Erika Jensen presents at the 2th annual Freshwater Summit.  Photo:  Mike Krebs, Record-Eagle

By Sheri McWhirter of the Traverse City Record Eagle - October 31, 2019

Invasive mussels in the Great Lakes are linked to problems with the freshwater seas’ food web, issues that impact both commercial and sport fisheries.

Speakers at the 12th annual Freshwater Summit in Traverse City dissected invasive mussels’ appearance since the mid-1990s and the accompanying alterations to the food web. They also talked about research into how the mussels can be combated not only in Lake Michigan, but across the Great Lakes and inland lakes and streams.

“The mussels have changed the fundamental way the lake functions,” said Steven Pothoven, research fish biologist at the Great Lakes Environmental Research Laboratory in Ann Arbor operated by the National Oceanic and Atmospheric Administration.

He said since first zebra and then quagga mussels began their infestation of Lake Michigan, researchers observed changes to the food chain from microscopic phytoplankton to top predator fishes.

Pothoven said invasive mussels reduce the productivity of offshore regions in the lakes by intercepting nutrients normally transported from near shore areas. They also selectively filter out algae, which he said can lead to blooms of the types of algae they don’t like such as harmful cyanobacteria.

Another impact is clearer water because of the mussels’ filtering, which in turn allows more algae to grow deeper into the water column. That can lead to increased claudophora blooms that are linked to avian botulism outbreaks, Pothoven said.

“These mussels create real management problems,” he said.

Changes to algae bloom cycles in the lakes — phytoplankton spikes shifted from spring to autumn during the last 30 years — are joined by rapid declines in the abundance of the macroinvertebrate diporeia, a small crustacean that resembles a shrimp and long served as a prey species. That historic Great Lakes creature is now practically vanished, Pothoven said.

There also have been changes to the zooplankton community, Pothoven said.

Altered spatial distribution and seasonal pattern changes have happened, mismatching when fish hatch and need to find food. Then there are booms in populations of spiny water fleas, an invasive predatory zooplankton.

“The problem is these eat zooplankton, the same thing fish eat,” Pothoven said. “It’s a zooplankton eating more zooplankton than fish are.”

Resulting declines in fish condition and growth have been observed, he said.

Another example of the disrupted food web is how whitefish eat the mussels, shells and all. It stunts growth, he said.

“It would be like if you ate your dinner plate with your taco,” Pothoven said, explaining how the whitefish become full but without as many calories consumed.

Summit attendee Rose Maylen, a stewardship technician with the Grand Traverse Regional Land Conservancy’s Americorps program, asked what Pothoven meant when he said invasive spiny water and fish-hook fleas are “lengthening the food chain” in connection to the invasive mussels.

Pothoven said invasive mussels facilitated invasions by those other invasive species which changed the food chain from algae-zooplankton-fish to algae-zooplankton-zooplankton-fish. He said with every added link in the food chain comes a 10 percent loss of energy and thus, diminished overall efficiency.