HOMEWOOD, Calif. – Off the shore of Homewood, the research vessel John LeConte takes regular measurements to help researchers track the status of Lake Tahoe. And as the seasons change, so does the state of the lake—its clarity, zooplankton levels, algae growth, and more shift with the weather.

“It’s like a health check or blood work for the lake,” said Brandon Berry, staff research associate at the UC Davis Tahoe Environmental Research Center (TERC). “Routine checks help us to build data and track changes more accurately.”

Berry, who captained the vessel during its most recent trip, is one of the staff members of the TERC responsible for conducting these trips, called Lake Tahoe Profiles (LTPs). They do roughly 25 LTPs per year, though there may be more or less depending on conditions—though Berry and TERC chemist Keeley Martinez both have gone out during brutally cold days to get measurements.

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Though there are also Mid-Lake Tahoe Profiles, Berry and Martinez take their measurements off the shore of Homewood, a spot that data has been collected from since the 1960s. This LTP marks number 1746.

The site also, according to Berry, “best represents the average measurements all around the lake.” This way, they can obtain data on the clarity, temperature, and productivity, among other measurements, that impact the lake’s health.

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As the season is changing, Berry anticipates that the lake clarity will increase as it typically does when winter arrives. The lake will also begin mixing as it cools, an important process that redistributes oxygen and nutrients throughout the lake, but crucially can only happen when the entire lake is roughly the same temperature. Right now, there’s a thermocline, or a distinct layer of temperature difference at about 40 meters down.

Another change that is occurring in the lake is the level of sunlight that penetrates the surface. As the angle of sunlight changes and days grow shorter, less light can travel into the depths of the lake, meaning temperatures drop and photosynthetic organisms like algae have less chance of growing.

“Lake Tahoe is ultraoligotrophic, meaning it should be a pretty low productivity lake, but it’s been going up,” said Martinez. She worked with Berry to collect water samples at different depths, which she then spikes with a carbon-14 isotope on the vessel’s lab. These sealed bottles then go back into the water on a marked buoy to measure carbon uptake, which can help predict how algae might grow.

Algae is the bottom of the food chain and does support the life in the lake when kept at manageable levels. A single step up from that are the zooplankton. While many of the native zooplankton were eaten by the introduced mysid shrimp, zooplankton measurements can help track what’s eating and what’s being eaten, and help understand how restorative efforts like returning the Lahontan cutthroat trout to the area affect the ecosystem. During the winter, because there’s less food, there also tend to be less zooplankton—though the TERC sends them to a different lab for counting.

The John LeConte carries complex equipment that is dropped to depths of 150 meters, helping to give a fuller picture of the water column—they measure particle size through lasers, photosynthesis through light panels, and oxygen through sensors.

But one of the simplest and most familiar ways to measure lake clarity is through a Secchi disk, a simple white disk that will seem to “disappear” as it passes through the water layers. This measurement is a central part of the TERC’s messaging about lake clarity; during this trip, it was clear down to 20 meters, which is typical of the winter.

These Lake Tahoe Profile results will be coalesced, along with other data, into the annual State of the Lake report from the TERC.

Eli Ramos is a reporter for Tahoe Daily Tribune. They are part of the 2024–26 cohort of California Local News Fellows through UC Berkeley.