Tahoe: Becoming Clearer

Visibility (the Secchi depth) is decreasing (UC Davis)

The waters of Lake Tahoe are renowned for their clarity, but in recent decades murkiness has risen.

The "Secchi disk" is a measurement tool that has been used for over half a century:

The white frisbee is the most well-known and influential method of studying Lake Tahoe’s clarity. Called the Secchi disk, it has been used by UC Davis scientists to measure Lake Tahoe’s clarity since 1968. The method is simple: Scientists lower a 10-inch disk into the water, lean over the edge of the boat and mark the moment when it disappears into the blue and they can no longer see it. There are other ways to measure how far light penetrates into a lake and how clear the water is, Hampton said. However, the Secchi disk’s simplicity makes it universally understood and resonate with people in a meaningful way, and researchers use the Secchi disk to study clarity in lakes all over the world.

Despite the cloudy long-term trend, UC Davis researchers say that the lake has become clearer beginning three years ago: [bold added]

For the last five months of 2022, Lake Tahoe was the clearest it has been since the 1980s. That is due in part to a resurgence of the lake’s native zooplankton. They’ve provided a natural clean-up crew to help restore the lake’s famous blue waters.

The findings are reported in the “Lake Tahoe Clarity Report 2022,” released today from the UC Davis Tahoe Environmental Research Center, or TERC, for the Tahoe Regional Planning Agency...

The primary factors affecting lake clarity are the concentration of particles in a specific size range, such as silt and clay, and tiny phytoplankton, or algae. The phytoplankton Cyclotella, a single-celled alga, is in this size range and has impacted clarity in most years.

Zooplankton are small, microscopic animals. Some zooplankton, particularly Daphnia and Bosmina, are specialized to consume particles in that critical size range.

“Daphnia and Bosmina largely disappeared from the lake after they were grazed down following the introduction of the Mysis shrimp in the 1960s,” said Geoffrey Schladow, director of the UC Davis Tahoe Environmental Research Center. “In late 2021, the Mysis population unexpectedly crashed, and it took 12 months for the Daphnia and Bosmina to build up their numbers and start their natural cleansing.”

Other factors are known to influence year-to-year changes in clarity. These include the magnitude of the runoff, the warming of the lake surface and the depth to which the lake mixes in the previous winter. The report examined all these factors and concluded that only the change in the zooplankton community could account for the magnitude of this year’s change.

It was too lazy to attribute the trend toward lower water clarity to the usual suspects of runoff, pollution, wildfires, and global warming. The growth in Mysis shrimp may have been the reason because of the negative impact on zooplankton, while the 2021 "crash" in Mysis shrimp resulted in a resurgence in plankton that "cleansed" Lake Tahoe. The 2021 shrimp die-off is so far unexplained and needs to be researched for a satisfactory understanding of the Tahoe ecosystem.

What was once clear is now cloudy, buty your humble blogger is confident that it shall become clear again.