Bottom Barriers

Synthetic bottom barriers are installed over the infestation and block out all light. The barriers are secured with rebar and left in place for a minimum of three months. Dependent upon environment, the barriers may be left in place for up to 18 months. This method is best used for well-established infestations in sheltered locations with a minimum of four feet water depth to avoid disturbance from wave action and boat traffic. Installation is carried out by certified divers and topside assistance is required to assemble barriers and maneuver the barriers in and out of the water.

The synthetic gas-permeable bottom barriers must be weighted down with rebar. Preparations include cutting rebar to length, carefully inserting rebar into sleeves on the bottom barrier, and bending additional rebar into u-shaped staples. Under water, specially trained divers install the barriers on the lake bed. Each barrier is 10′ by 40′ and weighs approximately 56 lbs when dry; they are installed with an overlap to help deter new plants from growing between the barriers. The divers secure the barriers to the lake bed with the rebar “staples” to ensure water movement does not displace them.

Throughout the growing season, the dive team returns to the site to perform maintenance on the barriers and remove any new plants that grow at the perimeter of the barriers. When it is time to remove the barriers, divers remove the rebar staples and lift the barriers up to the topside crew. The topside crew roll up the barriers and remove them from the water. The barriers can be reused for multiple seasons.

Diver-assisted suction removal

Under water, divers hand feed entire plants, including root systems, into a suction hose. The suction hose is mounted on a pontoon or watercraft, and the suction is powered by a gas motor.

The suction hose feeds invasive plant material into mesh bags. When the bags are full, divers remove the bags from the water and dispose of the material off site.

Hand Removal

Divers carefully remove the entire plant by hand, including all roots. If performed incorrectly, this method can lead to further spread of the plant through fragmentation, so expertise is required. It is generally used in conjunction with other methods to perform follow-up maintenance of a treated site, or in sparsely distributed infestations with immature plants that are not well-established.

This method is used in areas where plants are growing in a patchy distribution and also used to remove new growth outside of bottom barriers.

Pre-Treatment Monitoring

Pre-treatment monitoring includes general monitoring of the potential submersed aquatic plant habitat area, detecting infestations, scheduling/prioritizing areas to be treated and identifying which method(s) to implement within the area, and finally monitoring and characterizing the area to be treated prior to control implementation to ensure the appropriate methodologies are used and
installed/established to protect resources in the area. Resource protection includes knowledge of the substrate, existing subsurface utilities or hazards, native plant and animal species present in the area, cultural resources present in the area, public use and access of the area, and the existing quality and characteristics of the water in which the control action will occur.

Post-Treatment Monitoring and Control Maintenance

Post-treatment monitoring tracks whether treated areas have fully removed the infestation, and what type of plants or plant fragments remain. If treated areas are monitored and the monitoring identifies new plant growth, those areas can be re-treated, or maintenance measures applied to eradicate the infestation or prevent extensive re-infestation of species at a higher cost of control. If the affected areas can be maintained with lower levels of control effort, the cost of treatment and the potential for infestations to spread to other areas are minimized.

Post-treatment monitoring is conducted immediately following control implementation and annually following control implementation. This monitoring includes identification of the area being monitored and the control method(s) applied, the period of control implementation, and the post treatment success rate. If aquatic invasive species are identified during post-treatment monitoring, the species and number of plants are noted, including approximate plant size/maturity, and the location of the plants within the treatment area. Monitors also provide a recommendation as to maintenance methodology to keep reinfestation from occurring. Post-treatment monitoring is not only used to monitor treated areas to ensure they are maintained and avoid expensive and intensive control actions, this monitoring will also be used to identify the success rate of the control methods used, how the control method was or was not successful, potential reasons why new plants have re-established in the treatment area, and potential changes or improvements to the methods previously used.
 

Lake Tahoe Aquatic Plant Monitoring Program

TRPA coordinates annual lake-wide monitoring following the Lake Tahoe Aquatic Plant Monitoring Program: Aquatic Plant Monitoring and Evaluation Plan. The Action Agenda recommends monitoring strategies such as a broad spectrum near-shore-wide census every two years for six years, followed by once every five years, and in situ diver survey transects and drone surveys at 25 priority locations in intervening years.

Eyes on the Lake Program

The League to Save Lake Tahoe oversees the Eyes on the Lake citizen science program in which League staff train community members how to identify and report the location and presence of AIP in Lake Tahoe’s waters.

UV Light Pilot Control Project

Tahoe RCD leads aquatic invasive plant (AIP) control efforts in the Lake Tahoe Basin and continually seeks innovative technologies and methods to improve treatment efficacy and efficiency.

Attempts to locally control or eradicate AIP, specifically Eurasian watermilfoil (Myriophyllum spicatum) and curly-leaf pondweed (Potamogeton crispus), have been ongoing in Lake Tahoe since 2006. Gas-permeable bottom or benthic barriers and diver-assisted suction removal, when used in combination throughout the growing season (May until November), have proven successful. While this combination of methods is effective in an open water setting such as Emerald Bay, site-specific limitations do exist. Wave action, lake bed morphology, high boater use areas, and high turbidity can impede the effectiveness of these methods.

Research indicated that using ultraviolet C (UV-C) light, a short-wave electromagnetic radiation light that damages the DNA and cellular structure of aquatic plants and their fragments, could be an effective new method to kill and control AIP species, as laboratory tests resulted in complete mortality when exposure times of more than 5 minutes were applied. This technology was applied in Lake Tahoe to determine the full potential of UV-C light treatment as a new method to enhance and support current efforts in the treatment of AIP.

 The UV-C Light Plant Control Pilot Project (Project) was funded by the California Tahoe Conservancy (Conservancy) and Tahoe Fund and managed by Tahoe RCD. UV-C light was applied to three treatment areas: 1) closed marina system (Lakeside Marina or LSM); 2) adjacent open water or littoral environment (Lakeside Beach or LSB-Swim); and 3) an open water environment (immediately adjacent to the Lakeside Marina bulkhead with water taxi use (LSB-Taxi). UV-C light treatment was conducted between June 23, 2017 and September 11, 2017. Associated macrophyte surveys and biomonitoring (i.e., benthic macroinvertebrates, periphyton, zooplankton, phytoplankton and chlorophyll-a) were conducted pre-treatment, immediately post-treatment, and long-term post-treatment. Plant response to UV-C light treatment was measured one year after treatment in 2018, and through the 2018 growing season, with the following project milestones:

1. Pre-treatment 2017 to establish baselines; macrophyte surveys and biomonitoring (i.e.,benthic macroinvertebrates, periphyton, zooplankton, phytoplankton, and chlorophyll-a);
2. Immediate post-treatment 2017 to gauge treatment response and mortality; and
3. Long-term post-treatment 2018 to measure response to UV-C light treatment one year
   later and through the 2018 growing season.

UV-C light is an effective tool in treating microbes and other living organisms and is currently used in other applications such as food, air and water purification. The Project assists with determining the optimum intensity and duration of UV-C light treatment that is necessary for control of AIP, specifically Eurasian watermilfoil (EWM) and curly-leaf pondweed (CPW). AIP control efforts are anticipated to result in improvements to water quality, native fish habitat, and recreational access for swimming and boating. The Project provides a significant regional benefit by increasing the variety and application of methods available for controlling AIP, potentially at greater efficiencies and less cost.