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Lake Tahoe's clear water is due to tiny creatures called Zooplankton, researchers say

AYESHA RASCOE, HOST:

There's something in the water at Lake Tahoe. The freshwater lake between California and Nevada is the clearest it's been in decades, and researchers say that could be thanks to some tiny organisms called zooplankton. To tell us more about it, we turn to Geoffrey Schladow. He's director of the UC Davis Tahoe Environmental Research Center. Welcome to the program.

GEOFFREY SCHLADOW: Thank you. It's a pleasure to be here.

RASCOE: So first, can you tell us what these creatures are exactly? What are zooplankton?

SCHLADOW: Well, they're microscopic animals that live in all lakes, and they come in different forms. And they're parts of different families there. But basically, they eat the phytoplankton, the small, microscopic animals that live in the lake.

RASCOE: So how exactly are the zooplankton making the lake clear?

SCHLADOW: Well, these particular zooplankton, the Daphnia, are very indiscriminate feeders, meaning they eat everything, and they particularly eat everything that's very small. So very fine clay particles, very small phytoplankton - they consume that. They remove it. And it's those fine particles that are the root cause of the decline in clarity. So essentially, they're little Roombas cleaning up the lake.

RASCOE: OK. And then, like, why is lake clarity important?

SCHLADOW: So lake clarity is important for many reasons. It's an aesthetic quality, and it's a reason people come to Lake Tahoe. But it also impacts the drinking water quality, and clarity is a really important integrator of many things that impact all lakes. So things that wash into the lake from its watershed, things that fallen from the air affect the clarity. So by taking this one simple measurement, we can keep track of the daily and the seasonal changes that any lake is subjected to.

RASCOE: So, I mean, your recent report about the ecology of the lake says that the water is the clearest it's been since the 1980s. How do you measure something like that?

SCHLADOW: Well, we measure it in many ways, but the simplest one and the one that we have the longest record for is using a 10-inch white disc. It's called a Secchi disc, and it's lowered down into the water. And we measure the depth at which we can no longer see it. In many lakes, that could be just 10 or 12 feet. In Lake Tahoe, typically, the last few years it's been about 70 feet on average. But this last six months, it's been over 80 feet.

RASCOE: Oh, wow. So why is that? It's because are there more zooplankton now than normal?

SCHLADOW: What's happened is that there are different zooplankton now. And what we've had is a resurgence of the native zooplankton. There are a few of them, but the main one is called Daphnia. For many, many years, Daphnia were absent from the lake because of an introduction of invasive shrimp. But those shrimp have themselves now disappeared, and the Daphnia are back, and the lake very quickly has gotten clear.

RASCOE: Is this just a short-lived moment of clarity, or do you think this will be long term?

SCHLADOW: Well, that's the unfortunate part. We believe that it will be short term. It may last maybe two years, maybe a little longer because the shrimp will come back. And they're very aggressive, and they prey upon the Daphnia. And once they do that, Daphnia will disappear, and clarity will go back to what it had been.

RASCOE: That's Geoffrey Schladow, director of the UC Davis Tahoe Environmental Research Center. Thank you so much for joining us.

SCHLADOW: Thank you. It was a great pleasure. Transcript provided by NPR, Copyright NPR.

Ayesha Rascoe is a White House correspondent for NPR. She is currently covering her third presidential administration. Rascoe's White House coverage has included a number of high profile foreign trips, including President Trump's 2019 summit with North Korean leader Kim Jong Un in Hanoi, Vietnam, and President Obama's final NATO summit in Warsaw, Poland in 2016. As a part of the White House team, she's also a regular on the NPR Politics Podcast.