Lake Tahoe, one of America’s most iconic alpine lakes, is experiencing a remarkable shift in its ecosystem. Invasive shrimp, which have long dominated the lake and caused imbalances, are now dwindling in numbers. Instead, a variety of beneficial creatures, including zooplankton that feed on algae, are thriving. This mysterious change may be responsible for the lake’s legendary clarity. In just three years, Lake Tahoe’s average visibility has increased from 52.8 to 71.7 feet, with recent measurements reaching 80.6 feet. This level of clarity hasn’t been seen since the 1980s.
According to Geoffrey Schladow, director of the UC Davis Tahoe Environmental Research Center, the lake has not experienced such clarity since the 1980s. Researchers have been studying the lake’s food web and have observed the presence of different populations, including algae, predatory shrimp called Mysis, and the native zooplankton species Daphnia and Bosmina. In the past, the nighttime netting would capture around 100 to 150 shrimp, but now it only catches two or three. This dramatic decrease in the shrimp population is contributing to the lake’s increased clarity.
To measure the lake’s clarity, researchers use a 10-inch white plate called a Secchi disk, which they lower into the water. Between 2021 and 2022, the lake’s average clarity improved by 10 feet. This improvement is most noticeable when the zooplankton Daphnia and Bosmina, which were once almost nonexistent in the lake, are abundant. Schladow describes the changing dynamics as biologically fascinating and an ongoing puzzle that researchers are trying to understand.
Lake Tahoe, known for its breathtaking beauty, owes its clarity to the surrounding granite and its relatively small watershed, which minimizes agricultural pollutants. However, the lake’s waters have been progressively cloudier in recent decades, losing its renowned clarity at a rate of almost 1.5 feet per year. Concerned about this trend, management agencies have taken steps to reduce runoff from various sources, leading to a significant decrease in fine sediment and other clarity-harming contaminants entering the lake.
The introduction of non-native Mysis shrimp in the 1960s further contributed to the decline in clarity. These shrimp, originally brought in to provide food for Lake Trout, were not being consumed by fish due to their nighttime deep-water behavior. They successfully outcompeted the native zooplankton, especially Daphnia and Bosmina, and caused a significant imbalance in the lake’s food web.
However, recent research suggests that the Mysis shrimp population is now declining. Studies conducted since 2012 have found increased numbers of Daphnia and Bosmina in the lake. One possible explanation for this shift is that the shrimp are starving because their primary food source, copepods, is dying from fungal infections. The Tahoe Environmental Research Center is currently examining historic water samples to determine if the fungus is indeed the cause of the copepods’ death and the shrimp’s population crash.
The resurgence of Daphnia and Bosmina, now free from the threat of predators, has been vital in improving the lake’s clarity. These zooplankton species are highly efficient feeders, consuming algae and fine particles that contribute most to the lake’s clarity. Visitors to Lake Tahoe have witnessed the dramatic change, with underwater visibility reaching unprecedented levels.
While the increase in clarity is encouraging, Darcie Goodman Collins, CEO of The League to Save Lake Tahoe, emphasizes that other factors, such as urban landscape impacts and the overall ecological health of the lake, need to be considered. It is hoped that Lake Tahoe’s clarity will continue to improve in the coming years, potentially reaching levels similar to those seen in the 1970s. However, it is essential to recognize the ever-changing nature of the lake’s food web and the need for ongoing management efforts to maintain its pristine condition.
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