SOUTH LAKE TAHOE, Calif. — For more than a decade, eliminating the aquatic invasive weeds in the Tahoe Keys has been a priority for the Tahoe Keys Property Owners Association, The League to Save Lake Tahoe, the Tahoe Regional Planning Agency and others.

The Tahoe Keys is a residential development which consists of about 1,500 homes and approximately 170 acres of waterways connected to Lake Tahoe.

Since the 1980s, Eurasian watermilfoil has been prevalent in the Keys. In 2003, curlyleaf pondweed was also discovered there and has continued to grow.

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According to TRPA Aquatic Resources Program Manager Dennis Zabaglo, in 2014, curlyleaf pondweed accounted for 10% of the samples taken. In 2021, it overtook watermilfoil and now accounts for 55% of the samples taken.

The Tahoe Keys is a prime place for aquatic invasive species to thrive because the waterways are warm, stagnant and shallow. During the height of summer, AIS are found in nearly 100% of the waterways.

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“The Tahoe Keys are one of the biggest ecological threats to Lake Tahoe, its ground zero for aquatic invasive weeds, its spreading to the lake, there’s a lot of boating activity, there’s been hazardous algal blooms in the Keys since 2017, water quality is bad, it’s a big issue,” said Jesse Patterson, chief strategy officer for The League. “You have to address the Keys if you want to address the issue for the whole lake.”

A 2013 agreement between TKPOA and the Lahontan Regional Water Quality Control Board requires the Keys to address the weed issue in exchange for them being allowed to dig boating channels from the lagoons into the lake. While they have tried several solutions over the years, the issue has continued to grow.

To find long-term solutions, TKPOA teamed up with Lahontan, TRPA, The League along with the Tahoe Water Suppliers Association and the Tahoe Resource Conservation District. They developed a Control Methods Test, which is a 3-year program. In the first year, methods would be implemented to significantly knock back the prevalence of the weeds and years two and three would be maintenance methods to keep the weeds away.

Herbicides

One such solution proposed by the TKPOA are herbicides, which have never been used before in Lake Tahoe. The use of herbicides is controversial to say the least, and not a decision the agencies are taking lightly.

“We’re dealing with a different animal,” Zabaglo said in a presentation to the TRPA Governing Board. “This test is an opportunity to balance the fine line or urgency with getting it right.”

The CMT proposes the use of herbicides only once during the first year. In addition, a third-party environmental impact consultant was brought in to look at potential unintended consequences to the use of the herbicides.

“We’re super concerned about the use of herbicides too actually,” Patterson said. “Previous iterations of the proposal had a more aggressive use of herbicides, we were actually not supportive of … but having said that, we need to find out if they have a role to play in the broader system.”

Jim Good with Environmental Science Associates was part of the oversight committee and they recommended several safeguards be put in place.

The herbicides will be applied at a rate far lower than the approved label rates. In addition, anyone working with the herbicides will have required training and licensing. Finally, there will be a spill prevention and response plan and a plan for accelerated aeration to dissolve the herbicides if they find that they aren’t working.

The Sierra Club Tahoe Area Group has been the biggest opponent of the use of herbicides. During the presentation to the Governing Board, Tobi Tyler, former Toiyabe Chapter and Tahoe Area Group committee member said there is no way there will be a one-time use of herbicides, and it would open the floodgates to other areas of the lake being treated with herbicides.

“Applying aquatic herbicides is like applying a Band-Aid to a severed artery. The use of herbicides does not address the causes of the problem: decades of nutrient buildup from the heavily fertilized lawns in the Tahoe Keys, the numerous stormwater outfalls into the lagoons, and nutrient recycling from dead and dying weeds,” Tyler wrote in a blog post about the issue. “Herbicides or not, weeds will continue to flourish under these conditions until the conditions supporting the infestations are removed.”

However, Patterson countered that if the CMT is approved, it will be for one-time use of the herbicides, so if they wanted to use more, the group would need to seek further approval.

During the Wednesday TRPA Governing Board meeting, board member Hayley Williamson shared a personal anecdote about the lake she grew up on in the midwest.

The lake had been taken over by watermilfoil and herbicides were used to successfully knock-back the weeds.

“Herbicides saved the lake I grew up on,” Williamson said.

Herbicides are not the only solution proposed in the CMT and the hope is that the other solutions will supplement the use of herbicides after year one.

Other solutions

The CMT also proposes the use of laminar flow aeration and ultraviolet light in year one.

Pilot programs for the use of UV light have already shown some success. The UV light is lowered into the water and attacks and kills plants at a cellular level.

Its not selective and kills native species as well, but if the invasive species are gone, the native species are given the chance to grow back.

Laminar flow aeration bubbles air through several diffusers on the bottom of the lagoons which causes the water to circulate, increasing the amount of oxygen at the bottom of the lagoons.

The League oversaw testing of this method and Patterson said they’ve seen success.

The goal after year one is for the weeds to be knocked back by 75%.

Years two and three would maintain the progress made on problems.

Bubble curtains have been installed to prevent weed fragments from escaping into the lake and the use of UV light will be continued. Bottom barriers will also be installed to block light from the shallow areas of the lagoons and divers will hand-pull weeds that pop-up.

Its important to keep in mind that the Control Methods Test, is just that, a test. After three years, successful or not, the group will have to go through the approval process to continue using these methods or introducing new methods.

One thing Patterson hopes will come of this test is the possibility that other groups across the country can look at this test and learn.

“I’m hoping we learn something that works for Tahoe of course, but if it doesn’t, its something we can export to places, whether it works or not, to show people there’s way to use herbicides that’s not the way we use them now, because I think there’s way too many chemicals in the world and we rely too much on them,” Patterson said.

The Lahontan Water Board will be meeting January 12-13, during which approval of the test will be considered. If approved, testing will begin in the spring of 2022.