Twice a month Geoff Schladow, director of the UC Davis Tahoe Environmental Research Center, takes a boat out to the middle of the lake, where he lowers a 10-inch white disk into the cold waters until it disappears from view. The deeper it's visible, the clearer the water.
"Some days it just seems to stay there forever and ever," he said. But some days it doesn't.
Schladow's mission is to test the clarity of the lake, a crucial measurement for determining the health of the ecosystem. According to new data, that clarity declined by nearly a quarter in the summer of 2016 compared to 2015. Schladow's certain the decline is related to climate change. The way that's exhibiting itself in Lake Tahoe: hoards of red-blood-cell-sized plankton taking over the surface of the lake.
As the seasons change, the lake heats up and cools down. During winter, the temperatures at the surface and at the bottom of the lake are similar — that means that they're of similar density. So, there's a naturally occurring mixing that goes on as the water from the bottom moves to the top, and the top to the bottom.
This process is important, because nutrients and oxygen are driven from the surface to the lake floor.
When it's warm outside, the process is a bit different. Warm water remains at the surface as the cool, dense water stays below — and the rate of mixing slows.
"What climate change is doing is it's warming the surface of the lake, and this warming is starting earlier and earlier," Schladow said. Which is concerning, because the longer the warm spells, the less mixing that occurs.
Cyclotella love the warm water at the surface of the lake — so much that their numbers balloon. Even though they're one-twentieth the size of a human hair, they scatter light intensely, stopping it from reaching the depths of the lake — and decreasing its clarity.
At the bottom of the lake, without the mixing, there's a loss of oxygen.
"So, we have a lot of models running with predictions of what climate will be like over the next 100 years, and they're pointing to the fact that at some point, maybe 40 years from now, we're going to start running out of oxygen at the bottom of Lake Tahoe. We're creating a dead zone there," said Schladow. "Other lakes, other reservoirs are going to have this happening much sooner. So, Tahoe, it's still a ways down the road, but the fact that it will happen ... is something that we should be aware of and trying to avoid."
Correction: An earlier version of this story misspelled Geoff Schladow's name. KPCC regrets the error.