A mapping drone travels on Tahoe’s surface with a Tahoe Environmental Research Center boat in the background. Photo courtesy of TERC

Amid the expanse of Lake Tahoe’s crystalline water bobs a tiny metal boat carrying three researchers. They drop anchor 100 feet off the shore of what, for tourists, is an inviting beach, but the scientists suspect it’s a perfect spot to monitor the lake’s algae population.

Two divers check their scuba gear and tumble into the water to gather samples. The work is a regular part of the UC Davis Tahoe Environmental Research Center (TERC) and the decades-long effort to tell the hidden stories of Tahoe’s ecosystem. The researchers monitor a complex web of life that is constantly under threat from environmental pressures, natural and man-made.

TERC’s recent 2023 “State of the Lake” report revealed that the nation’s second-deepest lake is undergoing more rapid—and often unexpected—changes than the lake has experienced in decades.

Tahoe is suddenly clearer than it has been since the 1980s; populations of some invasive species have significantly decreased, while others have thrived. Fewer tiny particles and organic nutrients are flowing into the lake; microplastics infest the water; and increasingly warmer water temperatures have allowed floating algae species to flourish—then befoul miles of beaches when the blooms die, and decaying, smelly algae washes up on shore.

Each week in all seasons, TERC researchers keep tabs on an ecosystem of interdependent species and systems. Asian clams stimulate the growth of algae. The algae are consumed by filter-feeding zooplankton, organisms that strain the water for nutrients. Some filter-feeders are prey to mysis shrimp, an invasive species that got into the lake decades ago. In and around the lake, everything is connected to everything else. What changes one thing has a domino effect on many other species and natural systems.

The scientists study the lake from near and far. Teams aboard boats collect samples and monitor natural conditions. Buckets on buoys collect rainfall and the tiny particles of pollution carried by the drops. Divers swim in Tahoe’s depths. Unmanned vessels and drones collect data. Boats tow trawls that skim plastic debris from the water’s surface. Satellites and aircraft provide a comprehensive view of the 22-mile long lake, which is 12 miles across at its widest point.

“We have a helicopter that does full lake flights once a month and takes imagery of the entire (72-mile) shore line,” said Katie Senft, a TERC research associate who specializes in invasive species. “Every 10 days, we’re doing Secchi readings and dropping instruments in the lake to do profiles.”

A Secchi disk—which looks like a 10-inch white dinner plate dangling from a cable—is slowly lowered into the lake. The depth at which the disk is no longer visible is the measure of the transparency (clarity) of the water.

Tahoe is one of the most closely monitored bodies of water in the world. TERC research teams study pieces of a complicated puzzle that changes from season to season and year to year.

Working in all seasons

On a recent summer day at Tahoe, Senft and her team are monitoring algae. During the warmer months, metaphyton, an algae that floats, is the focus of their interest. That species is the green, stinky slime that washes up on some of Tahoe’s beaches. Metaphyton has been increasing in recent years, the researchers said, often in places that have a large population of Asian clams.

UC Davis researcher Katie Senft rinses the “manta trawl” and collects particles in a sample bottle after the trawl was towed across Lake Tahoe’s surface to collect buoyant plastics. UC Davis recently conducted a year-long, top-to-bottom assessment of microplastics in the lake. Photo courtesy of TERC

“When the water temperature rises, Asian clams excrete waste: nitrogen and phosphorus. It acts as fertilizer for metaphyton to grow,” said Senft. “It can grow without clams, but clams just speed up the process.”

The Asian clams, which were first detected on the south Tahoe shoreline in 2002, then found near the Sand Harbor boat ramp in 2012, are transported to the lake by boaters who didn’t wash their vessels before casting off at Tahoe. In order to stop the spread of invasive species, boat inspections have been mandatory at the lake since 2008.

“When we come out to these sites to monitor metaphyton, there are two components to it,” including diving and drone flights, Senft said. Senft and Brandon Berry, one of her research partners, program the drone. “Brandon flies the drone path that we lay out on an iPad, so it flies the same area near shore every time. When we don’t have that, we hop in the water and look around to find algae.”

One researcher dives down to the lake bed and collects samples. That’s done by placing a bottomless, weighted bucket on the spot to be sampled, and vacuuming up what’s inside. The samples are used to calculate the biomass of the whole algae patch.

Back in the lab, the algae samples are tested for nutrients, including nitrogen and phosphorus, and for chlorophyll, the natural compound that gives plants their green color. The algae also is analyzed to determine if it can be used as compost.

Nature has no sympathy for the TERC teams. From December through April, researchers dive in the icy water to collect periphyton, an algae that sticks to submerged surfaces.

Erik Young, a research associate helping Senft with the algal-monitoring project, noted that in the winter, the divers switch from wetsuits to dry suits, don thicker neoprene gloves, and wear heated vests while working underwater. The cold makes their jobs harder, Young said, but in all seasons, the researchers get glimpses of Tahoe’s landscape, below and above the waterline, that most visitors never see.

“One time, we were doing night work in February, and it was freezing cold,” Senft said. “We went to Emerald Bay, and I came out to start the generator, and the lake was glass. You couldn’t tell where the lake stopped and the sky started; it was like the boat was just floating in space.”

Such magical moments remind the researchers that their workplace—a lake like a vast sapphire set in a ring of snow-capped mountains—is a natural wonder.

“It’s been a year and a half working here, and this never gets old,” Young said. “It’s a great place to live and work, so I was happy that I landed here after (earning) my master’s.”

Young’s love of science and passion for discovery led him to his job at TERC, he said. “I am not a biologist,” Young said. “I have a master’s in environmental geochemistry, so I’m trained for aquatic science, but also the physics and chemistry behind it, rather than the biology and ecology side of it.”

He assists with all of TERC’s dive projects, including Senft’s, but Young is also working on a project at Clear Lake, northwest of Sacramento, where he uses sonar to map the lake’s bottom.

Senft, meanwhile, has a long history with Lake Tahoe.

“My grandpa loved the water here so much that he would fill up the back of his pickup truck with glass jugs of Tahoe water from spouts at the campground to bring back to Southern California,” she said.

Senft’s love for the outdoors and an interest in science led her to research invasive species. She counts herself lucky in that she spends every day doing what she loves while at the same time having a positive impact on the environment.

The researchers don’t have to punch a clock. They are often able to choose their work hours, but flexible schedules also may involve working nights, weekends or long shifts. Their time is split between field work and doing computer analysis of the data they collect. On field days, the team members are up before dawn.

“We have a lot of work to do, so we try to be where we need to be when the sun is coming up,” Young said. “Early mornings include loading all the equipment on the boat, getting our dive work ready, and heading out.”

Research teams usually spend six to 10 hours a day at their tasks, including investigating the algae sites, dropping off samples, setting up equipment to capture particles settling on the lake, measuring water clarity with Secchi disks, and doing a variety of other jobs before they head back to the dock or leave the lab.

UC Davis researcher Brant Allen prepares to lower the weighted Secchi disk into Lake Tahoe for a clarity measurement. This measurement is taken approximately every 10 days, with a long-term record going back to the late 1960s. High-tech instruments also are used to gauge water transparency, but don’t have the same long-term record of data as the disk measurements. Photo courtesy of TERC

Loving the lake to death

Major environmental threats to Lake Tahoe loom: global climate change, which is accelerated by human activities, and damage caused by the legions of people who flock to the natural wonder—and may be loving it to death.

“The number of people who want to be here to ski in the winter and then all summer seems like it’s going to increase every year,” said Young, who wonders if regulators will ever cap the area’s growth in visitors—a solution applied at Yosemite, although that isn’t as easy to do in an area that isn’t a national park. More visitors equals more pollution, which in turn fuels greater algae growth. If there’s no limit to the growing number of visitors, Young said, “I think what we are seeing with algae growth will continue to worsen.”

According to the Tahoe Regional Planning Agency, a bi-state agency created in 1969 to protect the area’s environment, Tahoe is host to 57,000 residents, and its economy now supports 15 million annual visitors.

The planning agency notes on its website that an alpine ecosystem as fragile as Tahoe’s can suffer devastating consequences even from small changes in air and water temperatures. They include more severe storms and droughts; winters with more rain and less snowfall; increased tree mortality; longer wildfire seasons with more intense fires; warming lake waters that are increasingly susceptible to algae growth; invasive species; and reduced water clarity.

The TRPA notes that the region’s transportation system is the largest contributor of greenhouse gasses in both California and Nevada. Those gasses, including carbon dioxide and methane, trap the sun’s heat, boosting average temperatures.

“A single drop of water in the lake takes 650 years before it leaves, since there’s no current and tides to move the water around. … The plastic in the lake has been here for such a long time, because it’s not getting flushed out.” —Katie Senft, TERC research associate

In winter, the top layer of the lake usually cools and sinks, carrying oxygen deeper and feeding microorganisms that live on the bottom of the 1,640-foot-deep lake. The colder and longer the winter, the more mixing occurs. But in 2016, mixing only reached down 262 feet, a much shallower depth than in previous years.

Since 1970, according to TERC, the water in Lake Tahoe has gotten 1.4°F warmer. Over the last 100 years, the Tahoe area’s average daily minimum temperature has risen 4.2° F, according to TERC. Those increases may seem minor, but because the lake’s ecosystem is so delicately balanced, the direct and indirect effects of global warming affect everything in and out of the water.

Warmer temperatures further diminish the lake’s clarity and have many other negative consequences, including jeopardizing the region’s $5 billion a year tourism industry, according to the TRPA. Water clarity is more than cosmetic. Clearer water also allows sunlight to reach underwater vegetation—plants that in turn generate oxygen, serve as habitats for fish and shellfish, and provide food for ducks, geese, fish and mammals.

The mystery of clearer water

As the lake has been warming over time, different algal species are becoming more dominant, especially a phytoplankton called cyclotella. That species has very small cells, which take longer to settle out of the water column. The tiny cells interfere with the light passing through the water and affect Tahoe’s clarity.

But even with cyclotella on the rise, Tahoe last year was clearer than it has been since the 1980s. In 2022, Lake Tahoe’s average annual clarity was 71.7 feet, compared to 61 feet in 2021. The greatest improvement in lake clarity was recorded from August through December, when the average Secchi depth was 80.6 feet. The swift increase in clarity was good news to scientists who want to eventually restore the lake to its historic 97.4 feet of clear water, but the recent gains may be temporary.

The clearer water coincided with high numbers of the lake’s native zooplankton, daphnia and bosmina. Those species, researchers said, act as a “natural clean-up crew” to help restore Tahoe’s renowned clear water. Mysis shrimp, an invasive species, usually keep the zooplankton population in check, but large numbers of the shrimp died out in 2021. That allowed the zooplankton to prosper and gobble up lots of the clarity-destroying algae, researchers theorize.

UC Davis researcher Brant Allen points at the meter wheel which indicates how deep the Secchi disk can be seen when lowered into Lake Tahoe. This must be set to zero at the lake surface before each measurement. Photo courtesy of TERC

The reason for the collapse of the mysis shrimp population remains a mystery. One theory is that their food source, a species of zooplankton called copepods, got fungal infections in the fall, and many died off. Left with a sparse supply of food, the shrimp may have starved to death.

“This summer, we had an intern who went back and looked at some historic zooplankton samples from back to 2016, and she was able to find individuals that had fungal infections every fall since then, but some years are worse than others,” Senft said. “It could also be wildfire smoke, a warming lake, or a phytoplankton species that could’ve been a poor food source the same year” that the shrimp died off.

The shrimp’s plight could be the result of a combination of factors, she said. “You know, with good research, you answer one question, but then you get 10 or 20 more, so that’s kind of where we’re at right now,” she said. If and when the shrimp population rebounds, the gains in clarity may disappear.

Some threats to the lake’s health are invisible to the naked eye.

The menace of microplastics

The journal Nature this year published the results of a study on plastic debris and microplastics in 38 lakes and reservoirs in 23 countries. Lake Tahoe had the third-highest concentration of microplastics among those sites.

Where are the tiny plastic particles coming from? There’s no specific evidence, but scientists theorize that the fragments devolve from plastic litter, which is broken down by the elements into smaller and smaller pieces that eventually are washed into the lake.

The lake’s litter problem is getting worse, as evidenced by this year’s July 4 holiday weekend, which set a record for the amount of trash left on the Tahoe shoreline. On July 5, volunteers with the League to Save Lake Tahoe collected 8,559 pounds of litter left behind by revelers on beaches and in nearby parking lots. That’s 2.5 times as much as the volunteers collected in 2022, when 3,450 pounds were hauled away.

What goes into the lake, stays in the lake.

“A single drop of water in the lake takes 650 years before it leaves, since there’s no current and tides to move the water around,” Senft explained. “… The plastic in the lake has been here for such a long time, because it’s not getting flushed out.”

From the 1860s to the early 1900s, loggers clear-cut most of the forest surrounding Tahoe to supply lumber for silver and gold mines, and to build Virginia City. They mowed down Jeffrey and sugar pine trees, which dominated the old-growth forest, leaving fir trees behind. Over decades, the survivors spread their seeds, creating a dense, fir-dominated forest that is more prone to fire and supports fewer plant and animal species.

The fire-prone forests contribute to Tahoe’s environmental woes, including algae growth.

“We’re monitoring the impact that smoke has on light penetration in the lake, and what we’re seeing is that it kind of has a greenhouse effect,” Senft said. “So algae grows better when you have all this wildfire smoke.”

The research team places metal buckets on buoys to collect particles in the air. During wildfires, a lot more particles rain down from smoke plumes, delivering more food—nitrogen and phosphorus—to hungry algae. “Also creeks, deposition and overland runoff bring in nitrogen,” Young said. “Especially when there are early rains in the springtime, all this stuff from the road washes off.”

Some positive developments

TERC’s State of the Lake report had other good news in addition to the unexpected, and perhaps short-lived, increase in water clarity. The Upper Truckee River, which is the biggest water source for Tahoe, carried just 11.1 metric tons of nitrogen into the lake this year, compared to its usual annual amount of 17.3 metric tons. That decrease indicates the area’s restoration and management efforts have reduced nutrient and fine particle loads entering the lake, TERC officials said.

Last winter’s cold temperatures—which froze the surface of Emerald Bay—were a boon for the lake’s health. In February, Lake Tahoe flipped, meaning it fully mixed vertically from top to bottom for the first time in four years. Mixing renews the water at the lake bottom with “fresh” oxygen-rich water from the surface and helps cool the bottom of the lake, which slowly warms due to geothermal heating.

As Lake Tahoe experiences rapid changes, advances in technology have allowed researchers to collect information and monitor conditions faster and more accurately than was previously possible. UC Davis keeps three research boats at the Tahoe City Marina: R/V John Le Conte, R/V Bob Richards, and R/V Ted Frantz. Each vessel is designed for a variety of research tasks. On field days, gliders and unmanned aerial vehicles patrol the water and air.

“Gliders are basically this big torpedo with no motor, so we have to pre-program waypoints on the lake where we want it to go,” Young said. “(The glider) controls its depth by changing its own density and using oil pumps, and has these wings that use (downward) movement to go forward. That movement gives the effect of the glider bouncing through the water column.”

The unmanned drones take flight to collect imagery of the lake, which is fed through a software program that classifies the results.

PHOTO/ZOE DIXON: UC researcher Erik Young holds a gooey sample of metaphyton, an algae that floats freely rather than sticking to rocks. That kind of algae befouled 16 Lake Tahoe beaches last year.

“So we can say one group of pixels is metaphyton, and another group wherever is sand,” Senft said. “That way, we can get an idea of percent coverage over a large-scale area and track how each changes over time.”

UC Davis TERC teamed up with the NASA’s Jet Propulsion Lab to keep five electronic buoys bobbing in Tahoe’s blue water. The buoys help calibrate satellite instruments and collect data on various environmental factors, including temperature, wind, humidity, radiation and other benchmarks.

NASA chose Tahoe because it is a perfect laboratory for the mission, researchers said. The lake is big enough to accommodate different satellite footprints, and its high elevation means there is less atmosphere to get in the way of communicating with satellites. The lake doesn’t freeze in the winter, and freshwater is much kinder to sensitive instruments than the salt water in oceans.

TERC programs also educate people about Tahoe’s environment. The center has partnered with Master Gardeners of Lake Tahoe, the Truckee Community Garden, and the North Tahoe Demonstration Garden to host garden workshops aimed at creating sustainable gardens.

Other public efforts include Tahoe Science Center, Green Building Tours and a monthly lecture series featuring experts on various environmental issues, scientific research, regional subjects and other topics.

But it’s the constant monitoring of the water, air and ground that helps scientists understand the delicate and often hidden relationships among the creatures and things that inhabit Tahoe’s web of life. Some queries are answered; most lead to many more questions.

“There are many complex processes occurring that we don’t fully understand,” said Geoffrey Schladow, UC Davis TERC director and professor of civil and environmental engineering.

“We need to better understand them to continue moving forward.”

RN&R editor at large Frank X. Mullen contributed to this report.

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