Update: Aquatic Plant Harvesting on Lake of the Isles this summer

On September 30, 2018, two young zebra mussels were found attached to an outgoing sailboat at Bde Maka Ska. The sailboat had been moored at a buoy on the lake for several months, so it is unlikely that the zebra mussels originated from anywhere other than Bde Maka Ska. In the weeks following the discovery, the Minnesota Department of Natural Resources formally designated Bde Maka Ska as infested with zebra mussels.

An MPRB aquatic plant harvester uses a conveyor belt to offload material onto a flatbed truck.

In recent years, the Minneapolis Park and Recreation Board’s (MPRB) aquatic plant harvester has been buoyed and offloaded in Bde Maka Ska, even when it was harvesting Lake of the Isles. The harvester made many trips back and forth from Bde Maka Ska to Lake of the Isles during a harvesting season.

The need for a modified aquatic plant harvesting plan

This year, moving the harvester between the lakes carries the risk of transporting zebra mussels into the upper chain lakes: Lake of the Isles, as well as Brownie Lake and Cedar Lake. These lakes have not been designated as infested since they are upstream from Bde Maka Ska. Zebra mussels are very weak swimmers, so it is unlikely that they could fight against the current and move into the upper chain lakes by themselves. Watercraft are the biggest threat for introducing zebra mussels into the upper chain lakes.

Thankfully, the most commonly-used watercraft in the Chain of Lakes — canoes, kayaks and paddleboards — are stored out of the water after each use. Because juvenile zebra mussels are highly vulnerable to drying, these watercraft are unlikely to transmit them when properly dried.

Zebra mussels could move up the chain, however, if they were attached to sailboats, docks, the aquatic plant harvester or other water-related equipment that is stationed for long periods of time in Bde Maka Ska.

The site for the aquatic plant harvester, buoy and offloading is at the northwest end of Lake of the Isles. It is planned for use for approximately two weeks in June and two weeks in August.

Therefore, MPRB has modified its aquatic plant harvesting plan this year. The goals of the plan are to:

  • Delay the introduction of zebra mussels to the upper chain lakes for as long as possible, while still managing nuisance growth of Eurasian watermilfoil
  • Harvest Cedar Lake, Lake of the Isles, and Bde Maka Ska twice each this summer
  • Eliminate directly moving the harvester from Bde Maka Ska to the upper chain lakes

Timeline: modified harvesting plan for the Chain of Lakes

1. June: the harvester is buoyed in and harvests Cedar Lake for one to two weeks.

2. Late June/early July: The harvester moves to the northwest side of Lake of the Isles, the lake near Peavey Fountain, where it is buoyed and offloaded for approximately two weeks. The conveyor belt is also onsite during this time.

3. July: The harvester and conveyor belt move to Bde Maka Ska. The harvester is buoyed and offloaded at this site for several weeks.

4. July/August: The harvester and conveyor belt are removed from Bde Maka Ska and air-dried for two weeks. The Minnesota DNR recommends a five-day dry period to kill zebra mussels on watercraft, so MPRB’s planned two-week dry period will ensure that all mussels are dead. After air-drying, the harvester is power-cleaned to remove any attached dead mussels.

5. August: Repeat steps #1-3 a second time.

Modified plan involves minimal impacts

Detailed view of the Lake of the Isles site.

MPRB inquired about this plan with the Minnehaha Creek Watershed District and the US Army Corps of Engineers. Neither agency requires MPRB to obtain a permit from them to use the Lake of the Isles site.

The Lake of the Isles site will be temporarily disturbed while equipment is stored there for approximately two weeks in June and two weeks in August. The site will not be permanently altered in any way. Grading will not occur; nor will gravel or other type of fill material be used. When harvesting is not actively occurring at Lake of the Isles, the harvester, conveyor belt, and all other related equipment will be removed from the site.

These activities will help delay the introduction of zebra mussels to Lake of the Isles, thereby protecting the lake for future generations of park visitors to enjoy.