A superficial test devoid of test tubes, lab coats and modern scientific analysis -- anyone with a boat, a rope, a white metal disk and good vision could carry out the task -- is perhaps the most important method observers use to assess Lake Tahoe's environmental health.
First used in the mid-1800s by Angelo Secchi, an Italian astronomer and scientific adviser to the pope, Secchi disks provide a basic metric to determine lake clarity. Researchers lower the disk, take note of the depth at which the disk disappears and compare the result to previous figures taken months, years or decades before.
The 191-square-mile lake gained its fame from its pristine water. Mark Twain, who made the lake an American icon with his book "Roughing It," called it an "enchanted mirror." But now Lake Tahoe's overall worsening clarity provides signals of how the warming climate is detrimentally affecting the region's ecosystem.
Observing those clarity clues and other indicators, scientists have observed several detrimental impacts likely driven by climate change: increased snowmelt runoff and erosion; distressed plant species, due to shrinking water supplies; the growing presence of invasive species; and a tinderbox-like environment, due to decades of fire suppression.
The latest "State of the Lake Report," an annual study compiled by the Tahoe Environmental Research Center, or TERC, part of the University of California, Davis, described many of these ecological trends spurred on by rising temperatures.
The surface water temperature in July 2013 was 65.6 degrees Fahrenheit, on average, the highest in five years, according to the report, and the local snowpack melted earlier in the year than is typical.
Nonetheless, the once-declining clarity has rebounded during the past decade -- even improving 13 feet during the last two years. Lake Tahoe's decrease in clarity has improved from 64.1 feet in 1997 to 70.1 feet in 2013. In 1967, viewers could peer 105 feet into the water.
"The importance of clarity is that it's an indicator," said Geoffrey Schladow, the director of TERC, in an interview with ClimateWire.
A 'bellwether' for other lakes
Clarity, officially measured in the center of the lake, can fluctuate from 50 to 120 feet, depending on shifting environmental factors, Schladow said. And Lake Tahoe can be a valuable test ground for developing climate change mitigation efforts to "export" to other lakes, according to Schladow.
"In many ways, Tahoe is a bellwether," he said.
TERC has six real-time monitoring stations around the lake -- collecting data on turbidity, algae, dissolved oxygen, organic material and more -- and aims to have 14 more running within the next 12 months. By comparison, the Great Lakes only have a five-buoy system, according to NOAA.
Undoubtedly aware of the lake's economic importance and international status, the federal government, the California and Nevada legislatures, local politicians and private businesses have poured $1.62 billion into the Lake Tahoe Basin since 1997 -- the year President Clinton established an interagency network to protect the area's health.
California, the federal government and private interests have contributed $636.2 million, $521.1 million and $299.6 million, respectively, according to a bill from Sen. Dianne Feinstein, the California Democrat.
Co-sponsored by the other three senators from California and Nevada, the "Lake Tahoe Restoration Act of 2013" would fund forest restoration efforts in the basin, fight invasive species (specifically singling out quagga mussels) and develop programs to reduce fire risk and more -- allocating $415 million over 10 years in total.
There are about 60,000 permanent residents in the Lake Tahoe Basin, Schladow said, and the lake and local ski resorts attract 2 million to 3 million visitors annually.
Threats to tourism
Starkly reliant on tourism revenue, Tahoe communities stand to lose a lot of revenue if invasive species make the lake unpleasant for tourists.
The North Lake Tahoe Resort Association, a local trade association, estimated in a 2009 report that aquatic invasive species -- including Asiatic clams, zebra and quagga mussels, and bloody red shrimp -- could cost the area $1.3 million in lost revenue if tourism visits dropped just 2 percent.
Local businesses would miss out on $3.2 million and $6.4 million if visits slowed by 5 percent and 10 percent, respectively, the report estimated. Projected five years ago, when invasive species were less established, these costs would probably be higher today.
The organization didn't focus on algae in its findings five years ago, but today, the blue-green gunk has grown into a gnawing concern.
"Climate change is tending to make our algae smaller," Schladow said. "They scatter light really effectively," deflecting light that otherwise might kill off algae.
Algae thrive in warmer temperatures and can grow aggressively when protected from ultraviolet rays by already-established algae clumps -- a phenomenon occurring along the lake's 70-mile shoreline as above-average temperatures create fertile circumstances for invasive plants.
No one wants to swim or play on a smelly, algae-infested beach, which Schladow said continues to develop.
The federal government owns 75 percent of the basin, which the Forest service oversees. The remaining 25 percent is privately owned and has been aggressively developed, Schladow said, which is environmentally "troublesome."
More erratic weather expected
The "built" environment is particularly concerning to Schladow and other scientists because it prevents precipitation from entering the ground naturally.
Rather, instead of penetrating the ground and helping the vegetation, it flows downhill across roads, parking lots and impermeable surfaces, carrying with it sediments that mar the lake's clarity and nutrients such as phosphorus and nitrogen, which feed algae and can generate damaging algal blooms.
Scientists expect the Sierra Nevada range to experience some of the most extreme climatic swings as the planet warms.
Jeff Brown -- director of the University of California, Berkeley's Central Sierra Field Research Station and manager of the Sagehen Experimental Forest, which was founded in 2005 as part of a web of local research stations -- has already seen those dramatic swings.
"The models that we see are bouncing all over the place," Brown said, explaining that temperatures are trending upward and will likely become more erratic.
Brown and his colleagues at Sagehen, which is about 20 miles northwest of Lake Tahoe and includes land within the Truckee River Watershed, an area north of Lake Tahoe, have chronicled the area's harshest winter and its most brutal summer on record just within the last four years.
The central region of the Sierra Nevada receives most of its water from winter snow, according to Brown, but the regional climate shifted to "rain-driven ecology" recently -- leading to "flashy" bouts of runoff and erosion.
Historically dependent on the snowpack's downhill trickle to seep into the ground -- a process Brown describes as a "tempered type of release" -- vegetation is suffering.
Harm from years of fire suppression
Aggravating the situation are two environmental factors.
Pines and conifers, the dominant tree species, contain a lot of resin, which creates slick coatings on the forest floor, speeding up runoff flow and accelerating erosion. Also, due to lengthy fire suppression, an accumulation of unburned debris on the forest floor, which is "very flammable" and becoming "thicker and thicker," blankets the ground, Brown said.
"Because fire has been pretty much excluded from the landscapes, we are also getting a buildup of what we call duff," Brown said. By suppressing fires over a long period of time, "you create a lot of impermeable surfaces," he said.
He added: "Now, the fuels are about as dry as they have been in history."
To Brown, implementing preventive measures to address the warming climate, rather than reactive cleanup efforts, is crucial.
"Everybody talks proactive, but everybody lives in the reactive world, and we need to shift that paradigm," he said.
The fires in the Western part of the country, according to Brown, have gotten hotter and more severe than ever before. "The fires we're seeing in the American West changed in 2001," he said. "Right now, we are in a third-year drought."
Clearcut and heavily logged in the 1800s -- left with built-up detritus that could have been eliminated with controlled burns -- the area's forests don't resemble those that stood before.
"Your perspective is not based on the history of the place," Brown said. "We don't know what a healthy forest looks like."
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