State of the Lake: 2022 saw rapid changes in clarity, algae growth

Pollen blanketed much of the Lake Tahoe basin in summer 2023. | Mike Bruno, UC Davis TERC
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By Emily C. Dooley, UC Davis

The Tahoe Environmental Research Center, TERC, at the University of California, Davis, released its annual Tahoe: State of the Lake Report in late July, describing 2002 as one of rapid biological change. Most notably in 2022, Lake Tahoe’s average annual clarity was 71.7 feet compared to 61 feet in 2021.

The 2023 report presents data collected during 2022 and puts it in context with historical records. It also serves as an important resource for restoration, management and monitoring decisions. The entire report is available at tahoe.ucdavis.edu/stateofthelake.

UC Davis researchers have been monitoring the second deepest lake in the United States continuously since 1968, and the report provides an update for nonscientists on a variety of factors affecting the health of Lake Tahoe.

Rapid changes
The report details changes in lake clarity and the possible links to a sudden decline in the Mysis shrimp population. Extensive algal blooms in the nearshore of the lake and new data on microplastics are also detailed. Other topics include new monitoring and modeling efforts, lowered nitrogen and phosphorus levels and research into the efficacy of no-wake zones.

“The lake is in a state of rapid change, which presents us the opportunity and the challenge to better understand the lake with more and smarter monitoring,” Schladow said. “Within 2022 we had more changes throughout the lake than we’ve experienced over decades.”

The biggest change in the lake had to do with the rapid improvement in clarity from August through December 2022, which was the best measured since the 1980s. This was preceded by the population collapse of Mysis shrimp, a species introduced to the lake decades ago, and other changes in the lake’s native phytoplankton and zooplankton.

“There are many complex processes occurring that we don’t fully understand,” Schladow said. “We need to better understand them to continue moving forward.”

As part of research on this topic, an acoustic doppler current profiler was installed in June at 300 feet below the surface on the west side of the lake to provide continuous water velocity measurements and “to track the expected resurgence of the Mysis shrimp in the coming years,” the report’s executive summary stated.


Read our story on how researchers monitor Lake Tahoe in this edition or at TheTahoeWeekly.com.


Nitrogen, algae changes
In other positive developments, restoration and management efforts continued to reduce nutrient and fine particle loads entering the lake. Total nitrogen coming into the lake from the Upper Truckee River, which is the largest source of water into the system, was 11.1 metric tons per year compared to the mean annual load of 17.3 metric tons per year.

Other biological changes included huge swings in the algal population and their rate of photosynthesis. As the water level fell in 2022 and temperatures increased, so did the makeup of the algal community, with a Cyanobacteria comprising the greatest number of phytoplankton. More areas of attached algae, or periphyton, were also noted in 2022. Some 16 miles of beach were fouled by decaying algae washing up on the beaches.

Schladow said that researchers are studying the link between the invasive Asian clams and increased algal growth. The Asian clams filter water “to bring in algae, they concentrate it, they use about 10 percent of it, and they excrete the rest. The result is that excrement is about 10 times as concentrated in nutrients,” which feeds the algal blooms, he said at a July 20 talk following the release of the State of the Lake report.

Other report highlights

  • Water levels in the lake change throughout the year based on weather, inflows and outflows. A wet November and December filled up the lake, bringing it almost to maximum capacity.
  • Microplastic pollution is an issue in Lake Tahoe, where samples show levels akin to San Francisco Bay. “What goes into Tahoe, stays in Tahoe,” Schladow said. “It’s a worry for every aquatic system.”
  • TERC has 25 stations around the Lake Tahoe Basin that relay real-time measurements, as well as autonomous underwater vehicles, satellites, drones and helicopters. New, machine learning technology helped to better monitor beaches and nearshore areas.
  • Extreme weather froze Emerald Bay and led to a longer period of vertical mixing in Lake Tahoe.

Katherine E. Hill contributed to this report.