In Green Lake, clearer water may be signs of trouble ahead

Zebra mussels have made a lake known for water clarity even clearer.
Zebra mussels have made a lake known for water clarity even clearer. The long-term impact of the change is a lot murkier.
Ann Arbor Miller for MPR News file

Green Lake is known for its walleye fishing and deep, crystal clear water.

Those are among the reasons why it was chosen to host the Minnesota governor's fishing opener this weekend.

But the improved clarity — due in part to the lake's growing population of zebra mussels — is changing the lake and its inhabitants in ways experts are still trying to understand.

"We have a lot of people who say to us, 'Well, isn't clearer water better water?'" said Charlene Brooks, aquatic invasive species program coordinator for Kandiyohi County. "But in reality, clearer water in many cases is less productive, meaning there's less food at the base of the food chain for the creatures — the fish that we like to fish for."

Green Lake has always had clear water. But since zebra mussels were discovered in the lake about four years ago, the water clarity has increased significantly.

"From 2016 to 2017, we've seen a pretty significant increase in the numbers of zebra mussels in the lake itself," said Gary Montz, a research scientist with the Minnesota Department of Natural Resources' ecological and water resources division.

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Green Lake's zebra mussel population is still in its early growing stage, Montz said, so it's difficult to predict its impact.

"If you have clearer water, you may see aquatic plants growing at deeper depths than you have seen before because the sunlight can penetrate deeper," he said. "You may see an increased abundance of the aquatic plants. You may see denser beds or stands of the aquatic vegetation."

Signs alert boaters about zebra mussels and Eurasian water milfoil.
Signs alert boaters about the presence of zebra mussels and Eurasian water milfoil at a public access on Green Lake in Spicer.
Kirsti Marohn | MPR News

Zebra mussels are found in more than 100 Minnesota lakes. They are filter feeders, straining out microscopic algae called phytoplankton that other species rely on for food.

Experts say it's not yet clear what impact zebra mussels are having on Green Lake's fish.

DNR surveys have found a decrease in the number of walleye in Green Lake, said Dave Coahran, Spicer area fisheries supervisor. But Coahran said there's also less fishing pressure for walleyes, as anglers switch to other species such as bass, sunfish and crappies.

"So there's less walleyes caught and harvested, but the people who target walleyes still do about as good as they used to," he said.

As Green Lake changes, it's facing other threats. Another invasive species, Eurasian milfoil, showed up about 18 years ago.

Just a few miles away, Lake Koronis is dealing with a troublesome invader known as starry stonewort, a pesky algae that forms dense mats, making it tough to boat or swim. There's a fear starry stonewort could spread to other area lakes, said Wendy Crowell, DNR consultant on aquatic invasive species management.

"Certainly people who are boating out on the lake should keep an eye out for an unusual plants, and clean all the plants off your boat and trailer," Crowell said.

Kandiyohi County and the Green Lake Property Owners Association are working to keep new invaders out. Inspectors check boats from sunup to sundown all summer at all six of the lake's public accesses.

Brooks said she worries about new invasive species showing up that aren't even on people's radars yet.

"We tend to talk about the same cast of characters, not realizing that there are countless species that we have not heard of or are not familiar with that are capable of causing a whole new level of damage," she said.