Higher lake levels in Minnesota this year mean less milfoil

A cool, wet spring and early summer brought a surprising benefit to much of Minnesota: less milfoil on lakes.

The Minneapolis Park and Recreation Board was able to mow and harvest the clingy, stringy invasive species from city lakes with just one mower this season, after expecting to need two, said Rachael Crabb, water quality supervisor for the board.

Tuesday, the Lake Minnetonka Conservation District pulled its mowers from the lake, ending a season that was several weeks shorter than usual. Executive Director Greg Nybeck said workers held off on going after milfoil in June because a late ice-out, cool temperatures and persistent cloudy, rainy weather reduced its early-season growth.

Generally, much higher lake levels for much of the season, particularly when compared with last year's drought levels, limited the widespread surface matting that makes milfoil infamous among swimmers and boaters. In many cases, lakes simply rose too high for milfoil, which grows from the lake bottoms up, to reach the surface, said Chip Welling, aquatic invasive species manager for the Minnesota Department of Natural Resources. Milfoil can grow about 15 feet tall, he said.

That situation was more common on larger lakes, Welling added, because water levels on smaller lakes tend not to fluctuate as much as on larger lakes. Because they get less runoff from large storms, they also get fewer nutrients that would spur milfoil growth, Welling added.

After a 2.79-inch rainfall July 13, Minneapolis' Lake Calhoun rose within two days to its highest elevation of the season, about a foot higher than normal, Crabb said. That would normally be a time when milfoil coverage would be at its worst, but this year the plants topped out below the surface.

Several Minneapolis lakes were blanketed at times this summer by a type of bright green algae that Crabb described as "cotton-candy-like." While it starved milfoil of light, limiting its growth, it's also a taint on the city's sky-blue waters. Crabb said what makes it proliferate isn't totally understood. Cold weather earlier this year, she said, may have left more nutrients available for the algae by slowing milfoil's start.

"We could see it uprooted and washed up along the shorelines," Geisen said.

This year hasn't seen such violent downpours and water level increases. So wild rice has been able to endure gently its "floating leaf stage," when it rises to the surface and flattens out, allowing it to absorb oxygen from the air and produce seeds, Geisen said.

The persistence of wintry weather into early May delayed the development of wild rice as well, but the crop across the region from Brainerd toward Duluth "is looking good again this year," Geisen said. Heavy rains to the northeast, into the Arrowhead, have made the crop there "spotty," she said.

Bill McAuliffe • 612-673-7646