A new study by UC Davis has reported that for the last five months of 2022, Lake Tahoe was the clearest it has been since the 1980s. This extraordinary clarity is due in part to a resurgence of the lake’s native zooplankton. These extraordinary creatures serve as a natural clean-up crew, helping to restore the lake’s famously blue waters.
The findings from this research can be found in the “Lake Tahoe Clarity Report 2022,” released by the UC Davis Tahoe Environmental Research Center (TERC).
According to the report, zooplankton Daphnia and Bosmina specialize in consuming a type of phytoplankton called Cyclotella. As zooplankton gobble up the phytoplankton, water clarity dramatically improves. Unfortunately, an unforeseen foe entered the picture when Mysis shrimp were introduced to the lake in the 1960s. The shrimp decimated the zooplankton population, leading to a significant loss in clarity over the years.
In a long-overdue stroke of luck, the Mysis shrimp population unexpectedly crashed in late 2021. It took 12 months for the Daphnia and Bosmina to build up their numbers and start their natural cleansing.
“Clarity is measured as the depth to which a 10-inch white disk, called a Secchi disk, remains visible when lowered into the water,” the report explains.
In 2022, Lake Tahoe’s average annual clarity was 71.7 feet compared to 61 feet in 2021. This number jumped to 80.6 feet when measured from August to December of 2022. This coincided with the highest numbers of the zooplankton Daphnia and Bosmina.
California and Nevada have an ultimate goal to restore lake clarity to its historic 97.4 feet.
This is just the beginning
“We expect the impact of Daphnia and Bosmina to grow over 2023, and clarity may return to 1970s levels—despite the expected large runoff from this year’s record snowpack,” TERC boat captain and Secchi disk observer Brant Allen said. “These events support the hypothesis we put forward several years ago that the food web is a major factor in controlling lake clarity.”
Despite all the good news, Mysis shrimp populations are expected to rebound and clarity will likely return to what we have seen in the past 20 years. In the meantime, researchers are monitoring the lake in the absence of Mysis in order to better understand their effect on lake clarity.