The number of zebra mussels on the floor of Lake Mille Lacs -- Minnesota's most popular walleye lake -- has increased 41 percent from last year.

In one spot at Three Mile Reef, scuba divers last week counted 7,700 zebra mussels per square foot -- or about 53 per square inch.

"It's just a carpet of zebra mussels,'' said Tom Jones, Department of Natural Resources large lake specialist who dove three times last week to assess the zebra mussel population.

The invaders from Europe attach to rocks, so rocky areas of Mille Lacs are covered with them as far as the eye can see. "It's just amazing,'' Jones said. "They just go on and on and on.''

The mussels are mostly absent where the bottom is sand and mud. Jones also saw zebra mussels attached to vegetation -- which is why boaters must be diligent in removing weeds, he said.

He and coworkers have checked the same areas since zebra mussels were first found in Mille Lacs in 2005. This year, those areas averaged about 1,300 zebra mussels per square foot, compared to 920 per square foot last year. That's nearly 1.5 times as many.

"We expected a lot worse,'' said Jones, perhaps a 10-fold growth. Warmer lake temperatures this summer might have slowed the population explosion, he speculated.

"We've seen zebra mussels turn turquoise, then white, and we think that is stress-related,'' Jones said. "The water temperature was 78 degrees even below the surface; that's pretty toasty.''

But divers aren't finding lots of dead zebra mussels. And it's also possible the slower growth rate is because zebra mussels have filled the best habitat, Jones said.

"There are areas with no vacancy,'' he said, "and other areas with lower densities because there is no place for them to live.''

Still, the population explosion has been phenomenal: In 2009, they counted just four zebra mussels per square foot; in 2010 they counted 14 per square foot. Now, two years later, that number has increased nearly 100 times.

Jones doesn't know how high the zebra mussel population will get, but scientific literature says peak densities can hit 10,000 per square foot. He expects the numbers to decline somewhat after peaking.

Officials don't know what impact the zebra mussels will have on Mille Lacs, its famed walleye fishery or other infested waters. But the good news at Mille Lacs is that water clarity hasn't increased -- at least not yet -- something that usually occurs.

Zebra mussels filter up to a quart of water daily and consume algae, which is food for zooplankton, essential food for small fish. Clearer water wouldn't be beneficial to walleyes and could spark vegetation growth.

But Jones said it's too early to draw any conclusions about how zebra mussels might affect the lake. "We'll just have to wait and see.''

July, August and September is when zebra mussels produce microscopic larvae called veligers, which could be spread to other lakes in minnow buckets, livewells or boat bilges.

With more zebra mussels, there's bound to be more veligers.

"It's certainly not time to be complacent,'' Jones said. "The biggest problem is spreading them [zebra mussels] from Mille Lacs.

"The risk factor is higher because the population is higher. More vigilance than ever is required.''

He said boat lifts and docks pulled from Mille Lacs "almost certainly would be infected.''

Meanwhile the other problem with zebra mussels is their impact on swimmers. Their shells are extremely sharp, whether the mussel is alive or dead and the shells have washed ashore.

"They slice like razor blades,'' Jones said. "My fingers are all cut up from counting them.''

Doug Smith • dsmith@startribune.com