UV lights used to zap Lake Tahoe's invasive plants, home to human-introduced giant goldfish

Lake Tahoe's famously clear blue water is under attack from a common household plant – Eurasian watermilfoil and curlyleaf pondweed, two plants found in many household fish tanks. 

After years of ineffectively battling the plants — which provide breeding grounds for invasive fish including what have morphed into extremely over-sized goldfish — preservation groups, scientists and inventors are partnering on a pilot program using UV lights to zap the troublesome plants. 

“The principal idea is, if you can control the plants, you can control the fish,” said Sudeep Chandra, director of the University of Nevada, Reno’s Global Water Center.

Eurasian watermilfoil and curlyleaf pondweed are “two of the most notorious plant invaders in the country,” according to Dennis Zabalgo, Aquatic Resource Program Manager for Tahoe Regional Planning Agency.

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Watermilfoil was introduced to Lake Tahoe in the 1980s and the introduction of pondweed can be traced back to the early 2000s, according to Chandra.

The plants hover along the lake’s south and east shores, Chandra said, particularly at the Tahoe Keys, Lakeside Marina and Lakeside Beach. More than 170 acres of Tahoe Keys waterways are overrun with the invasive plants.  

Swimmers and boaters may have experienced the pesky plants as they wade into the water to swim or launch watercraft.

The plants excrete phosphorous, promoting algae growth, which reduces water quality and creates a habitat for non-native fish such as bass, bluegill and goldfish.

According to a 2017 U.S. Department of Agriculture report, goldfish were likely introduced to the lake by “well-intentioned pet owners.” The goldfish may also be contributing to the lake’s murkiness by producing excrement that fuels algae growth.

When aquariums get dumped, plant life that grows in fish tanks – such as milfoil and pondweed – are also deposited into the lake. Milfoil and pondweed also spread through boats entering the lake with pieces of plant material on them.

As lake management agencies became aware of the rise of invasive aquatic plants in the lake, they instituted programs to mitigate the spread of the invasive plants, such as a 2008 mandatory boat inspection program for all watercraft entering the lake.

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But, management agencies are playing catchup on the problem.

In the 1980s, an effort to manually remove watermilfoil was made in the Tahoe Keys to improve waterways for boaters and swimmers. The treatment plan inadvertently increased the number of plants in the area.

“For this particular species, if you clip the plant either through a boat propeller or through harvesting controls that are applied in the Tahoe Keys, the plant fragments can float and move around the lake,” Chandra said. “Once they hit the sediments they can reattach with the root system and grow. This explains why they’re so prolific and can move around the lake rather quickly within a matter of decades.”

By 2005, the Eurasian watermilfoil had spread to 15 other locations in the lake, and management agencies began applying bottom barriers and diver-assisted suction removal. The treatments had little success, according to Chandra.

In 2015, Inventive Resources President John Paoluccio pitched a new idea – treating the plants with intense doses of UV light. The idea spurred from a 2002 project he piloted at Mercer Caverns in the foothills of the Sierra Nevada using UV rays to remove algae from the caverns’ walls.

The program launched at the lake in 2017, and now, driving a 12- by 32-foot barge around the lake, Puoluccio and a small crew blast the invasive plants with intense UV lights mounted on the bottom of the boat for 15 to 20 minutes at a time. The light treatment can penetrate the water up to depths of 15 feet. Within a couple of weeks, the plants collapse.

The UV treatments target only plants, not fish, but eliminating the invasive plants can help mitigate non-native fish populations.

The UV treatment also doesn’t tap into seed pods stored in the lake’s bottom, meaning the plants will continue to grow if seeds remain. The seeds can germinate for up to a decade, Paoluccio said.

“You’ve got a seed bank in this marina that’s going to last many, many years,” Paoluccio said. “It’s going to take years of treatment of any type to get this under control.”

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Paoluccio and his crew treated Lakeside Beach twice last year. This year, the focus has been on nearby Lakeside Marina, and the focus will soon spread to the Tahoe Keys – the lake’s largest area of invasive aquatic species, Zabaglo said. Additional treatment methods will include application of aquatic herbicides and aeration.

“Tahoe’s famous for the cleanest water in the world, and we want to keep it that way,” Paoluccio said.

Amy Alonzo covers Mason Valley and rural Nevada. Reach her at aalonzo@gannett.com or (775) 741-8588. Here's how you can support ongoing coverage and local journalism.