Chemical treatment no "silver bullet" against lake weeds

MADISON WI - Treating an entire lake, instead of just a portion, with a chemical to kill the invasive, nonnative plant Eurasian water-milfoil is gaining popularity in some states, but has shown only limited success in Wisconsin and a host of adverse side effects, recent research shows.

Experimental whole-lake treatments of the chemical fluridone on four Wisconsin lakes temporarily knocked back extensive infestations of the stringy weed, but they quickly returned to pre-treatment levels or worse on three lakes – even when followed up by additional chemical treatments and manual harvesting to target areas of regrowth.

The treatment also eliminated beneficial native plants critical for fish and other aquatic creatures, which in turn was suspected of triggering significant decreases in water clarity and increases in algae on three of the lakes. The decrease in beneficial plants and increase in algae is also suspected of contributing to low dissolved oxygen levels that threatened fish survival on one lake over winter.

“Some people have been saying whole-lake treatment is the silver bullet,” says Jennifer Hauxwell, a Department of Natural Resources aquatic ecology researcher and one of the principal investigators. “But our research shows that’s not the case – and that there’s good reason to be methodical in determining which lakes are good candidates for this approach.

“Ecologically, dosing a whole lake with a chemical is a big step up from the “spot” treatments we routinely permit.”

DNR routinely authorizes chemical treatments on small portions of a lake, typically less than 10 acres, as one method of trying to manage excessive native aquatic plants and invasive exotic plants such as Eurasian water-milfoil. That invasive plant, first documented in Wisconsin in the 1950s and now found in more than 400 lakes, sometimes forms thick mats at the water’s surface that can interfere with boating, swimming and other recreation, can impair fish habitat and crowd out native plants, and reduce the economic value of a water body.

In recent years, however, the agency has fielded a growing number of requests from lake associations and others to allow entire lakes to be dosed with the herbicide fluridone to address extensive milfoil infestations.

Since 1997, DNR has approved experimental whole-lake treatments on four lakes with the chemical fluridone to try to control Eurasian water-milfoil: Potter Lake in Walworth County, Random Lake in Sheboygan County, Bughs Lake in Waushara County and Clear Lake in Sawyer County.

The lakes have been monitored over the intervening years, and Hauxwell and fellow researcher Kelly Wagner analyzed the data from one year after treatment, and also from four to seven years after treatment, to learn how the Eurasian water-milfoil, native aquatic plants, and water clarity responded.

“Before you use any tool you want to understand positive and negative outcomes, and have realistic expectations of what it can achieve,” Hauxwell says. “This is especially true with a whole-lake treatment.”

Chemicals now used on Eurasian water-milfoil and other nuisance-level plants kill plants and organisms that aren’t the targeted species. With a partial treatment, however, those unintended effects are confined to a small area and susceptible species can survive in refuges elsewhere in the lake. .

In addition, because fluridone is a relatively slow-acting herbicide, the dosage must be maintained for 60 to 90 days to kill the plants. The herbicides used in spot treatments, including 2,4-D, endothall, and diquat, are usually fast-acting and kill the plant on contact.

The four Wisconsin whole-lake treatments applied concentrations ranging from 6 to 16 parts per billion of fluridone to the lake surfaces and maintained levels greater than 4 parts per billion in the water for periods ranging from 40 days to nine months.

Data collection in subsequent seasons determined that the whole-lake treatments, followed by subsequent spot treatments and manual removal in spots with regrowth, offered relief from the Eurasian water-milfoil that ranged from one growing season to four, Wagner says.

Among the unintended consequences resulting from the treatment was a shift in the native plant communities on all four lakes after treatment. Plants susceptible to fluridone – among them coontail, native milfoils, elodea and naiads, all important in providing foraging, hiding and spawning areas for some fish – were significantly reduced or eliminated while native plants not susceptible to fluridone sometimes increased, as did curly-leaf pondweed, another invasive species.

The researchers conclude that the shifts may have contributed to significant decreases in water clarity on two of the three lakes for which data was available. Potter Lake, where native plant populations declined the most, experienced the largest drop in water clarity, from 6.5 to 3.25 feet.

If beneficial native plants die, not only are they no longer sequestering nutrients such as phosphorus and nitrogen, but their decomposition adds more nutrients to the water, Hauxwell says. Algae are able to quickly respond to increased nutrients. As their populations explode, “water clarity goes down and the odor associated with certain types of algae may go up.”

DNR staff are also concerned with potential consequences if fluridone kills the kind of toxin-producing blue-green algae naturally present in many Wisconsin lakes. Such toxins are released into the water when the blue-green algae die, and can pose a threat to humans, pets and domestic livestock.

Wagner says the Wisconsin results, and the researchers review of whole-lake treatments in 10 other states, show that it’s important for people to be realistic about what whole-lake fluridone treatments will achieve, and that some lakes won’t be good candidates for the treatment.

“This is a nuisance relief tool, not an eradication tool for Eurasian water-milfoil, and it has some unintended effects that make it a poor choice for many lakes,” she says. Lakes dominated by native plants susceptible to fluridone and fertile lakes with high levels of nutrients are poor candidates, as are flowages and drainage lakes, where the slow-acting, long-lasting chemical treatment couldn’t be easily confined to the target lake.

FOR MORE INFORMATION CONTACT: Jennifer Hauxwell (608) 221-6373; Kelly Wagner (608) 221-6338