents and repopulate upstream areas. Despite these challenges, invasive species representing both types of planktonic lifestyles are well established in the Mississippi and Illinois rivers. The methods by which populations are maintained may make species such as the zebra mussel amenable to control in large-river systems, whereas other species such as Daphnia lumholtzi are likely to prove nearly impossible to control.
In large-river systems such as the Illinois and Mississippi, zebra mussel populations are patchy, with upstream populations producing young that drift with the currents to replenish downstream populations. A system of populations connected by dispersal can be studied and described within a metapopulation framework. Metapopulation models have long been used in marine systems to help conservation biologists understand the size, number, and spacing of protected source populations (refuges) required to sustain the overall meta-population of desirable marine organisms. For the past several years, researchers at the Illinois Natural History Survey and the State University of New York have been developing metapopulation models for zebra mussels in large-river systems. Eventually, we hope to apply the inverse of conservation recommendations to affect control of zebra mussels on an ecosystem-wide level.
In the Illinois River system, Lake Michigan likely serves as the ultimate source population that sustains riverine populations. Multidirectional currents and high retention times in Lake Michigan allow populations within the lake to be self-sustaining with some larvae passing through the lock and dams and into the Chicago River and canal system. During peak production times, as many as 13 million larvae per second flow from Lake Michigan into the Illinois River system. These larvae settle out and form adult populations which in turn produce larvae to seed and replenish zebra mussel populations farther downstream. In the upper Mississippi River, Lake Pepin likely plays a role similar to that of Lake Michigan. Adult zebra mussels are rare to absent above Lake Pepin, but increase dramatically below the lake. Larval abundance exhibits a similar pattern, with more than 100 million larvae per second flowing past sites below Lake Pepin on frequent occasions, and flux as high as 1.5 trillion larvae per second estimated during peak production.
Control of zebra mussels in the Illinois River might be possible by greatly reducing or eliminating the drift of veligers from Lake Michigan into the river system. An electric dispersal barrier is currently being constructed to reduce the exchange of exotic fish species between the river and Lake Michigan. Plans are under way to eventually develop a barrier that will also be effective against various planktonic organisms such as zebra mussel veligers. If proven effective in the Illinois River, similar control tactics could feasibly be applied to the Mississippi River, focusing on areas such as Lake Pepin.
Unlike the zebra mussel, control of Daphnia lumholtzi may never be possible. Daphnia lumholtzi seems better able to withstand the high summer temperatures and high sediment loads characteristic of Illinois River backwater lakes than can zebra mussels. These backwater areas can maintain populations of Daphnia lumholtzi during periods of isolation from the main channel, and provide Daphnia lumholtzi to the main channel during periods of connectivity. It is not likely that dispersal barrier technology could feasibly be applied to the myriad of floodplain lakes and sloughs along the Illinois River. Perhaps even more important than its ability to survive and flourish in the floodplain is the ability of Daphnia lumholtzi to produce resting eggs, which are covered in a protective coating called an ephippium. Ephippial eggs can survive dessication and may remain viable for decades or even more than a century. Because they can survive out of water for long periods of time, ephippial eggs may be transported overland in all directions via various natural and human-mediated vectors. Even if the adult Daphnia lumholtzi were somehow eliminated, hatching of ephippial eggs would reseed the populations for years to come. Although species such as the zebra mussel may be amenable to control in large-river systems, species such as Daphnia lumholtzi emphasize the importance of preventing exotic species from becoming established in the first place.
Jim Stoeckel, Center for Aquatic Ecology
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