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Study: Lake Minnetonka's water quality changing since zebra mussels arrived

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We don't know how to get rid of zebra mussels, but five years worth of data from Lake Minnetonka might help researchers learn how to manage the invasive species.

The Minnehaha Creek Watershed District has been monitoring the lake closely since the invasive species arrived in 2010.

Wednesday they released a report highlighting the findings of their research, which they've done in conjunction with Blue Water Science.

Zebras take a Goldilocks approach to algae

While algae is one of the main foods of zebra mussels, the watershed district says mussel numbers are biggest in bays where algae levels are moderate – not too low, nor too high.

They contrasted Halstad Bay, which has high algae and 28 zebra mussels per square meter, with Wayzata Bay, where the algae level is moderate and there are 200,000 zebra mussels per meter (those numbers are from 2014).

The watershed district's report says where algae levels are very high it tends to be blue green algae, which is not a good food source for the mussels.

How has the water changed?

According to the report, there are two main changes in the areas that are thick with zebra mussels: algae levels are down and water clarity is up.

The effects of those changes are still being studied but Eric Fieldseth, who manages the district's Aquatic Invasive Species program, tells KSTP the zebra mussels are reducing the food available for native mussels and other organisms.

"You think of a food web, you start eliminating some of the base of the food web, it's going to start affecting organisms all the way up to fish," Fieldseth says.

Given the popularity of lake among boaters, there were fears when zebra mussels arrived that Minnetonka would be an "exporter" of the invasive species.

But Lakeshore Weekly reports boat inspections have limited their spread and nearby waters such as Lake Minnewashta and Piersons Lake remain free of zebras.

The Department of Natural Resources has more information about zebra mussels and a list of infested waters in Minnesota.

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