2909 17-Mile Drive © 1999 by Leo Poppoff

Pebble Beach, CA 93953

\(831\) 375-3302

December 9, 1999

Basin Watch

by

Leo Poppoff

THE LAKE'S HEALTH REPORT

Every few years, TRPA issues an annual report on water quality in the
Tahoe basin. Note that this is a report on the health of the lake. It's
not a report on the potability of lake water. The levels of pollution by
nitrates, phosphorous and iron that affect the "blueness" of the lake
and its ecology are far below drinking water standards. Most
municipalities in the world would love to have drinking water sources as
pure as Lake Tahoe.

This year's Annual Water Quality Report was circulated a couple of
months ago. It covers stream and lake measurement up to 1996.
Unfortunately, stream data for the last three years, the wettest in a
long time, were not included.

The report is a gold mine of Tahoe water quality history. Thirty-two
years of lake measurements and sixteen years of stream measurements are
compiled. Over the years, support for the basin's water measurement
program has waxed and waned. Fortunately, the program seems to be in
very good shape currently.

The Lake Tahoe Interagency Monitoring Program now involves some nine
agencies. They all do a part, but most measurements in this report were
made by scientists from the U.S. Geological Survey and the U.C. Davis
Tahoe Research Group (TRG).

Water clarity is measured by watching a white, dinner-plate sized disk
as it is lowered in the water. The depth that it disappears is recorded,
along with the depth at which it can be seen again as it is raised. The
average of those two measurements is called the Secchi depth. This
technique was invented by Professor P.A. Secchi for his research on the
Adriatic Sea. It's deceptively simple, but very repeatable, and has been
a standard technique for oceanographers and limnologists for 133 years.

In 1968, lake water clarity measurements showed that the Secchi Disk
could be seen at depths of 31 meters (a little more than 102 feet). In
1989, The disk couldn't be seen deeper than 20 meters (about 66 feet).
Clarity varies from year to year, depending on algal growth and
suspended sediment in the lake. For example, clarity decreased by some
six meters between 1968 and 1973. Yet, from 1973 to 1980, there was very
little change. Clarity dropped from 1980 to 1985 and then was relatively
unchanged for the next five years. If we had measurements from just one
of those periods, we might have concluded that there is no change in
clarity or that clarity is decreasing much faster than it actually is.
Without the long term record we now have, we really wouldn't know how
the lake is doing.

Tahoe's water clarity pretty much follows algal productivity --- but
with opposite trends. As algal productivity increases, clarity
decreases. Algal productivity is measured by noting how fast the lake's
algae sop up carbon. That rate is controlled by the supply of nutrients,
mainly nitrogen, phosphorous and iron, available to stimulate growth.

The water quality report shows that algal productivity has almost
quadrupled since 1968. Algal productivity also varies from year to year,
depending upon how much nutrient gets to the layer in the lake where
algae grow. There are several sources of those nutrients. Much of the
nitrogen is carried by winds from the Central Valley. Most of the
phosphorous and iron wash in from the watershed.

Through various routes, nutrients settle to the depths of the lake ---
far below the layers where algae grow. But every year, the lake mixes
vertically, bringing some of those nutrients back up to where algae can
use them. The depth of mixing varies from year to year, depending upon
winter weather conditions. The amount of nutrient washing in from the
hillsides also depends on weather, as does the transport of pollutants
from the Central Valley.

Every few years, the lake mixes clear to the bottom. The largest algal
blooms on record occur then, which indicates that deep mixing brings
lots of nutrients back to the surface. The degree of mixing seems to
control algal productivity more than any other event.

When you consider all those variables, it's no surprise that algal
growth and clarity vary from time to time. And it emphasizes the need
for long term measurements to be sure of trends. With new, aggressive,
erosion control programs on the horizon, these records will be
invaluable for determining progress in stemming the decline in water
clarity.

In a table showing attainment or non-attainment of TRPA's Environmental
Thresholds, lake water clarity is listed as being in attainment of an
interim target. Considering that clarity has steadily declined over the
past three decades, a finding of interim attainment is puzzling. Maybe
this is one of the thresholds that needs to be reconsidered. More on the
water quality report next time.