Mussel Populations In Iowa River Lower Than Researchers Hoped

Aug 27, 2018

Early results from a survey of the Iowa River show mussel populations are lower than researchers hoped. Scientists are monitoring the animals to better understand water quality in the river. 

Earlier this month, dozens of researchers and volunteers from Iowa, Illinois and Minnesota fanned out on the Iowa River to dig for mussels. The bivalves, which can live for decades and even a century in some cases, form the bedrock of freshwater stream ecosystems. 

Biologist Dan Kelner with the Army Corps of Engineers compares the animals to coral reefs, saying thy're integral to the overall health of the waterway.

"They provide that base for substrate, which then attracts other invertebrates, fishes, which then colonize the mussel beds," Kelner explained. "[They] attract invertebrates, crayfishes, other non-game fishes, darters, and then other predatory fishes that feed on them. So mussels really are the bedrock of many aquatic systems."

"Every kind of thing that a human can throw in a river, they've thrown in this river," - Scott Gritters, Iowa Department of Natural Resources

Combing through the sand and rock on the river bottom, researchers hoped to find evidence that mussel populations in the Iowa River are growing. But early results from the recent survey of the river suggest numbers are down from past years.

The team found 22 distinct live species, compared to 20 live species in 2014, but divers encountered fewer mussels overall. Sites which registered the highest density on the entire river in 2014, with 3.2 mussels per square meter, had zero mussels this year. And there was just one sighting of the rare Higgins eye mussel, a federally endangered species that scientists reintroduced in the Iowa River in 2000.

"Without doubt I was hoping to find more and expected to find more," said Scott Gritters, a biologist with the Iowa Department of Natural Resources, who organized the survey.

In past surveys, researchers have found evidence that Higgins eye mussels are reproducing on their own, a sign that water quality is improving by some measures. 

“If we find mussels in a river, or we find them coming back to a river, we know that it can sustain aquatic life," Kelner said. "Because mussels are longer-lived sedentary animals. So they can’t move out of the way of perturbations, harm.”

"If we find mussels in a river, or we find them coming back to a river, we know that it can sustain aquatic life." - Dan Kelner, Army Corps of Engineers

But naturally-occuring and human-caused threats to the mussels remain. Kelner considers the Iowa River to be one of the better quality rivers in the state, based on other surveys of mussel and fish populations by the Iowa DNR. Still, trash rests on the river bottom and dots the banks, which are shored up with hunks of old road surfacing, and in some places, entire cars.

"There's old cars...every kind of thing that a human can throw in a river, they've thrown in this river," said Gritters, warning his volunteers to keep an eye out for potential hazards. 

Kelner says pesticides from agricultural runoff and silt erosion from neighboring farmland pose a significant threat to mussels, as well as the channelization and damming of rivers. On some parts of the Iowa River, farm fields run up to within a few feet of the river's edge.

“We have row crops right up to the bank. Which those banks, then when the water gets high, erode into the river," Kelner said.  "There’s still major problems with a lot of rivers in the Upper Midwest, United States and elsewhere.”