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.
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.
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.
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.
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.
M.Rydel, Marine Taxonomic Services; S. Matthews, Tahoe RCD; Google Earth.