About 200,000 zebra mussels per square meter cover the bottom of Lake Minnetonka's Wayzata Bay — thriving on the water's moderate algae levels and affecting its quality, a study of the lake has found.

The Minnehaha Creek Watershed District released the findings of a five-year study on Thursday that looked at the conditions in which the invasive pests thrive, knowledge that could help slow their spread.

The invasive species, first found at the popular recreational lake in 2010, is less numerous in waters with high or low algae levels, according to the study. The zebra mussels' concentration in Wayzata Bay is "off the charts," said Eric Fieldseth, a program manager with the watershed district, compared with only 28 per square meter found in western Halsted Bay.

Dense populations of the dime-size zebra mussels can increase water clarity and decrease levels of chlorophyll and phosphorus, threatening fish and native mussels. They also damage boat motors and slice swimmers' feet.

Zebra mussels typically live four or five years. At the conference where the findings were released, a photo of another lake showed a stripe of washed-up zebra mussels on its shore. As the population booms and busts, Fieldseth said, the species will die off and then re-establish again.

Fieldseth added: "It's likely it'd happen on Lake Minnetonka over the next couple years." The key is to detect the species early and prevent their spread in Lake Minnetonka or across other chains, he said.

Zebra mussels' long-term effects on Lake Minnetonka still aren't known. The district plans to add more monitoring sites to better assess their effects, as well as other invasives such as hybrid milfoil and common carp, on lakes across the state.

Lake Minnetonka is "viewed as an exporter of [aquatic invasive species] research throughout the state," Fieldseth said. "It's equally as important to reduce the spread of AIS to other lakes. Just a few strands on a water tractor or trailer could contain zebra mussels."

Looking ahead, the district plans to study the adult zebra mussel population on Lake Minnetonka's floor.

"Just because you have an invasive on the lake, it doesn't really act invasive," Fieldseth said. "That [new research] will help us determine the number of zebra mussels it takes to actually cause these changes."

Natalie Daher • 612-673-1775