Bay-wide chemical treatments worked well to kill Eurasian water milfoil on Lake Minnetonka, but they won't be expanded this year because of other troubling changes in the water, the Department of Natural Resources has ruled.

Milfoil was nearly eliminated on Grays Bay and greatly reduced on Phelps Bay after chemical treatments last summer. But some desirable native plants disappeared with the unwanted weeds. Water clarity also dropped on Grays Bay.

It's not certain that the chemical affected water clarity, but both developments have given the DNR pause about expanding the treatments, said Chip Welling, DNR coordinator of aquatic invasive species management.

"There is some cause for concern and we hope that in 2010, the vegetation in Grays Bay will rebound," Welling said. "We don't want the other bays to do this until we have a better sense of how milfoil can be managed without doing more harm than good."

Eurasian milfoil is a thickly growing water weed whose stems tangle into unsightly floating matts that discourage boating and swimming.

The decision has disappointed waterfront property owners on St. Albans and Gideons bays who had raised money to dose their bays this summer, said Bob Tenner of Excelsior, who lives on Gideons Bay.

"It seemed to be such a success in the other bays and now that is being questioned by the DNR," said Tenner, who solicited residents to back the effort financially. "We are all disappointed because everyone was enthusiastic about attacking the problem."

Mayors of cities on the two bays -- Greenwood, Excelsior, Shorewood and Tonka Bay -- have asked legislators for a reconsideration.

This will be the third summer of a five-year study of the effects of chemical treatments on weed-plagued Grays, Phelps and Carmen bays. Carmen opted out of treatments last year and will resume this year. The U.S. Army Corps of Engineers and the DNR are doing the study.

Welling said treatments will not be extended to other bays until study is done on the effects of this summer's spraying in the original bays. One concern is the use of the chemical Triclopyr, which caused three species of native plants to decline, Welling said.

Spot-checks found that the presence of clasping-leaf pondweed dropped by 50 percent from September 2007 -- the year before the treatments began -- to last September.

The presence of flat-stem pondweed dropped by 49 percent and the presence of fern pondweed dropped by 15 percent from 2007 to last year. Welling said. Water lillies were also damaged. The effect on native plants is a concern because a key reason to control milfoil is to promote growth of native plants that provide habitat for fish and wildlife and improve water quality, he said.

The Lake Minnetonka Association, representing lakefront property owners, started the weed treatments in 2008 with DNR permission. Its executive director, Dick Osgood, was enthusiastic about last year's success and eager to expand the program to St. Albans and Gideons Bay this year.

He considers the DNR decision overly conservative and said other factors could explain the loss of native plants and the decline in water clarity.

In Grays Bay, three species of native plants increased and four decreased, while in Phelps Bay five species of native plants increased and three decreased, Osgood said. "That is inconclusive."

While the water in Grays Bay was less clear last August than in 2008, Osgood said, "There has been no determination at all what caused that. If the cause were the treatment that would concern me."

The decline in water quality could be tied to lake levels being down a couple of feet last year and wind and waves stirring up the bottom, Osgood said. Increased boating encouraged by less milfoil could have made the water murky, he said.

Not treating milfoil and allowing it to spread also kills native plants, Osgood said.

Herbicide treatments in Grays, Phelps and Carmen bays will start in late April or early May, Osgood said.

Grays will be treated with Endothall to control curly leaf pondweed because the Eurasian milfoil was nearly eliminated last summer.

Phelps will be treated for both milfoil and curly leaf pondweed. Carmen Bay will be treated for milfoil. Chemicals will be applied at slightly reduced strength and a few weeks earlier to see if it helps native plants, Osgood said.

Laurie Blake • 612-673-1711