The decrease in clarity is being attributed to, according to a UC Davis report, a one-two punch of the end of a five-year drought followed by a winter of record-high precipitation. More sediment washed into the lake in 2017 than in the previous five years combined.
For the past half century the University of California, Davis Tahoe Environmental Research Center has conducted continuous monitoring of Lake Tahoe as part of the center’s measurement programs, while also presenting current research on emerging issues.
The center recently released its annual State of the Lake Report for 2017, which showed a dramatic decrease in Tahoe’s clarity as well as record high average surface temperatures.
During 2017, a year marked by extreme weather, the average annual clarity dropped to a record low average depth of 59.7 feet — a 9.5-foot decrease from 2016 and the lowest value ever recorded at Lake Tahoe. Clarity is measured using a 10-inch opaque white disk, called a Secchi disk, which is lowered into the lake until it can no longer be seen by the observer.
The decrease in clarity is being attributed to, according to the report, a one-two punch of the end of a five-year drought followed by a winter of record-high precipitation. More sediment washed into the lake in 2017 than in the previous five years combined.
“While 2017 may be viewed as an anomalous year, it has reinforced the fact that progress toward environmental restoration of Lake Tahoe will be punctuated by extreme years in future decades,” Geoffrey Schladow, director of the UC Davis Tahoe Environmental Research Center, said in a statement. “Monitoring these extreme years and applying the lessons learned will be critical to ensuring that the lake and its watershed has the resilience needed to thrive under future conditions.”
Clarity readings from the first part of 2018 have shown levels are back in the normal range, according to the report, suggesting 2017 was an outlier. Federal and state regulators have targeted 97.4 feet as the lake’s clarity restoration goal.
“The plan for clarity is literally a decade’s long plan, so it’s going to be 30 or 40 years until we get the clarity where we want it. It was predicted there would be years like this when the clarity would be worse than it was when we began, just as there are going to be years when it’s suddenly way better,” said Schladow.
“I have to keep warning people every year it can fluctuate wildly, so we’re tracking that long-term trend and that trend is very different than the trend was when all this started 15 years ago. If we continued the trend it was on (15 years ago), the clarity would be a lot worse, so things have improved.”
The report also stressed extreme climatic and hydrologic events may become more common in the future and that current monitoring reports need to be reviewed and upgraded.
“What I’ve tried to argue is this was a number of things coinciding, and shouldn’t be taken as representative of where the lake is going,” said Schladow. “The fact that 2018 is back to, let’s call it normal, is good. That is encouraging, but we could get these wet winters again and I think the lake is going to keep warming.”
Warmest lake temps on record
In all but one month of 2017 air temperatures were higher than average for 1910-2017, while Lake Tahoe’s average surface water temperature for the year was 53.0 degrees compared to 50.3 degrees in 1968.
In July 2017, the water temps reached a record of 68.4 degrees, which was 6.1 degrees higher than the previous year.
The center also applied downscaled future climate projections, which suggest air temperatures will rise in the Tahoe Basin by 7 to 9 degrees between now and the end of the century. Researchers also suggest the watershed will dry substantially, particularly in the north and east sides of the basin, which will add to forest stress and wildfire risk.
Another factor in warming water temperatures are Tahoe’s invasive species. Millions have been spent on the removal and prevention of invasive aquatic species in the lake, but warmer temperatures can make it easier for these species to take hold, while native organisms could find it more difficult to thrive in the changing ecosystem.
“As it gets warmer, some (native species) may get stressed, and so it makes them more prone to disease,” Schladow said. “It also creates new openings for species that couldn’t have existed in Tahoe 20 years ago, but now it’s started opening up a niche for them. It’s making the possibility of new invasive species surviving higher than it would have been in the past.”
Ridding the lake of one invasive species, the Mysis shrimp, has resulted in the return of the native zooplankton Daphnia, which has coincided with several feet of clarity improvement in both summer and winter, the report says. A pilot project in Emerald Bay is testing whether Mysis numbers can be reduced sufficiently to sustain such a clarity improvement.
With the summer of 2018 winding down, Schladow said measurements from this year have shown similar surface water temperatures as last year.
“They’re just as warm this year as they were last year,” he said. “Last year’s warming wasn’t a one-off event.”
While Tahoe’s rising temperatures are cause for concern, Schladow said at least it has raised awareness of the issue along the shores of the lake and the greater area.
“We’ve had this warming that really got everybody’s attention, and people are starting to be sort of agents in the basin and starting to reevaluate their programs,” he said. “Are they doing everything they can be doing and are all their assumptions still appropriate? I think it’s really healthy that everybody is going through that process right now.”
lack of deep water mixing
The lake’s cycle of deep water mixing, which brings oxygen to deep waters and releases large amounts of heat, hasn’t happened in six years, according to the report.
In winter, surface waters cool and sink, which brings oxygen deeper into the lake, promoting aquatic life throughout the water column.
“We’re at the point where it’s the longest it’s been since it has mixed deeply,” said Schladow. “The big concern is if it doesn’t mix at the bottom of the lake, the oxygen just continues to get consumed, and so at what point will that happen?”
“How many years of non-mixing does it take to lose that oxygen? Its something we’ve been working on for a couple of years now, and we still don’t have the answer.”
Compounding the issue in 2017 was a record level of nitrogen and phosphorous loads entering the lake, the report says, due to high streamflow.
“These two nutrients are actually starting to increase in the lake, and that’s worrisome because this issue of deep mixing and the lake losing oxygen,” said Schladow. “The best way to work against that happening is to slow the rate at which oxygen is being used in the lake. We can’t stop the rate of warming, but what we’re trying to avoid is losing all of the oxygen.
“What causes that loss is algae growing and eventually dying and sinking to the bottom. The less nutrients we add to the lake, the less algae there will be and the less decomposition … if we cut back on nutrients there will be less algae.”
When algae and other organisms decompose, as Schladow explained, oxygen is used as part of the chemical reaction.
In order for the lake to go through a deep mixing, the area needs to go through several months of very cold temperatures.
“What we need is a really cold winter,” Schladow said. “The winter season is when that happens. The winter is actually getting shorter and summers are getting longer. With a shorter winter period, there’s just not enough time for it to mix to the bottom.”
Tree mortality
Surveys of the forest from 2009-17 showed increased tree mortality throughout the Tahoe Basin, according to the report, of which, large stands of sugar pine along the north shore were particularly affected by mountain pine beetle.
The center’s forest and conservation biology lab collected seeds from diverse sugar pine trees that survived the drought and attacks from pine beetles, and are therefore likely more resilient to future droughts and insect assaults. Those seeds will yield 10,000 trees to be available to revegetate impacted stands.
