While out enjoying an afternoon on one of Lake Tahoe’s sandy beaches over the past few years, you might have noticed large mats of decomposing algae washing up or floating nearby. The lake’s famed blue waters are facing another threat while the battles of climate change and invasive species wage on — and it’s all very much connected. 

Nearshore algae blooms are a burgeoning ecological threat to Tahoe. Not only do they impact the experience for beachgoers, but they also degrade water quality and, in some cases, pose a threat of toxicity. 

Over the last 50 years, the rate of algal growth has increased sixfold, according to U.C. Davis Tahoe Environmental Research Center’s 2022 State of the Lake Report. Between 2021 and 2022 alone, the amount of algae growing in the lake jumped up 300%. 

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There are two general communities of algae in Tahoe: periphytic algae — the slimy green algae clinging to rocks — and the free-floating metaphytic algae. 

“We’ve got the same amount of slime on our rocks as we had roughly in the early 80s,” explains Dan Segan, principal natural resource analyst at the Tahoe Regional Planning Agency. “The nuisance algae washing up on the beaches is a more recent occurrence.” 

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Though the algae is native, in the past, normal conditions have kept the population levels low. Tahoe is an oligotrophic lake, meaning it’s deep, cold, clear and low in nutrients that are needed to support large populations of aquatic plants, animals or algae. 

But as the climate has become warmer, drier and characterized by extreme weather variability, the lake’s temperature has continued to rise. The average water surface temperature of Lake Tahoe in 1968 (the year monitoring began) was 50.4 degrees Fahrenheit, but in 2021, it was 53.1 degrees, the third warmest year on record. 

“The lake is warming every year along with climate change, which is great if you’re someone wanting to swim on our beaches — we get to 70 degrees in our nearshore and shallow waters of Tahoe these days — but it also makes it more hospitable for algae and algae growth,” notes Segan. “There’s clearly a dynamic there between the warming waters and the ecological change in the lake.”

INVASIVE ISSUES 
So far, most of the algal blooms have been concentrated in South Lake Tahoe and Tahoe City. In South Lake Tahoe, a large toxic bloom popped up near the mouth of the Upper Truckee River in fall 2021. The following spring, a much larger bloom of attached algae grew along six miles of Tahoe City’s shoreline. 

U.C. Davis Tahoe Environmental Research Center (TERC) has been monitoring the lake’s algae — among many other environmental metrics — for decades. Their latest research project is examining the relationship between the rise in algal blooms and the growing population of invasive Asian clams. 

These non-native bivalves were first discovered on Lake Tahoe’s South Shore in 2002. Since then, despite numerous agencies’ best efforts, the population has spread around the lake, with the largest numbers concentrated on the southeast shores. The clams eat algae from the water, but their excrement creates a concentration of nutrients that allows the algae to grow beyond its normal means.  

“We first started seeing clams in real numbers around 2008 in Marla Bay, and two years later, we had a fairly large algal bloom there. It washed up on the beaches, and there was a lot of speculation at the time because we hadn’t really identified the linkage between the clams and the algae,” says Brant Allen, the field lab director with TERC. “As the clams spread around the lake and became fairly high in density, we started seeing these algal blooms following the high density of clams around the lake.”

UPPING THE TECH

In 2017, TERC expanded its algae-monitoring program to include aerial data collection from an instrumented helicopter and drone. Combined with the original practice of divers collecting samples from a fixed set of locations, the researchers are now able to better understand how the areas affected by algae are shifting as water levels, temperature and clam populations fluctuate. 

TERC is also exploring ways to mitigate the algal blooms, including getting at the source of the issue that they can control: the invasive clams. Bottom barriers laid across the areas where populations are high cut off the clams from oxygen in the water, but the process is costly and slow. The mats need to sit in place for a month during the summer or 3-4 months during the winter to kill the clams, and it’s not 100% effective. 

“It’s very difficult to make sure that you’ve covered all of the clams. Unless you have a very localized population, the bottom barriers are very difficult to use for eradication,” notes Allen. 

The agency is in the preliminary stages of another approach for algae control, which would use a vacuum-like device to suck the algae from the water. After collecting the bloom, TERC would dry out the algae and compost it for resale to offset the cost of the process. 

TOXIC BLOOMS 

Across the country, toxic algae blooms have made headlines over the last few years. 

“We’re lucky in Tahoe because the harmful algal bloom concerns are primarily for stagnant, warmer waters. But the area where we’ve seen the most concern is in the Tahoe Keys,” explains TRPA’s Segan, pointing to the high-concentration of invasive aquatic plants impeding the flow of water in the South Shore neighborhood’s already slow-moving, man-made lagoons. (Numerous mitigation efforts are underway, and have been for years, to address the Key’s high concentration of invasive species that have proliferated in the mostly stagnant water.) 

The vast majority of the algae that you’ll see on Tahoe’s beaches and in open water is not a human health concern, says Segan. However, last summer, harmful algal blooms were discovered on the South Shore along Regan, Barton and Kiva beaches and in the Tahoe Keys waterways. 

Formed by tiny organisms called cyanobacteria, these blooms can make the water green, white, red, or brown. In Lake Tahoe, the harmful algal blooms can look like a thick layer of paint floating on the water or a scummy mat along the shoreline. Most often it is a bright shade of green. Testing of Tahoe’s water and algae is ongoing, and signage will indicate advisory levels from “Caution” to “Warning” to “Danger.”

“We can’t control the water temperature, but there’s pretty robust plans in Tahoe to prevent nutrients from getting into the water,” notes Segan. “All of the watershed protections we have in place prevent the nutrients from getting into the lake that cause algae in the first place. Prevention is the best cure.”