Zebra mussels are impacting Lake Minnetonka’s water quality

Zebra mussels have caused significant changes in Lake Minnetonka’s water quality, according to findings of a study by Minnehaha Creek Watershed District (MCWD). The District has been monitoring the lake’s zebra mussel population since they were first detected there in 2010. Five years into the study, which was developed in partnership with Blue Water Science, MCWD released its findings on Thursdsay, April 21 at its AIS Spotlight, a gathering of community leaders at Minnetonka Community Center.     

The District has discovered the biggest water quality changes are occurring among bays with the highest number of zebra mussels. In Wayzata Bay, which the population topped out at an estimated 200,000 zebra mussels per square meter in 2014, there has been an increase in water clarity and a decrease in algae (Chlorophyll) and Phosphorus. Those changes are not as prevalent in bays with lower zebra mussel populations. In Halsted Bay, which has 28 zebra mussels per square meter, there has been little change in water clarity, Chlorophyll and Phosphorus.

MCWD has also learned what conditions zebra mussels need to thrive. The highest populations are in bays with moderate levels of algae, an important food source for zebra mussels, and the lower populations are in bays that either have very high or very low levels of algae. Lakes with very high algae levels tend to be dominated by blue-green algae, a poor food source for the mussels, and lakes with very low algae content simply don’t have enough food available.

“This detailed data on how zebra mussels spread in different types of lake conditions will be valuable for predicting the severity of infestations in lakes across the state, and the impacts they may have on water quality,” said MCWD AIS Program Manager Eric Fieldseth. “This is new information in the field of zebra mussel research.”

The long-term impacts of zebra mussels on Lake Minnetonka are not yet known. As water quality changes in some bays, they could alter the food web which support fish communities and other aquatic life. Changes in algae communities could also lead to more harmful blue-green algae blooms. The native mussel population is also at risk. In parts of the lake, native mussels are being suffocated by their invasive cousins.

“Lake Minnetonka is a uniquely valuable place to study the effects of zebra mussels because of the diversity of water quality, depths and lake bottoms among its numerous bays,” said MCWD Director of Research and Monitoring Craig Dawson. “The findings are extremely valuable in helping inform future lake management decisions.”

The District plans to gather additional data as part of this study. It will increase the number of monitoring sites on Lake Minnetonka to assess the long-term impacts of zebra mussels.

For more information about MCWD’s AIS Program, visit www.minnehahacreek.org/AIS.