U.S. Water News Online
PEORIA -- Sediment in the Illinois River may be the bane of
boaters, but it's likely contributing to a die-off of zebra mussels,
Zebra mussels invaded the Illinois and Mississippi rivers more
than a decade ago, threatening native mussels, clogging water-intake
systems and causing other damage. The small clams from northern
Europe attach themselves to hard surfaces, including native mussel
shells and locks in the river systems.
Though no scientific surveys of zebra mussel populations have been
done lately in the Illinois River, scientists believe they're not
"We had good populations through 1995. Since then, they seem to
have not been doing very well," said Jim Stoeckel, a biologist from
the Illinois Natural History Survey at Havana.
John Sullivan, water quality specialist with the Wisconsin
Department of Natural Resources, said the zebra mussel population
also has dropped in the upper Mississippi River lately.
"They're definitely impacted by something. We suspect a
combination of things," he said.
Scientists believe high sediment loads in the rivers and high
water temperatures killed many zebra mussels. They filter out and
consume bacteria and algae in the water but "don't like things that
are sandy or gritty," Sullivan said.
In fact, expelling the grit uses up their energy.
They also cannot survive for long in water above about 86 degrees
Fahrenheit, Sullivan said. The temperature of the upper Mississippi
was that high for several days this year.
It's the same story on the Illinois River.
"We found zebra mussels are not well adapted to high sediment
loads that we see here in the Illinois River," Stoeckel said.
Floods in 1993 and 1995 caused abnormal conditions and allowed
them to thrive, he said. But since then, they've died off.
They also die off when river temperatures rise.
"The question is, why did they do so well back in those early
years?" Stoeckel said.
He has been monitoring the larvae in the Illinois River, and those
numbers have dropped off. The larvae attach themselves in late May or
June, "but they die back by the end of the summer. So populations
here are transient. Adults don't seem able to survive very long,"
In the right habitat, they can live up to five years.
The reduced zebra mussel population has benefited the native
mussels, many on the threatened species list. When zebra mussels
attach to native mussel shells, they suffocate the natives.
The natives long ago adapted to water temperatures in the Illinois
River, though high sediment loads and pollutants are not good for
them either, Stoeckel said.
"We need to find out whether the zebra mussel die-off is lack of
recruits, insufficient numbers coming downstream or plenty of larvae
that died after settling out," Stoeckel said.
He's studying samples and may have definitive information soon.
In the upper Mississippi River, zebra mussels are thriving in a
cold, 16-foot-deep lake on the Wisconsin-Minnesota border that is
part of the river, Sullivan said, so the supply of larvae likely is
On the Mississippi, the conditions lately "have definitely set
them back," Sullivan said. "Time will tell whether they will build up
to levels we saw earlier."
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