Environment

“People are just amazed and wowed at the optical blue that you see from pure water itself,” said Sudeep Chandra, a limnologist at the University of Nevada, Reno, who collaborates with Girdner. “That blueness is the reflection of the hydrogen and oxygen hanging out together without any material in it.”

Since 2010, however, Girdner and his colleagues have noticed an unexpected change in the Secchi data: Despite the day’s slightly cloudy reading, Crater Lake’s clear water is getting even clearer.

This might sound like a good thing. After all, the lake’s remarkable, glasslike transparency and brilliant hue are major draws for the half-million tourists who visit every year. But it might also indicate that something is going wrong with the lake’s physics, chemistry and ecology, and it could be a harbinger of changes to lakes across the world in the age of climate change.

As the planet warms, summers are growing longer and winter nights aren’t getting as cold as they used to. As a result, the surfaces of many deep, temperate lakes are warming even faster than the air. This shift to the energy flux of the top layer of water can set in motion a series of physical changes that add up to a breakdown of lake mixing — a fundamental process that acts like a heartbeat for deep, temperate lakes that don’t freeze in winter. Lake mixing is driven by physical properties such as wind, air temperature, water temperature and salinity, and on seasonal or annual cycles it circulates water between the surface and the depths. When mixing stops, oxygen and nutrients don’t get distributed throughout the water column, which can kill fish, trigger unsightly and dangerous algal blooms and invite invasive species to take over.