In a remarkable turn of events, Lake Tahoe is experiencing a resurgence in its famed clarity, thanks to a shift in its ecosystem. The invasive shrimp population, which had disrupted the lake’s delicate balance for years, is dwindling, making way for beneficial creatures that feed on algae and other fine particles. As a result, the average visibility in the lake has significantly increased from 52.8 feet to an astonishing 71.7 feet over the past three years. Even more impressively, the visibility reached 80.6 feet in the last five months of 2022, a level not seen since 1988. Geoffrey Schladow, director of the UC Davis Tahoe Environmental Research Center, remarked, “We haven’t had this level of clarity since the 1980s.”
To study this transformation, researchers aboard the vessel John Le Conte carefully monitor the lake’s food web. They track the presence of algae, the invasive shrimp known as Mysis, and a variety of helpful native species of zooplankton, particularly Daphnia and Bosmina, which are indicators of good water quality. Previously, the researchers would capture around 100 to 150 shrimp during nighttime netting operations. However, they are now only catching two or three. “You can name them!” Schladow joked. Additionally, the researchers measure lake clarity by employing a method using a 10-inch white plate, called a Secchi disk, which is lowered into the water to determine visibility. Although data from 2023 is not yet available, the lake’s average clarity increased by 10 feet between 2021 and 2022. Schladow stated, “Biologically, things are changing. Things that weren’t in the lake before, suddenly they’re there. Things that were in the lake—they’ve disappeared. It’s very interesting, and we’re trying to piece it together.”
Lake Tahoe, nestled amidst the Sierra Nevada on the Nevada/California border, has always been renowned for its breathtaking beauty. Its crystal-clear waters have captivated many, famously inspiring writer Mark Twain, who described it as “so singularly clear…that the boat seemed floating in the air. The water was not merely transparent, but dazzlingly, brilliantly so.” The lake owes its exceptional clarity to the surrounding granite and its relatively small watershed, which helps keep it free from agricultural pollutants. However, in recent years, the lake’s waters had become increasingly murky, with clarity decreasing by approximately 1.5 feet annually. Concerned by this trend, management agencies in the region have implemented measures to reduce runoff from various sources, effectively preventing over 500,000 pounds of sediment and contaminants from entering the lake each year. However, another culprit emerged: the non-native Mysis shrimp, whose abundance wreaked havoc on the lake’s ecosystem. In the 1960s, these shrimp were introduced into the lake with the intention of providing a food source for Lake Trout. However, the shrimp’s preference for darkness and their vertical migration to the surface during the night meant they were largely uneaten by sight-feeding fish. Consequently, they proliferated and decimated the populations of important zooplankton species like Daphnia and Bosmina, which disappeared from the lake by 1971.
But now, a glimmer of hope has emerged. Starting in 2012, researchers discovered small numbers of Mysis shrimp in the lake’s Emerald Bay. Further study in other areas of the lake has revealed a resurgence of Daphnia and Bosmina populations. Schladow exclaimed, “For 50 years, Mysis ruled. Then they disappeared. Gone. We didn’t know why.” Microscopic analysis in the UC Davis Tahoe Environmental Research Center’s laboratory has shed light on a potential cause: the shrimp are starving. With the decline of Daphnia and Bosmina, the shrimp have shifted their diet to copepods, a different type of zooplankton. Unfortunately, these copepods are now dying in large numbers due to fungal infections. The laboratory is conducting further research to determine if this fungus is the primary factor behind the copepods’ demise. It remains unknown why the copepods are experiencing higher mortality rates at present. Nonetheless, the re-emergence of Daphnia and Bosmina is incredibly promising. These species’ eggs can remain dormant in the lake sediment for up to a century, and now they are hatching and thriving in the absence of predators. Schladow explained, “They’re very, very, very efficient feeders. The lake’s cleanup crew, they just shovel things into their mouths, and the food they eat, the algae and very fine particles, are what impact the lake’s clarity the most.”
These positive developments have not gone unnoticed by visitors to Lake Tahoe. Swimmers, like A.J. Kohn, described the experience as “serene” and “crystal clear,” while paddleboarder Heather Pratt called it “absolutely beautiful.” However, despite these improvements, Darcie Goodman Collins, CEO of The League to Save Lake Tahoe, urges caution. While better annual clarity averages are a cause for celebration, they do not necessarily indicate overall improvement. Collins stated, “A lot of the impacts are coming from our urban landscape and could have a significant impact on our ecology, including the Mysis shrimp and our native populations of Daphnia.” Lake management agencies in California and Nevada envision a future where the lake’s visibility is restored to levels reaching 90 feet. Nevertheless, the delicate nature of the lake’s food web means that the resurgence of Mysis shrimp is a possibility, which could jeopardize this progress. Nonetheless, researchers are hopeful that strategies to control the shrimp population could be implemented, ensuring a more balanced ecosystem. Schladow expressed optimism, saying, “If we can get the lake back into some balance, then the system is pretty resilient. And that gives us hope.”
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