2909 17-Mile Drive © 2000 by Leo Poppoff

Pebble Beach, CA 93953

\(831\) 375-3302

December 21, 2000

Basin Watch

by

Leo Poppoff

IS A PESKY AQUATIC WEED INJECTING NUTRIENTS INTO LAKE TAHOE?

An immigrant from the East has been upsetting some folks at Tahoe
because it's relentlessly claiming territory around the lakeshore. Folks
would like to stop its spread. In fact, they'd like to send it into
exile. But it has great survival skills.

We're talking about a weed that thrives in the Tahoe Keys. It's known as
Eurasian milfoil or Myriophyllum spicatum. It has infested and defied
control measures in lakes all over the U.S.

Reports of aquatic weeds showing up along the Tahoe shoreline date back
to the 1950s, but Lars Anderson points out that without samples or more
definite identification, those sightings can't be confirmed to be the
pesky milfoil. Dr. Anderson is with the U.S. Department of Agriculture's
Exotic and Invasive Weed Research Lab at UC-Davis. He's been studying
Lake Tahoe's milfoil colonies for several years.

During the 1970s and 1980s, Tahoe Research Group researchers spotted
milfoil at several spots around the lake. And it was clogging the Tahoe
Keys waterways. How it got to the Keys is not clear.

Since many other waterways in California are infested with milfoil, it's
logical to suppose it was brought in with a boat that had been used
elsewhere. One unconfirmed story blames an aquarium owner who had the
weed in his tank and by accident -- or on purpose -- dumped water, with
the weed, into the Keys.

Either way, milfoil found a comfortable home in the Keys. Milfoil likes
deep sediment, full of nutrients, and protection from turbulent waters.
When the Keys were created, marsh vegetation was buried in the canals.
The vegetation subsequently decomposed and formed a rich sediment bed.
Milfoil flourished in that environment and slowly moved around the lake.

Milfoil probably migrated to other Tahoe marinas by hitchhiking on
boats. Yet, Anderson also finds milfoil fragments floating around the
lake, so it also might have migrated that way. Fragments are unavoidably
produced when milfoil is harvested from the Keys. Harvesting milfoil is
currently the only control used at Tahoe. Crayfish also harvest milfoil,
and their grazing also creates fragments. Additionally, crayfish help to
enrich the sediment bed.

So far, according to Anderson, milfoil is found at some 17 locations
around the lake. It's apt to be found wherever there are sediments and
protected waters. The most recent appearance is in the Sunnyside Marina.

In order to grow, aquatic plants extract nutrients from sediments.
Because milfoil doesn't have a waxy coating, its nutrients are easily
leached into surrounding water. So, nitrogen and phosphorus that had
been buried in the sediment are sucked up by milfoil and recycled into
the lake.

When milfoil dies and decomposes, it releases nutrients to the water and
carbon to the sediment, thus enriching its own habitat. When it grows
thickly, it shades the water and that makes it hard for native aquatic
plants to survive. So, it slowly takes over the area.

The question today is this: does milfoil's recycling of nutrients
contribute to Lake Tahoe's eutrophication? Anderson, along with
researchers at UC-Davis, conducted some experiments last year to get a
handle on that question.

One of their experiments looked at the transfer of tagged phosphate from
the roots of individual plants to the surrounding water. The plants were
grown in glass tubes with sealed compartments for the roots. Researchers
compared the results with those from similar investigations with a
native aquatic plant (Elodea canadensis).

Another experiment investigated milfoil and E. canadensis planted in
plastic containers. They were rooted in Tahoe Keys sediment and grew in
unfiltered lake water. Tagged carbon was used for these experiments to
see if algae bloomed more profusely in tubes with milfoil. In both
experiments, results were also compared with controls.

The scientists found that milfoil exuded some 0.45 percent of the tagged
phosphorus that had been added to the sealed root compartment. That was
nine times as much as exuded by the native plant. They also found that
algal growth was stimulated more in water surrounding milfoil plants
than in water surrounding native plants. More nitrate was also found.
Both plants stimulated more algal growth than the controls.

These kinds of studies had been done by others in lakes with much higher
nutrient concentrations and algal growth than Tahoe has. Anderson and
his associates showed that similar results are found in Lake Tahoe,
which is still almost sterile. But, are milfoil colonies serious
contributors to Lake Tahoe's algal growth problem? Scientists who work
on the algal growth problem at Tahoe don't believe that recycling by
milfoil is significant when compared with other nutrient sources.

However, milfoil is still a big pain to the folks at Tahoe Keys. Their
dilemma is how to get rid of the stuff without affecting the lake with
harmful chemicals or biological controls. Anderson and his group are
working on that problem.