2909 17-Mile Drive © 2000 by Leo Poppoff

Pebble Beach, CA 93953

\(831\) 375-3302

October 19, 2000

Basin Watch

by

Leo Poppoff

WHAT'S CAUSING DECREASED CLARITY IN LAKE TAHOE?

As we've heard and read many, many times, the clarity of Lake Tahoe's
waters is steadily diminishing. A current research question is: What's
causing it? A couple of years ago, I would have written that it's
increasing populations of algae. And that's still a very important
factor, but researchers are learning that it's not the whole story.

Tahoe Research Group (TRG) scientists are looking more closely at what,
in Tahoe's waters, absorbs and scatters sunlight -- and affects clarity.
According to a paper by Alan Jassby in TRG's recent progress report,
water clarity in Lake Tahoe changes with the season. Lowest clarity, as
measured with the Secchi disk, occurs in June and again in December. The
Secchi disk is a white, dinner-plate size disk that's lowered into the
water until it disappears.

Jassby explains that "the June low is due to the accumulation of mineral
particles carried in by the melting snow pack; a spring increase of
phytoplankton also contributes." Generally, he adds, "the larger the
snowpack, the bigger the decline in clarity." The December low is caused
by winter mixing that brings nutrients, algae and mineral particles from
lower depths.

According to Jassby, the December low didn't exist when monitoring
started at Lake Tahoe in 1967. The low has become evident and stronger
as algal growth and sediment flows have increased. He points out that
it's important to understand more precisely what constituents are
causing the lake's clarity loss. "It will be of no help," Jassby writes,
"to control 99% of erosion if the microscopic particles most responsible
for the clarity decline are still entering the lake."

Jennifer Coker and Geoffrey Schladow have been investigating the nature
of particles responsible for clarity loss. They found that most of the
particles floating in the lake were smaller than two microns. A micron
is one-millionth of a meter.

Ted Swift is determining the absorption and scattering caused by these
tiny particles. He points out that a two-micron particle settles (in
still water) about one-foot per day. That means it would take some three
years to settle to Tahoe's average depth (1000 feet). Smaller particles
will float near the surface, and impact clarity, much longer.

According to Coker and Schladow, the ratio of organic particles (algae)
to mineral particles (silt) varies during the year. Early in the year,
numbers of algae and mineral particles are about the same. In succeeding
months, they find that the ratio of algae to mineral particles increases
to about two-to-one in June. This, Coker and Schladow believe, is due to
warmer temperatures and influx of nutrients during the snowmelt.

The ratio then decreases till early October, and increases again,
peaking in November, then falls again. This is consistent with the
deeper winter mixing that brings algae and nutrients up from the depths.
These results are from a station offshore of Sunnyside. Measurements at
the mid-lake station, south of Stateline, show much more variance
throughout the year. Similar ratios were found there in December. Yet in
June, it appears that the ratio reverses with mineral particles becoming
dominant. Also, particle concentrations were greater at the mid-lake
location.

Here's how Ted Swift and his associates describe the optical effects of
particles and algae in the lake. Pure water absorbs light in the red
portion of the spectrum, and scatters very little light. The result is a
bright blue color. Mineral particles scatter more blue light than red.
So sediment laden water appears reddish-brown.

Algal particles scatter some light, but mostly they strongly absorb red
and blue light. This is because algae grow by absorbing light and carbon
dioxide and producing green chlorophyll. As you might expect,
chlorophyll makes water appear green.

Swift pointed out that light penetrates to 105 meters, and possibly
deeper, so algae can and do grow at great depths in the lake. He also
explains that tiny mineral particles settle slowly, and some are mixed
back up in winter. So, a fraction of particles washed into the lake each
year is still around when a fresh batch moves in during the next
snowmelt. Over the years, then, the concentrations of mineral particles
accumulate and increase.

Swift and his colleagues collected particles from the lake at several
depths. They then measured the absorption and scattering characteristics
of these particles. With this information, they are developing a model
of the optical effects of particles in the lake. Preliminary results
show that calculations of scattering and absorption with depth correlate
remarkably well with Secchi disk measurements.

Swift, et al, conclude that "inorganic (sediment) particles contribute
disproportionately to clarity loss due to their higher scattering
efficiency and slow settling rate." This is a new, and fascinating,
wrinkle in knowledge about the causes of Tahoe's clarity loss. And, as
Jassby pointed out, it needs to be considered seriously as new erosion
control techniques are developed.