Critical juncture reached in fight with aquatic invasive weeds (Opinion)

David Peterson
Guest column

We are at a critical juncture in the ongoing fight against the aquatic invasive species that are threatening Lake Tahoe’s shorelines.

The Tahoe Keys Property Owners Association has been fighting invasive weeds for four decades, and despite our efforts, the invasive weeds now cover over 85% of the lagoons. Complicating matters, a new species, curly leaf pondweed that was found in 2003, is establishing a foothold. This is significant, as curly leaf represents a greater threat to the lake, growing in colder, deeper water and reproducing aggressively. Now is the time to arrest the spread, and we must use all effective methods available.

The Tahoe Keys Property Owners Association and collaborating organizations – the Tahoe Regional Planning Agency, the League to Save Lake Tahoe, the Tahoe Resource Conservation District, and others — have explored the entire gamut of mechanical, physical, and biological control methods for invasive and nuisance aquatic plants. Together, we have invested millions of dollars and thousands of staff and university experts’ hours to find a solution. The goal is to identify feasible options and develop an integrated management plan to control weeds in the Keys in order to help stop the spread of invasive weeds into the lake.

A proposed small scale control methods test to evaluate the most viable treatment alternatives will be before the Lahontan Regional Water Quality Control Board. The CMT will evaluate the use of safely applied aquatic herbicides and experimental ultraviolet light, alone and in combination, inside the lagoons of the Tahoe Keys. Laminar flow aeration and other small-scale treatment alternatives will also be tested.

The CMT is a three-year testing project in which the small-scale herbicide test (16.9 acres out of 170 acres, or 10%) will be in only the first year and only in the interior of the lagoons when spring runoff is moving water from Lake Tahoe into the lagoons. The test will help determine which methods may be effective in achieving our goals: a 75% reduction in biomass, a reduction of viable plant fragments entering Lake Tahoe, and providing sufficient clear space for vessel movement.

The CMT project has been thoroughly reviewed at the federal, state and local regulatory levels, by independent university scientists, and by concerned stakeholders. Only EPA approved (federal and state) aquatic herbicides are proposed for use. These herbicides have a successful, safe and effective “track record” based on their use for several decades throughout the U.S. on the same target invasive and nuisance plants that are found in the Key’s lagoons. They have specifically been deemed secure for use in the Sacramento-San Joaquin Delta, Big Bear Lake, and dozens of other water bodies in California. The herbicides only target the invasive plants, leaving native plants, fish and wildlife unharmed.

After a decade of study and five years of planning and analysis alongside our partners, we are recommending that herbicides should be tested in the warm, shallow, obstructed channels of the Tahoe Keys, where other methods haven’t worked — not in the lake. The proposed test is less about the products themselves, and more about the application, monitoring, and containment within the Keys. With double barrier curtains, the herbicides will be isolated (in the lagoons) until the natural degradation process makes them non-detectable, usually after about two to four weeks. Waters near the lake and ground water sources will be monitored to ensure no effect to water supplies.

It has also been implied by some that ultraviolet light treatment is an environmentally preferable alternative. UV-C has shown some promise in clear waters of the lake with sandy bottoms. And while we fully supported its testing and further development and expect it to have a future for Lake Tahoe, it has limited scale and has struggled to deliver effective results in the more turbid water of the Keys lagoons. Divers with suction devices and bottom barriers were used successfully to contain a small outbreak of invasive weeds in Emerald Bay. These techniques are being used extensively today outside the Keys and will continue to be essential tools. That said, these methods do not work at large scale, and are extremely difficult to apply inside the Keys.

Knocking down the weeds inside the Keys is an essential first step to stopping the spread of invasive weeds into the lake, but it’s only the beginning. Many factors contribute to the weed growth in the lagoons, including the weeds themselves when they release nutrients into the water during die-off at the end of the growing season. A comprehensive, lasting solution will involve the use of many tools to keep the weed population in check. And it will likely include other techniques like laminar flow aeration and water circulation to improve water quality and reduce the occurrence of harmful algal blooms. The Keys is committed to working with partner organizations to define such a plan and implement it with urgency. Time is our enemy here.

About five years ago, the Tahoe Keys Association began changing its aquatic weed removal practices and added skimming boats to collect loose aquatic plant fragments from the water surface of the lagoons. With the support from the League to Save Lake Tahoe, we also instituted a boat back-up station in the West Lagoon to dislodge plant fragments from boats before entering Lake Tahoe and installed bubble curtains to reduce drift of fragments into the lake.

These “on the water” improvements were complemented with a Tahoe Keys-wide phosphorous fertilizer ban, new water conservation requirements, and annual association member and landscape contractor education activities. Recently, the Tahoe Keys Water Company developed a long-term water facilities improvement plan that will require water meters and water smart irrigation controllers for all water customers, resulting in an anticipated 30% or more reduction in landscape water use, thereby reducing landscape runoff into the lagoons.

This will be complemented by plans for improved stormwater run-off management, landscaping guidelines being developed (with funding from TRPA) for the homeowners and landscape contractors, and other water conservation best management practices specific to the Keys.

Collaboration runs on trust, and on that front, we have excelled. We have an unqualified appreciation for the progress made by the stakeholders over the last few years on the extensive environmental analysis and the design of the Control Methods Test. But change runs on courage, and here we are challenged. Regulatory policies and practices and misinformation about control methods have tended to favor the status quo. But in the case of the invasive weeds, we face a status quo that is unacceptable. The data on the spread of the invasive plants, and especially the curly leaf pondweed, shows that we are losing this battle and we must change tactics to resolve it. We need to proceed carefully, but deliberately. And we need to use all the tools available to us.

The permit decisions for the control methods tTest will come before the TRPA and Lahontan Boards in January 2022. Without action these invasive species will continue to spread and further threaten Lake Tahoe’s ecology, the recreation industry, and our economy. An approval allows the collaborative effort to move forward.

The public comment period on the Lahontan tentative permit is open through 5 p.m., Nov. 1. We urge residents to review the facts and submit comments in support of the CMT and specifically the herbicide test.

For more information, please attend our AIS Open House at the Tahoe Keys Pavilion from 3-6 p.m. Friday, Oct. 29, or review the information on AIS and the CMT at

David Peterson is president of the board for the Tahoe Keys Property Owners Association.

Support Local Journalism

Support Local Journalism

Readers around the Lake Tahoe Basin and beyond make the Tahoe Tribune's work possible. Your financial contribution supports our efforts to deliver quality, locally relevant journalism.

Now more than ever, your support is critical to help us keep our community informed about the evolving coronavirus pandemic and the impact it is having locally. Every contribution, however large or small, will make a difference.

Your donation will help us continue to cover COVID-19 and our other vital local news.