Zebra mussels have been found in a popular recreational lake in the Twin Cities' west metro area, the Minnesota Department of Natural Resources said Wednesday.

Christmas Lake had already been considered at high risk for infestation because it is across Hwy. 7 from sprawling Lake Minnetonka, where the fast-spreading aquatic invasive species has been seen since 2010.

As a result of the finding, the DNR will designate the 235-acre lake as infested and update warning signs at its public launch in Shorewood.

Even before the discovery, boat inspectors had been working at the public launch site daily from 6 a.m. to dusk from spring ice-out to the end of October, funded by the Watershed District, the city of Shorewood and the Christmas Lake Association.

On Saturday, staffers with the Minnehaha Creek Watershed District found four zebra mussels attached to a device suspended in the water below Christmas Lake's public dock.

In a subsequent search, they found more zebra mussels on underwater rocks and along the shoreline near the launch. DNR staffers visited the site this week and confirmed the presence of zebra mussels.

Zebra mussels crowd out and compete with native species for food.

"Because this infestation appears to have been detected early, Watershed District is working with the DNR and the city of Shorewood on a rapid-response plan which may help to prevent a lakewide infestation," the DNR said.

The Watershed District had been conducting what it called "rigorous monthly monitoring" of Christmas Lake. It's been submerging samplers, searching the shoreline in kayaks and snorkeling to check underwater rocks, and testing water samples for the presence of zebra mussel larvae, called veligers. In the last check on July 29, staffers found no zebra mussels, and none of the samples contained veligers, the district said this week.

The nonnative, fingernail-sized zebra mussels crowd out native mussels and compete with other aquatic creatures for food. They attach to boat hulls and docks, and their sharp shells create a hazard for swimmers.

For Minnesota's 2.3 million boaters, such a DNR designation on a lake means more watercraft inspectors and other measures, including mussel-sniffing dogs.

Public concern and awareness have risen dramatically in the past couple of years as zebra mussels infest more Minnesota lakes. As a result, local funding to fight aquatic invasive species has steadily increased over the past three years, with watershed districts, park districts, cities, counties and other local governments spending millions of dollars on it. Enforcement measures, fines and warnings have also been amped up.

For more information about zebra mussels and a current list of Minnesota waters that have been designated as infested, go to www.mndnr.gov/ais.

Staff writers Kelly Smith and Pamela Miller contributed to this report.