Suddenly, zebra mussels are knocking on the door of the Boundary Waters Canoe Area Wilderness.

The recent discovery of the invasive critters in a small Iron Range mine pit lake poses an ominous threat to northeastern Minnesota lakes, including those in the BWCA. And because many northeast lakes already have low nutrient levels, biologists say zebra mussel infestations there could potentially affect the fisheries even more than in other state lakes.

Officials don't know how zebra mussels got into little Lake Ore-Be-Gone, also called Gilbert Pit, near Gilbert -- the farthest north to date the invasive species has been found in the state.

But they are concerned.

"The spooky part is the proximity to Burntside Lake [near Ely] and Lake Vermilion [near Tower]," said Rich Rezanka, Department of Natural Resources invasive species specialist. "All of a sudden it's not a four-hour drive from an infested water to those bodies, it's a short drive.

"Zebra mussels in Vermilion or Burntside would be catastrophic," he added. "The whole BWCA watershed would be susceptible."

Zebra mussels in the relatively sterile northeast lakes pose a greater risk for a fisheries food chain collapse, biologists say. Zebra mussels filter up to a quart of water daily and consume algae, also the main diet of zooplankton, tiny animals essential to small fish.

Algae eat phosphorus, so in lakes with low phosphorus levels -- and thus low algae levels -- zebra mussels could further reduce algae levels to the point that there is no longer enough food to sustain zooplankton populations. And the northeast lakes tend to be low in phosphorus.

"The zooplankton could starve, then there would be no food for the little fish, and there could be a collapse of the food chain," said Tom Jones, a DNR fisheries biologist.

No one knows if that will happen. But the potential is there, biologists say. "Food resources are scarce to begin with, so if you put someone else at the table, it's that much more food gone," said Rezanka. "They pose a serious threat."

Added Jones: "We need to monitor lakes with varying levels of phosphorus to better understand potential impacts of zebra mussels and other invasive species."

Rezanka said officials don't know how zebra mussels were introduced into the 223-acre flooded mine pit, but the mussels found were older ones that appeared to have been there a couple of years. They likely were accidentally brought there by people, officials say.

The lake has been stocked with rainbow trout for years, and an unknown number of anglers fish it. But because it is clear and 450 feet deep, it also is a popular scuba diving spot. Divers discovered the zebra mussels two weeks ago, and the DNR confirmed their presence last week.

Until now, the closest lakes to the BWCA that have been infested with zebra mussels are Pike Lake in Duluth and Lake Superior -- both lakes that anglers and scuba divers use.

Lake Ore-Be-Gone, a popular swimming hole, also has another invasive species, Eurasian water milfoil, which has been there for years. "What the heck is it doing in a mine pit 100 miles from the nearest infestation?" asked Rezanka.

The lake is fed by springs and no streams enter or leave it.

Signs were posted at the lake last week notifying users that it is contaminated with both invasive species, and that special rules now apply. It will be added to the list of waters infested with zebra mussels.

DNR conservation officers will increase patrols at the lake access, said the DNR's Capt. Ken Soring.

"It's just another step in the wrong direction," he said of the infestation.

State law requires boaters to drain watercraft and clean them of vegetation when leaving a lake. They also must leave their drain plugs out while transporting boats.

"The vast majority of people are doing what they are supposed to do, but there's always the ones that don't, and they [zebra mussels] were introduced because of that," said Rezanka. "Our biggest concern is that those same people go to Lake Vermilion next."

The discovery shows northeast residents that zebra mussels aren't just a problem in Brainerd, Alexandria or Minneapolis, he said.

"That's why the rules are in place."