"When mussels start to disappear, it's a sign that the river's health is on the decline."
Tharran Hobson, Illinois River Restoration Manager with The Nature Conservancy
When engineers reversed the flow of the Chicago River in 1908, they had no idea that the change would contribute to the decline of a small organism 200 miles downstream. Their goal was to keep Lake Michigan clean for drinking water, but changing the course of nature often has unintended consequences — in this case, moving excess sediment downriver and ushering in an era of massive land changes across the state. Even if turn-of-the-century science could have predicted some of those effects, protecting mussels in central Illinois would not likely have been a priority.
Fortunately, mussels — a keystone species in a functioning freshwater environment — are one of The Nature Conservancy's priorities. Freshwater mussels are imperiled worldwide, and many species are nearly extinct. But a new project, the first of its kind attempted on the Illinois River, could provide a solution to this large-scale problem.
Sixteen years ago, a survey found only a single butterfly mussel in the Illinois River. In April 2009, Conservancy biologist Tharran Hobson, along with Dan Sallee of the Illinois Department of Natural Resources (IDNR), decided to see if they could bring this species back from extinction in the Illinois River. Their experiment began in earnest when they started to propagate butterfly mussels in a controlled backwater environment at the Conservancy's Merwin Preserve at Spunky Bottoms, a wetland restoration located along the Illinois River.
The role of mussels in the ecosystem is not fully understood, but it's clear that they are big contributors to biological diversity — and with some mussels living more than 100 years, those contributions can be quite long-lived. Hobson says, "when mussels start to disappear, it's a sign that the river's health is on the decline." Nearly 50 species of freshwater mussels once lived in the Illinois River, but, according to Hobson, "we only have roughly half that number today. This experiment could start to turn that around." As these small, sedentary creatures feed, they filter in plankton for food, which can change the clarity and chemistry of water. Their waste provides nourishment for important algae populations. Although butterfly mussels grow to only about 5 inches in diameter, they can filter up to 18 gallons of water a day.
The Conservancy's river conservation director, Douglas Blodgett, says, "Because of the Clean Water Act, the Illinois River is a lot cleaner now than it was in the early to mid-1900s when mussel populations started to decline. We still have a long way to go toward a sustainable system, but mussels are an important factor in increasing biodiversity."
The excess sediment transported downstream due to the river reversal, combined with environmental changes during the 1900s like habitat loss, dams and levees and expanded agriculture along the Illinois River, made life especially difficult for mussels. They are extremely sensitive to environmental changes, and their inability to move quickly makes them particularly vulnerable to stresses. Sediment can smother them and pollutants can also cause deadly toxins to build up in their delicate flesh.
In parts of the Upper Mississippi River, butterfly mussels are still populous. This is where Conservancy and IDNR scientists gathered specimens for brood stock. They hope this brood stock will produce offspring that can later be transferred to the Illinois River.
In the wild, newly hatched mussel eggs, called glochidia, attach as parasites to a fish host, where they grow until they become juveniles. Butterfly glochidia are picky parasites and will only attach to a specific fish species, the freshwater drum. Fortunately, the freshwater drum remains populous in the Illinois River and is unharmed by the mussel, brightening long-term prospects.
The scientists in Illinois are recreating this process by siphoning the glochidia out of the female mussels with a syringe before "infecting" the gills of the fish with the microscopic mussels, a process which does no harm to the mussel. Weeks later, when the glochidia reach what is known as transformation stage, they will fall off the fish and onto the bottom of specially designed rearing cages. Hobson says, "Our hope is to recreate a self-sustaining population in the Illinois River."
Conservancy scientists are learning from similar efforts to propagate different mussel species in main stems of the Upper Mississippi River, rather than in controlled backwaters. Blodgett calls this "a direct attempt to build biodiversity from the riverbed up. If a self-sustaining population can be created, it could have huge repercussions in big-river conservation."
It will take several years before success can be determined. Freshwater mussels, though more diverse in the United States than anywhere else in the world, are one of the most imperiled groups of organisms on Earth. If the Conservancy and IDNR scientists succeed, they could provide a model for other big-river restoration projects not only in Illinois but around the world.