by Lisa M. Krieger
In the middle of the night, researchers aboard the John Le Conte—an old 37-foot diesel-powered salmon trawler—drop a net into the freezing cold water.
What was surprising was not what they caught. Here’s what they haven’t done: Invasive shrimp, which have long occupied America’s most famous alpine lake, disrupting its balanced ecosystem.
In a rare piece of good environmental news, harmful predators are nearly extinct. In their place are helpful creatures that safely return to eat algae and other fine particles. Scientists say this mysterious change may have restored the lake’s famous clarity.
Lake Tahoe’s average visibility has increased from 52.8 feet deep to a staggering 71.7 feet in the past three years, although this summer’s runoff has created temporary darkness. That’s like looking seven stories underwater. In the last five months of 2022, visibility increased to 80.6 feet, a level not seen since 1988, when it was 81 feet.
“We haven’t had this level of clarity since the 1980s,” said Geoffrey Schladow, director of the UC Davis Tahoe Environmental Research Center.
To study the dramatic changes in the food web in Lake Tahoe, the Center’s research vessel counted the presence of three different populations: algae; the predatory shrimp, called Mysis; and an abundance of beneficial native species of algae-eating zooplankton, especially Daphnia and Bosmina, which are indicators of good water quality.
In the past, nighttime netting would catch 100 to 150 nocturnal shrimp, which rise to the surface of the lake in the dark.
Now only two or three have been caught. “You can name them!” Schladow joked.
The lab also monitors the lake’s clarity by measuring the depth to which a 10-inch white plate, called a Secchi disk, remains visible when lowered into the water. While 2023 data is not yet available, average lake clarity jumped 10 feet between 2021 and 2022.
Scientists say that the lake is the clearest when the zooplankton Daphnia and Bosmina, which were once almost extinct, are the most abundant.
“Biologically, things are changing,” Schladow said. “Things that weren’t in the lake before, suddenly they’re there. Things that were in the lake—they’re gone. It’s very interesting, and we’re trying to put it together.”
The mountain-surrounded lake, located on the Nevada/California border in the Sierra Nevada, has long been considered a natural wonder.
Amazed by its crystal waters, the writer Mark Twain declared it “so clear…that the boat seems to be floating in the air. The lake is due to the clarity of the surrounding granite. In addition, it has a relatively small watershed and is mostly free of agricultural wastes.
But in recent decades, the lake’s waters have become increasingly cloudy —losing their world-renowned clarity at a rate of about a foot and a half a year.
Lake Tahoe seems destined for the future to look like any other lake: a dark mess in the mountains.
Alarmed, regional management agencies are taking steps to reduce runoff from roads, gardens, golf courses and construction of new multimillion-dollar homes. They report that more than 500,000 pounds of fine sediment and other pollutants that harm clarity are kept from the lake each year by road maintenance and erosion control projects.
But a natural factor also emerged: the non-native Mysis shrimp.
The introduction of shrimp is an idea that has gone terribly wrong. In the early 1960s, the California and Nevada Departments of Fish and Game imported them from the Great Lakes, believing the shrimp would provide food for Lake Trout, which is Tahoe’s primary sport fish.
But shrimp are sensitive to light—and once in the clear waters of the lake, they spend their days in the dark, deep lake. Every night, they make a great vertical migration to the surface.
Most fish are “visual feeders,” and don’t occupy the same water column during the day, said Katie Senft, research associate on the Center’s staff. So the shrimp is not edible. And with some predators, they thrive.
Heavy consumers of zooplankton, shrimp destroy Daphnia and Bosmina, he said. By 1971, those two important species had largely disappeared from the lake.
But now something new is happening.
As of 2012, only a few Mysis shrimp have been found in samples from the lake’s Emerald Bay. In the work that is now being extended to other locations in the lake, Daphnia and Bosmina reappeared in large numbers.
“For 50 years, Mysis ruled. Then they disappeared. Gone,” Schladow said. “We don’t know why.”
Microscopic studies in the Center’s modern laboratory, nestled in the tall pines of Incline Village, revealed a possible cause: The shrimp were starving.
When the population of Daphnia and Bosmina decreased, the shrimp switched their diet to another type of zooplankton, called copepods. And those copepods died from fungal infections, the lab suspects.
Enlarged, dead copepods look like a chia pet. To better understand if fungus is the underlying cause of today’s population crash, Center intern Katie Fielder scanned historical water samples.
There is likely always fungus in the lake, and the Center has seen these “fuzzy” copepods before, but never in such density. It’s not known why so many are dying today, Senft said.
During these years, Daphnia and Bosmina patiently waited. Their eggs can sit dormant in lake sediment for up to a century, Senft said. Now the eggs have hatched—and, free of predators, they thrive.
“They are very, very, efficient feeders,” Schladow said. Crews cleaning the lake, “they only know the things in their mouths—and the food they eat, the algae and fine particles, is what affects the clarity of the lake the most.”
Swimming underwater at Sand Harbor Lake the weekend of July 4, “it was peaceful. Crystal clear. The little minnows were doing something in the water, even though it was so cold I’m not sure my brain was working,” said AJ Kohn, who was visiting from Minneapolis.
“It’s pretty cool,” said paddleboarder Heather Pratt of El Dorado Hills.
Darcie Goodman Collins, CEO of The League to Save Lake Tahoe, welcomed the research, saying that “learning more about this system will help us really understand the ‘trigger points’ and help us make management decisions…
“It’s very unlikely that the shrimp population is the only part of the improvement or loss of clarity,” Collins said. “Many of the impacts are from our urban landscape, and can have a significant impact on our ecology, including the Mysis shrimp and our native Daphnia population.”
Lake clarity is expected to grow by 2023, and may return to 1970s levels, despite expected large runoff from this year’s record snowpack. Lake management agencies in California and Nevada envision future restoration of up to 90 feet of visibility.
Unfortunately, nature’s food web is a tale of flux. Mysis will begin eating the newly emerged zooplankton and re-establish itself, scientists predict, returning the lake to its disturbed and more cloudy state.
“It’s temporary,” Schladow said. Once introduced, “you can never get rid of things completely.”
But the discovery could support strategies to deliberately remove the shrimp, which can be caught while hovering in its tight 50-foot-deep nocturnal band. Although it cannot be deleted, the numbers can be monitored.
“If we can get the lake back into balance, then the system is relatively stable,” Schladow said. “And that gives us hope.”
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Citation: Scientists: Lake Tahoe regaining legendary clarity (2023, July 23) retrieved 23 July 2023 from https://phys.org/news/2023-07-scientists-lake-tahoe-regaining-legendary.html
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