Climate change affecting Tahoe’s clarity, scientist says

Climate change is warming Lake Tahoe’s cobalt-blue waters, which could complicate efforts to maintain its famed clarity, according to a scientific expert on the lake.

The lake has warmed about 1 degree Fahrenheit over the past 30 years, a result of it not releasing as much heat back into the atmosphere due to carbon dioxide blanketing, said limnologist Charles Goldman. Limnology is the study of inland waters.

“These are serious times, and this carbon dioxide question is just enormously serious,” he said during a presentation June 7 at the Tahoe Center for Environmental Sciences building in Incline Village. “There is probably no problem that has faced humanity to the degree that this has since the ice ages,” he added.

about the effects of climate change on Lake Tahoe in the 21st century, Secchi disk depth transparency has declined by 10 meters since 1968, according to a UC Davis report from 2010. Meanwhile, the rate of algae growth measured by carbon-14 is increasing at about 5 percent annually, the report says. Clarity is measured by the depth at which 10-inch white Secchi disks are no longer visible.

Yet there are occasional years when lake clarity improves. According to UC Davis’ 2012 State of the Lake Report, the annual average lake clarity showed a “marked improvement” from 2010, increasing by 4.5 feet.

“Such large year-to-year functions are not unusual in the long-term record, and are part of the reason we advocate focusing on the long-term changes rather than annual or even short-term changes when trying to evaluate effectiveness of management programs,” the report states.

The report indicates winter clarity has improved over the past decade. That might be the result of recent efforts to reduce urban stormwater flows to the lake.

Still, summer clarity continues to decline, with 2011 marking the second-worst summer on record.

Lake Tahoe mixes to the bottom about every fourth year, Goldman said Friday, bringing nutrients up to the surface that promote algae growth.

This deep mixing also takes oxygen from the surface and distributes it throughout the lake to support aquatic life, according to the 2008 Science Daily story “Global warming could radically change Lake Tahoe in 10 years.”

In that story, Geoffrey Schladow said that if greenhouse-gas emissions continue at current levels, deep mixing could become less frequent and even nonexistent. That would not only affect aquatic life, he said, but could cause locked-up phosphorous in the lake-floor sediments to be released, which would fuel algae growth at the lake’s surface. Schladow is the director of the Tahoe Environmental Research Center.

When Goldman came to Lake Tahoe in the late 1950s, he said, the nitrogen to phosphorus ratio was 1:1. That wasn’t sufficient for normal plant growth.

“That’s why Tahoe was so beautifully clear,” he said.

Today, that ratio has moved to 40:1 because there is so much nitrogen in the atmosphere, Goldman said.

While lake clarity is affected by the influx of phosphorus and nitrogen, it’s also affected by fine sediment particles, which come from land disturbance and urbanization, according to the 2010 UC Davis report.

Political decisions also can have a large impact, Goldman said.

“Keep in mind that the younger generation are really going to inherit part of the mess we left them, and it’s really up to us to inspire their environmental enthusiasm,” Goldman said in the closing of his presentation.