© 2001 by Leo Poppoff

July 12, 2001

Basin Watch

by

Leo Poppoff

THE BIG PICTURE

We spend a lot of time delving into the details of the Tahoe Basin's
environmental issues. That's fair enough, but we can lose sight of the
big picture when we burrow into the minutiae of things. So, once in a
while, it's good to step back and look at the whole enchilada.

For several decades, scientists with the UC-Davis Tahoe Research Group
(TRG) have been looking into aspects of Lake Tahoe's decline. And in a
recent publication, they've put it together in a very readable article.
"Lake Tahoe: Diagnosis and rehabilitation of a large mountain lake" is
the article. It's written by Alan Jassby, Charles Goldman, John Reuter,
Robert Richards, and Alan Heyvaert.

We'll hit some highlights, but the entire article is well worth reading.
It's published in "The Great Lakes of the World (GLOW): Food-web, health
and integrity," a compilation edited by M. Munawar and R.E. Hecky -- an
Ecovision World Monograph, published by Backhuys Publishers in Leiden,
The Netherlands.

Though Lake Tahoe is very large, clear, and relatively sterile, it's
quite vulnerable. The watershed, composed of forested granitic and
volcanic soils and drained by 63 small streams, doesn't supply much
nutrient to the lake, naturally. The amounts of additional nutrients
released by lumbering and urbanization might not be large compared to
pollution flows found elsewhere. But they're large compared to natural
conditions, so they can have significant impacts. The only drainage is
through the Truckee River, so flushing nutrients from the lake is very
slow (estimated at 650 years).

Observations of Tahoe's clarity were made more than a century ago by
Professor John LeConte, and later by a fishery expert, Chancey Juday,
and a few other scientists. But, the first regular monitoring was
started by Professor Charles Goldman in 1967. He was also the first to
measure the growth rate (primary productivity) of algae; its continued
measurement provides the most sensitive index of the lake's
deterioration.

In the beginning, Tahoe's algae were stimulated by nitrogen nutrients.
But by the 1980s, more nitrogen was falling into the lake than its algae
could use, so algae began to respond to phosphorus nutrients instead.
During this period, according to the TRG article, the kinds of algae
growing in the lake also changed. Before nutrient inputs increased,
Tahoe's algae were mostly a type called centric, with round silica
skeletons. Currently, the dominant algae are pennate, with elongated
skeletons.

The populations of some types of algae increased while nitrogen was the
key nutrient, then disappeared when phosphorus became the key stimulant.
Other species increased as the lake responded to phosphorus.

By fooling around with fish populations, fishery experts managed to mess
up the lives of Tahoe's native aquatic critters. The introduction of
Mackinaw trout helped to extinguish the native cutthroat trout. The
planting of opossum shrimp for Mackinaw food doomed a species of
zooplankton called cladocerans.

Clarity measurements are made by watching a white disk, called a Secchi
disk, disappear as it's lowered into the water. That depth, on average,
has decreased from 31.22 meters in 1968 to 20.53 meters in 2000. But, is
clarity directly related to concentrations of algae, as we had earlier
supposed?

More recent work, cited in the article, shows that fine mineral
particles also affect clarity. And, clarity measurements with Secchi
disks don't take all the algal growth into account. Even when lake
clarity is greatest, the disk disappears at depths that are relatively
shallow compared to the depth of a significant layer of algae. The
article notes that Secchi depth measurements made in late winter might
be more representative of average particle concentration in the lake.
That's when the lake mixes vertically and it's before summer
stratification sets in.

As the authors explain, atmospheric sources of nutrients have become
very important in recent decades. Nitrogen deposition has doubled, and
atmospheric fallout is now the major source of nitrogen nutrients.
Phosphorus from atmospheric sources has also become important, though
not to the extent that nitrogen has.

Nutrients find their way to the bottom of the lake, where they're sealed
in sediments. By analyzing lake bottom sediments, scientists find that
the lake can actually recover from heavy nutrient influxes in a much
shorter time than it would take to flush them down the Truckee River.
Apparently, Lake Tahoe recovered within a few decades from nutrient and
sediment influxes caused by the Comstock logging operations. And that
impact was similar to current conditions.

If phosphorus stimulation of algae is the major contributor to Tahoe's
clarity decline, TRG researchers believe that the lake could recover
within a couple of decades -- if the nutrient is controlled. On the
other hand, if fine particles are the major cause of declining clarity,
the lake's recovery could be even faster. That's good news, but
controlling nutrients and fine particles is a challenge that we have yet
to meet.

There's more in the TRG article. But, we're out of room.