Eurasian watermilfoil is shown in this photo.
Blindly feeling along a section of fabric mat underneath the water’s surface at Lakeside Marina, a diver grabs a U-shaped piece of rebar and begins hammering away.
At only a handful of feet below the water’s surface visibility drops to zero, making work difficult, but it’s an essential job and a possible vital piece in the puzzle to solve Lake Tahoe’s aquatic invasive species problem.
During this past week, a crew of six divers from Marine Taxonomic Services spent four days installing approximately 1 acre of bottom barrier mats at Lakeside Marina in South Lake Tahoe in order to starve invasive plant species of sunlight.
“We work in marinas mainly and have other projects around different beaches at the lake to set bottom barriers to control invasive species satellite populations,” said Corrine Bricker, a diver for Marine Taxonomic Services.
“So far, we can tell that bottom barriers are an effective way to control invasive species,” she said. “In Tahoe City, we’re seeing very little growth. Those mats were in about a year and a half ago and we are seeing nothing left, which is a really good sign.”
One by one the team rolled out the 40 foot by 10 foot plastic fabric mat sections out on the lake floor, which were then “stapled together” and held down by pieces of rebar. The mats will then be left for eight to 12 weeks, killing the invasive weeds beneath them.
The work done this week brings the total mats installed in Lake Tahoe to 4.5 acres of the permitted 5 acres for the lake.
Boost for the barriers
“Last year Tahoe Water Suppliers Association recognized we were short the full 5 acres of mats for the bottom barrier work,” said Tahoe Fund CEO Amy Berry. “They came to the Tahoe Fund and challenged us. They said, ‘Every dollar that you can raise from the private community, we’ll match dollar for dollar until we hit the goal of $52,000.’”
The $52,000 was then used to purchase the remaining bottom barriers, which cost roughly $300 each, including rebar. A large portion of those bottom barriers were then installed during the week as the Tahoe Fund, Tahoe Water Suppliers Association, and the Tahoe Resource Conservation District continued the fight against aquatic invasive species.
Funding for projects to fight invasive species also comes through pier and buoy fees on the lake through a California state senate bill passed in 2013.
“When you pay pier and buoy fees to state lands, before this bill was passed the money went into the general fund, now when you pay your pier and buoy fees the money stays right here and gets reinvested into projects like removal of aquatic invasive species,” said Berry.
History of invasive plants
Since the early 1970s, Eurasian watermilfoil has pervaded areas of South Lake Tahoe and the Tahoe Keys, and was followed by curlyleaf pondweed roughly three decades later.
The two plants likely entered the lake through the aquarium trade and have since negatively impacted water quality around Tahoe’s shoreline. Curlyleaf pondweed is the hardier of the two plants and presents a unique set of problems the conservation teams are still working to unravel.
“Pondweed is the bigger challenge, because it’s a more vigorous plant,” said Tahoe Water Suppliers Association Executive Director Madonna Dunbar. “It’s a survival-type plant. It’s got a thicker structure as its growing, and then it puts off these seed pods call turions that can be dormant in the soil for years, and then they’ll sprout. It’s got this colonization technology in its biology. It’s like a super weed.”
The mats do not have an effect on the turions, said Dunbar, noting the seeds can lay dormant for up to a decade.
“We have a seed bed in the water that’s been here for years,” she said. “And it could exist for 10 years dormant … nothing has an effect on the turion until it sprouts. That’s the big challenge.”
Moving forward, there’s planned research for treating the seeds, said Dennis Zabaglo, aquatic resources program manager for the Tahoe Regional Planning Agency.
“The conservation district has received money from the California Tahoe Conservancy to do further research on these turions to find out what other mechanisms may treat that more effectively,” said Zabalgo. “Is it UV light or some other kind of methods?”
Bottom Barriers
Currently bottom barriers have been installed in several parts of Lake Tahoe, ranging in locations from the Tahoe Keys to Sand Harbor.
Two types of mats are used in the process. In Sand Harbor a thicker non-permeable mat was used in the fight against Asian clams, while in the Tahoe Keys, which is among the worst areas in terms of invasive species, the mats like the ones installed at Lakeside Marina have been used.
An area has to be treated each year, according to Dunbar, but so far the technique has shown signs of success.
“Crystal Shores was really successful,” said Dunbar. “It was a couple of seasons in a really similar environment, very small (home owners association) marina, and they just came in early with the bottom barriers and did several seasons, and the weed problem was eliminated in that location … these are really effective methods but they’re expensive and they’re labor intensive. They do require a couple of years of treatment, but then it seems to really be working.”
The mats installed this week in South Lake Tahoe mark the fourth year the marina has undergone treatment.
“We’ll basically come back in the spring and reassess and say, ‘OK, what came back? How much percent cover?’’ said Tahoe Resource Conservation District Executive Director Nicole Cartwright. “And these divers will help us determine what type of treatment we’ll do.”
Cartwright said moving ahead, the district is looking at an area of the Truckee River as a place to install the roughly half acre of remaining bottom barrier inventory.
