A year of testing for zebra mussels at Whirlpool Lake in Riding Mountain National Park has found no evidence of the invasive species.

DNA traces of zebra mussels were found in the lake in 2017, prompting the closure of the lake and its campground as a precaution.

Whirlpool Lake is 210 kilometres northwest of Winnipeg, in the 3,000-square-kilometre national park on Highway 10 north of Brandon, Man.

Richard Dupuis, visitor experience manager at Riding Mountain National Park, said the lake is now open to the public.

"People can currently go there. There's snowshoeing, backcountry skiing and excellent opportunities for wildlife photography," he said.

Dupuis added there is no clear reason why zebra mussel DNA got into the area.

"Actual zebra mussels were not found at Whirlpool Lake or Riding Mountain National Park, just the DNA," said Dupuis.

"There's very low risk in opening the area."

Testing will continue to ensure the invasive species doesn't show up in the waters in the future.

Zebra mussels were first found in Manitoba when they appeared in Lake Winnipeg in October 2013.

Zebra mussels have since been detected in the Red River, in Cedar Lake (440 kilometres northwest of Winnipeg and just north of Lake Winnipegosis), possibly in Singush Lake in Duck Mountain Provincial Park, and were suspected in Whirlpool Lake.

Researchers think the invasive species came to Manitoba in the Red River, in the form of mussel larvae, or as adults attached to debris or plant material.

One female mussel can produce up to one million eggs during a single spawning season. The mussels can choke out other organisms in a lake, make lakes clearer and thus promote algal growth and clog water intake systems.

Zebra mussels can also damage boat engines, power plants and public water pipes.