Margaret Moran
mmoran@sierrasun.com

Scientist: Climate change impacting Tahoe clarity

INCLINE VILLAGE, Nev. — Climate change is helping warm Lake Tahoe’s cobalt blue waters, which could complicate efforts to maintain its famed clarity, said one of America’s most respected scientists and an expert on the Sierra lake.

Over the last 30 years, Lake Tahoe has warmed about a 1 degree Fahrenheit, a result of the lake not releasing as much heat back into the atmosphere due to carbon dioxide blanketing, said renowned limnologist Charles Goldman.

“These are serious times, and this carbon dioxide question is just enormously serious,” Goldman said during his well-attended “The Impact of Climate Change and Global Warming on Inland Waters of the World” presentation Friday 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.”

According to a UC Davis 2010 report about the effects of climate change on Lake Tahoe in the 21st century, Secchi disk depth transparency has declined by 10 meters since 1968, while the rate of algae growth measured by carbon-14 is increasing at about 5 percent annually. 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 stated.

The report indicates winter clarity has improved over the past decade, which may be due to recent efforts to reduce urban stormwater flows to the lake, but more studies are needed to confirm it.

Still, summer clarity continues to decline, with 2011 marking the second-worse summer on record, and the cause may be related to impacts of climate change, the report stated.

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 article, “Global warming could radically change Lake Tahoe in 10 years.”

In that article, Geoffrey Schladow, director of TERC, said if greenhouse-gas emissions continue at current levels, deep mixing could become less frequent and even non-existent. That would impact not only aquatic life, he said, but could cause currently locked up phosphorous in the lake-floor sediments to be released, which would fuel algae growth at the lake’s surface.

When Goldman came to Lake Tahoe in the late 1950s, he said the nitrogen to phosphorus ratio was 1:1, which 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 of so much nitrogen in the atmosphere, Goldman said.

While lake clarity is impacted 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 can also have a large impact, Goldman said, evident of global lake decisions such as the one to divert river inflows into the Aral Sea for irrigation purposes, causing one of the largest lakes in the world to practically dry up.

“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.


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The Record Courier Updated Jun 17, 2013 02:46PM Published Jun 12, 2013 11:10AM Copyright 2013 The Record Courier. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.