Crystal Shores: a history of infestation
The Crystal Shores marinas complex consists of three adjacent marinas, owned and maintained by three separate homeowners’ associations. The site is located on the north shore of Lake Tahoe, Nevada, and offers access for the homeowners to the idyllic crystal-clear waters of Lake Tahoe.
The location also has a history of supporting aquatic invasive plants. In 1995, the first documented lakewide aquatic plant surveys were carried out by Dr. Lars Anderson, working with the USDA Agricultural Research Service. Between 1995 and 2006, six aquatic plant surveys were conducted along the shoreline of Lake Tahoe. Eurasian watermilfoil was documented at Crystal Shores during each of these surveys. Again, in 2009 and also in 2012, Eurasian watermilfoil was documented at Crystal Shores during lakewide aquatic plant surveys.
Weather patterns and fluctuating lake water levels may impact the size of aquatic plant infestations in Lake Tahoe; however, Eurasian watermilfoil infestations have consistently been recorded in this location.
What is Eurasian watermilfoil and why is it a problem?
Eurasian watermilfoil (Myriophyllum spicatum) is a feathery submerged aquatic plant that is native to Europe and Asia. It was discovered in North America in the 1940s and has since spread to nearly every U.S. state and at least three Canadian provinces. It was first documented in Lake Tahoe in the 1980s. It spreads when plant pieces break off and float on water currents. This reproduction through fragmentation poses unique challenges to treatment.
If left unattended, a single Eurasian watermilfoil fragment can produce thick mats of vegetation in a relatively short time.
In Lake Tahoe and in other systems, this can cause unwanted ecological and economic impacts. In North America, high density populations of Eurasian watermilfoil impact native species and can cause unwanted effects on water quality, recreation and other ecosystem services. In the nearshore regions of Lake Tahoe, this species has been found to alter phosphorus cycling in the water column, potentially leading to increased algae production. Additionally, it may provide habitat for invasive warm water fish species, further disrupting Lake Tahoe’s ecosystem. Eurasian watermilfoil forms thick mats that are navigational and aesthetic nuisances to homeowners and recreationists alike. In addition, studies have found that infestations of this species are associated with a decline in lakefront property values.
How do you control Eurasian watermilfoil?
A variety of mechanical, biological and chemical methods have been used to control Eurasian watermilfoil across North America. Mechanical means to reduce plant biomass such as harvesting, cutting, hand removal or mowing are often used in localized regions such as marinas or swim areas to clear navigation or recreation use zones. Unfortunately, these methods tend to promote the spread of the plant because watermilfoil can grow from broken-off stems. For this reason, Tahoe RCD has focused on using a combination of bottom barrier installation, diver assisted suction removal, and hand removal in Lake Tahoe.
For this reason, Tahoe RCD has focused on using a combination of bottom barrier installation, diver-assisted suction removal, and hand removal in Lake Tahoe.
In 2018 and 2019, pilot projects to test the use of UV light and laminar flow aeration were initiated and will possibly be added to the tools used to treat infestations in the future.
Bottom Barriers
Synthetic bottom barriers are installed over the infestation; the barriers are air-permeable but 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 environments. It is also necessary to have 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 air-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.
Each barrier is 10' by 40' and weighs approximately 56 lbs when dry.
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, 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.
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.
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.
Making a Plan for Crystal Shores
In 2014, the Crystal Shores East homeowners’ association and Nevada Division of State Lands contacted Tahoe Resource Conservation District (Tahoe RCD) in hopes of addressing a rapid Eurasian watermilfoil re-infestation of their marina after a maintenance dredging. Nevada Division of State Lands provided funding to plan and implement the project, and the Crystal Shores East homeowners’ association also contributed funding for implementation. Tahoe RCD began treatment immediately by hiring a contractor to install barriers in the Crystal Shores East marina for the remainder of the season. Tahoe RCD pursued additional funding through the Nevada Division of State Lands (NDSL) license plate grant to treat all three of the marinas; due to the close proximity of the marinas, it was determined that they were best treated as a single site.
In early summer of 2015, Tahoe RCD awarded a contract to Marine Taxonomic Services LTD. to perform surveys and implement treatment. The contracted dive team conducted full surveys of the three marinas in the Crystal Shores complex. Eurasian watermilfoil infestations were documented throughout Crystal Shores East marina and Crystal Shores Villas marina, with only sparsely distributed infestations within the Crystal Shores West marina. Tahoe RCD developed a plan and began treatment in 2015. Due to differing characteristics of the infestations, different treatment plans were developed for each of the marinas.
The contracted dive team carried out surveillance visits at this site from 2016 to 2019. Multiple visits were carried out each year spanning the course of the growing season, May through October. After initial treatment, no Eurasian watermilfoil was detected at this location.
Post treatment: Crystal Shores West, 2018.
Crystal Shores East
In July of 2015, the contracted dive team installed bottom barriers at Crystal Shores East.
Once installed, the barriers covered the lakebed throughout the marina (depicted in photo to the right). In contrast to prior treatments, the barriers were left in place for an entire year before removal. Tahoe RCD's contracted dive team visited twice throughout the growing season to perform maintenance on the barriers and remove any new plant growth at the perimeters of the barriers.
The contracted dive team utilized a combination of diver assisted suction removal and hand removal to remove any new plants that had grown at the perimeters of the barriers or settled on top of the barriers.
In September of 2016, the barriers were removed from Crystal Shores East marina after 18 months of being in place. At time of removal, no plants were growing in the marina.
The barriers were carefully rolled and stored for future use.
In 2017, the contracted dive team carried out three sets of surveillance visits throughout the growing season. The previous years’ treatments were successful - no Eurasian watermilfoil was detected at any of the sites in 2017. During one of four surveillance visits in 2018 a single rooted Eurasian watermilfoil was detected in the Crystal Shores East marina. Likewise, a single rooted Eurasian watermilfoil was detected in the same marina during one of the six surveillance visits carried out in 2019.
Post treatment: Crystal Shores East, 2018.
Crystal Shores Villas
At Crystal Shores Villas, the infestation was dense and covering the majority of the marina. However, treatment was postponed due to a dredging operation that was being implemented by the homeowners’ association.
Tahoe RCD's contracted dive team reassessed the site in 2016. The maintenance dredging carried out by the homeowners' association left the water within the marina opaque. The team was unable to perform surveillance until the end of the growing season; no returning Eurasian watermilfoil growth was detected.
From 2017 to 2019, the contracted dive team continued to carry out surveillance visits at this location throughout the growing season. No Eurasian watermilfoil was detected at this location after the maintenance dredging.
Post treatment: Crystal Shores Villas, 2018.
Lessons for the Future
On both occasions that newly rooted Eurasian watermilfoil was detected, these plants were removed by the dive team during these visits with minimal effort; however, if left unchecked they could easily reestablish into a vigorous infestation. A study in Emerald Bay, Lake Tahoe (Shaw 2016), indicates that while initial treatment costs of aquatic invasive plant removal can be high, surveillance visits are a fraction of the cost and allow for efficient treatment to be carried out before plants become reestablished.
The treatments at Crystal Shores complex are considered a great success; however, the historic occurrences at this site, as well as the detection of rooted plants during surveillance visits indicate that this site may be prone to re-infestation in the future if left unchecked. The following important recommendations may reduce this threat, keep the Crystal Shores complex weed-free, and protect this investment and maintain property values.
Recommendations to prevent re-infestation:
1) Annual visual plant survey
- Designated HOA staff or member participation in League to Save Lake Tahoe “Eyes on the Lake Training (EOL)” to learn how to identify aquatic invasive plants and conduct a plant survey of your Marina,
- Complete annual EOL plant survey of the Marina each year during the peak of growing season (June-September),
- Submission of annual survey report to the League, NDSL, and Tahoe RCD.
2) Plant fragment control
- Designated HOA staff or member conduct daily skimming of Marina during aquatic plant growing season (May through October).
3) Annual diver survey
- One to two diver surveys completed each year during the peak of growing season (June through September) to detect and remove (hand removal) any aquatic invasive plants observed.
Further Resources & Reading
Credits:
M.Rydel, Marine Taxonomic Services; S. Matthews, Tahoe RCD; Google Earth.