Officials say Tahoe Keys invasive weeds test is working

Staff Report
The turbidity curtain before being removed.

SOUTH LAKE TAHOE, Calif. – Officials say the Tahoe Keys Property Owners Association test project to control the largest infestation of aquatic invasive weeds in the Tahoe Basin through the use of herbicides is proving successful. 

A foul odor with extensive algae and widespread discoloration in the Tahoe Keys are due to decomposing invasive weeds, as well as the prolonged isolation and lack of circulation, officials said on Friday. 

Conditions have improved somewhat over the last week and should continue a return to normal as air and water temperatures begin to drop, said a news release.

With herbicide levels now at non-detect status, turbidity curtains in Area B of the Keys, in place since May to restrict herbicide movement, are being removed this week, also allowing for boating activity. Forecasts in Area A of the Keys suggest levels will reach non-detect status by mid-to late September, with curtain removal there soon after. 

“This is the first step in the effective implementation of the Control Methods Test and a major milestone in the battle against aquatic invasive weeds which threaten Lake Tahoe’s clarity,” said Dr. Lars Anderson, an aquatic ecology/invasive species specialist working on the project. “The results certainly portend major progress as the two applied herbicides acted specifically and effectively on the target invasive weeds: Eurasian watermilfoil, curlyleaf pondweed and coontail, while ensuring safety guidelines for people, pets and wildlife.” 

The three-year field test project was preceded by a multi-year collaborative public planning process and extensive environmental review by the Tahoe Regional Planning Agency and Lahontan Regional Water Quality Control Board. 

On May 25, TKPOA began trials of EPA-approved herbicides, in conjunction with UVC light treatment, and The League to Save Lake Tahoe initiated the laminar flow aeration program, a process to aerate the lagoon bottom sediments with small bubbles to reduce plant growth in designated test areas. 

TRPA simultaneously launched independent rigorous monitoring for the project to collect data on the efficacy of treatments, water quality, and overall data on how the natural environment responds to the various treatments. 

“TKPOA and the Control Methods Test team are committed to following the permit conditions and stringent guidelines of the thorough and intensely monitored program,” said Anderson. 

The herbicides levels degraded by over 99% within four to six weeks to about five to six parts per billion, with the last half-percent to reach one ppb now realized.

“It’s also important to note that the CMT permit-driven herbicide threshold to allow curtain removal is 400 times lower than EPA-stated safe ‘receiving water limits’ established for other lake systems throughout the U.S.,” he added. “To grasp the ‘scale’ of one part per billion, it is equivalent to one inch in 16,000 miles; or three seconds in a century; or a penny in $10 million; or about an inch and half compared to the circumference of the earth. The data that is being collected will allow us to determine long-term solutions for not only the Tahoe Keys lagoons but Lake Tahoe. We certainly appreciate the understanding and patience of the homeowners and public.” 

“Preliminary hydroacoustic scans and imagery of the weeds are encouraging,” said Pete Wolcott, Chair of the TKPOA Water Quality Committee. “We have tens of thousands of data points this year alone, and we will be looking closely at this data to assess the first-year results of the test.”

A status report on initial testing efforts will be released this fall with the preliminary results at No additional herbicide application or boating restrictions will occur with the project, but extensive monitoring will continue alongside non-chemical methods for the next two years. The results of the three-year test will inform long-term management plans for the Tahoe Keys and for addressing aquatic invasive weeds in Lake Tahoe.

In an unrelated circumstance, algae and harmful algal blooms have been noted this summer throughout California’s aquatic eco-systems, including beaches around Lake Tahoe and in the Tahoe Keys due in part to the low lake levels. While no area at the lake is currently considered in a danger zone, caution is suggested. Inside the Keys during the test period, algae growth continues to be extensively monitored, and to date, “warning level” HABs have only been observed outside the Tahoe Keys CMT Project test areas. 

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