Number range CHAPTER© 2004 by Leo Poppoff

June 24, 2004

Basin Watch

by

Leo Poppoff

WHAT ABOUT THAT ICKY STUFF GROWING ON THE ROCKS?

Twenty years ago, Tahoe Research Group (TRG) scientists surveyed the
periphyton growing along the lakeshore. Periphyton is the brownish,
greenish, whitish, slimy stuff that grows on rocks at the lake's edge.
It's a kind of algal growth, stimulated by nutrients. Instead of
floating out on the lake, as other algae (phytoplankton) do, periphyton
anchors itself to rocks, and other things, and grows in colonies of
filaments.

Sunlight and carbon dioxide work together to produce chlorophyl, the
green stuff in plants. Nutrients stimulate their growth. So, periphyton,
which is a plant, grows in the shallows of the lake where it gets plenty
of sunshine. And, it grows best where it gets a good supply of
nutrients.

Most of us don't go out on the lake. Even if we did, we wouldn't notice
the loss of clarity that concerns us. But, we can certainly see that
slimy stuff growing on the rocks -- a very visible indicator that too
much nutrient is getting into the lake.

TRG scientists identify two zones along the lake's edge where periphyton
likes to grow. The first, and most evident, is the area between high and
low lake level -- the splash zone. Monitoring programs pay special
attention to this zone. The other zone lies below the lake's low water
mark and reaches depths as low as 600 feet, according to TRG's 1986
report.

Researchers use a special tool to sample periphyton. It's placed over a
patch of periphyton which is scraped off and captured in a syringe.
Samples are kept wet and cool until they're analyzed at a laboratory.
Several methods were tried, but determining the mass of chlorophyl in a
sample has proven to be the most appropriate index for monitoring at
Tahoe.

Periphyton was monitored from 1982 to 1985. TRG scientists did an
intensive study of its seasonal and spatial distribution, nutrients, and
solar radiation. They also checked water flow and seepage near the algal
colony. Samples from several depths were analyzed, and the degree of
development along the adjacent shoreline was noted.

The greatest growth of periphyton was found along shorelines adjacent to

commercial areas, subdivisions and golf courses. Obviously, lots of
nutrients wash or seep into the lake in those areas. For example, the
average annual concentration of periphyton adjacent to Dollar Point was
7 times as great as at undisturbed Sand and Deadman Points.

Periphyton growth on shoreline rocks appeared to be a good indicator of
the amount of nutrient flowing to the lake, and of the degree of
adjacent watershed disturbance. So, researchers recommended that
monitoring be continued. But, funds were scarce, and the idea was
shelved.

Now, two decades later, California's Lahontan Water Quality Control
Board commissioned TRG to do another survey (from 2000 to 2003) to see
if, and how, things might have changed. John Reuter, Scott Hackley,
Brant Allen, and Debbie Hunter repeated the monitoring of periphyton at
the same locations, and did the same kinds of analyses. However, they
added one more monitoring location, Tahoe City. How do current
measurements compare with two decades ago? Are things getting better --
or worse?

Researchers learned that periphyton colonies were still thick near
developed areas and much thinner near undeveloped areas. They concluded
that "the relative relationships between the sampling locations appeared
generally consistent over the 20-year period. Locations that were low in
1982-85 were generally low in 2000-03 and sites with elevated chlorophyl
in 1982-85 also showed elevated chlorophyl in 2000-03."

Reuter and his colleagues also noted that "many of the sites in 2000-03
were comparable to 1982-85." You might want to conclude, then, that
conditions haven't worsened during the last 20 years. But, and this is
an important "but," 1982-85 was a relatively wet period and 2000-03 was
a relatively dry period. The flow in Ward Creek, for example, was 2.3
times greater in 1982-85 than in 2000-03. Since, periphyton colonies are
fed by nutrients flowing into the lake, you might imagine that if
2000-03 had been as wet as 1982-85, periphyton would have been a lot
thicker -- and the conclusion would be that things have, indeed, gotten
worse. Even though 2000-03 was a relatively dry period, the average
annual concentration of periphyton along the Tahoe City shore was quite
high -- the highest recorded during that period.

Periphyton growth is affected by a multitude of factors, such as lake
level, precipitation, sunshine, and nutrient availability. Since they
all vary with climatic conditions, it's really necessary to have
long-term records to determine trends.

The reason we have so much confidence in lake clarity measurements, for
example, is because we have a several-decade record that reveals a
long-term trend, even though year-to-year records do not. To really know
what's going on, a continuing monitoring program is needed. And, Reuter
points out, periphyton is an aesthetic problem as well as a scientific
interest. So both the public and scientists should be involved in
setting an environmental standard for periphyton.

Comments? Send them to <basinwatch@sbcglobal.net>