An infestation of an invasive aquatic species, known as milfoil, has sprouted in Steel Lake.

Eurasian watermilfoil was found in the lake June 14. The submersed aquatic plant originates from Europe and Asia and is considered a noxious weed. If not managed properly, it could lead to deteriorating water quality, algae blooms and danger to swimmers. Currently, the lake remains open for recreational use, with the exception of the infested areas. But city staff are again reminding boaters to do their part in helping to prevent the spread of the weed.

The milfoil infestation is the largest seen in the lake since 2000, said Dan Smith, Surface Water Quality program coordinator. It is limited to two private residential shorelines. Staff is watching closely to make sure it does not spread further. Milfoil is known for being particularly invasive and tricky to control, according to the Washington State Department of Ecology’s (DOE) website, www.ecy.wa.gov.

“It’s not a big problem right now but it’s something we continually have to keep an eye on,” Smith said.

Milfoil is spread primarily by boats used in multiple bodies of water, he said. The plant can become attached to the bottom or propeller of a boat and then be transferred from one area to another. Once the weed takes root, it can break and spread.

Signs at Steel Lake warn boaters and lake users of the dangers posed by milfoil. The signs ask boaters to remove this and other invasive species from their recreational vehicles before enjoying the lake. The signage is not always obeyed and sometimes leads to infestations, such as the one underway.

If milfoil is not contained, it can disrupt the ecosystem. It is not uncommon for the plant to smother out natural vegetation that provides food for wildlife in and around the lake, Smith said. The weed can produce dense masses of vegetation on the water’s surface, according to the DOE website. It can also deplete oxygen levels in the water and increase the sedimentation rate, according to the DOE. An overgrowth of milfoil may also be dangerous for swimmers, who can get caught up in the long weed.

“Swimmers, especially smaller children, can become entangled in it,” Smith said.

Steel Lake is one of two lakes in Federal Way that is monitored by a lake management district (LMD). The second LMD is located at North Lake. The LMD at Steel Lake was created in 2003. Residents living along the lake have agreed to pay an annual fee that goes toward keeping the lake clean and free of weeds like milfoil. Through the LMD, the lake is annually surveyed. This is how the infestation was discovered.

Treatment of infestations and public education about the dangers of invasive aquatic species is funded through the LMD. Contractors plan on applying herbicide to get rid of the milfoil in Steel Lake.

The herbicide is not harmful to animals or lake life, Smith said.

“All the herbicides that are used are vetted through the Department Of Ecology,” he said.