2909 17-Mile Drive © 2000 by Leo Poppoff

Pebble Beach, CA 93953

\(831\) 375-3302

November 9, 2000

Basin Watch

by

Leo Poppoff

LAKE TAHOE'S SHRIMP PROCESS A LOT OF NUTRIENTS

Tahoe's algae are famous for slurping up nutrients and reducing water
clarity. Yet, they also play an important role in the lake's aquatic
ecosystem. Floating algae (microscopic plants also known as
phytoplankton) are the bottom link of the lake's food chain.

Here's the simplest way to look at it. Phytoplankton are grazed by tiny
aquatic animals called zooplankton. In turn, zooplankton are gobbled up
by little shrimp. And shrimp are snacks for smaller fish, which in turn
nourish big lake trout (Mackinaw).

Still, things aren't that simple. Crawdads snack on algae, newspapers
and lake trout eggs. Some of the bigger fish also reach down the chain
and eat zooplankton or shrimp and probably top it off with a few
crawdads. So, it's more like a food web than a chain.

Nutrients cycle and recycle through the web -- and eventually end up on
the bottom of the lake. Nutrient recycling by kokanee and mysis relicta
was investigated by Jason Van Tassell, David Beauchamp, Gary Thiede, and
Christine Gemperle of Utah State University, and Robert Richards and
Charles Goldman of Tahoe Research Group.

Kokanee escaped to Lake Tahoe in 1944 from a north shore hatchery. Then,
hundreds of thousands of kokanee were planted in the lake to provide a
new sport fishing opportunity. Kokanee like the open lake and eat
zooplankton rather than smaller fish. They're a landlocked variety of
sockeye salmon and act as though Lake Tahoe is the ocean. After two to
four years in the lake, kokanee migrate up Taylor Creek, turn red, lay
and fertilize eggs, and become snacks for bald eagles.

Opossum shrimp (Mysis relicta) were brought to Lake Tahoe from the
Waterton Lakes in Glacier National Park in the mid-1960s. It was a
well-intentioned effort to help Tahoe's Mackinaw. Mysids, as they're
called by researchers, are cute little critters, about 3/4 inch long,
almost transparent, with big black eyes and a pair of long antennae.
They love the deep, dark, frigid waters of Lake Tahoe.

In other, shallower, lakes, mysids fatten kokanee, which in turn fatten
large Mackinaw. But, in Lake Tahoe, it didn't quite work that way.
Kokanee live in the upper part of the lake and feed during daylight.
Mysids can see in the dim light of Tahoe's depths, but they're sensitive
to sunlight. They spend daylight hours at the murky bottom of the lake
and swim to the surface to feed at night. So, kokanee and mysids seldom
cross paths.

As a result, both kokanee and mysids prey on Tahoe's zooplankton. The
pressure has been disastrous for some of Tahoe's tiny crustacean-like
zooplankton (copepods). Daphnia (water fleas) were essentially
extinguished and bosmina populations now rise and fall. Because copepods
eat phytoplankton, it's thought the disappearance of daphnia and bosmina
played a role in the growth of Tahoe's algal populations. Bringing
mysids to the lake has been called an environmental disaster.

Nitrogen and phosphorus nourish phytoplankton which are eaten by
zooplankton. Mysids come up to the surface at night and eat the
zooplankton. Then mysids dive to the bottom of the lake while digesting
their meals and excreting their waste. Along with waste, they excrete
nutrients.

The Utah State and TRG researchers determined how much of the copepod
population is eaten by kokanee, and how much is eaten by mysids. Then
they calculated how much nitrogen and phosphorous is removed and
recycled by each species. They found that nutrient cycling by kokanee
was not significant.

But, mysids were another matter. They're, by far, the largest consumers
of copepods. Mysids and copepods sort of regulate each other. After
mysids consume most of the copepods, and run low on food, mysid
populations diminish. With diminished mysid populations, copepod
populations increase. The cycle repeats over and over again.

One-half to one-third of the lake's annual loading of nutrients are
assimilated and sequestered by mysids. But much more is recycled. During
the study period, Utah State researchers found that the recycled amount
of nitrogen was equal to about two-and-one-half times the amount that
enters the lake each year. For phosphorous, the ratio was almost two.
This also shows that the lake contains much more nutrient than the
annual loading to the lake. Part of the mysid excretion might be
converted back into nutrient forms that algae can use -- but part of it
sinks to the bottom, taking nutrients out of circulation.

Mysids process tons and tons of nutrients. Their activity acts as a
pump, according to the researchers, moving nutrients from the surface to
the bottom of the lake. But it's not known how much of the lake's load
of nutrients is completely removed by mysids. Nutrients moving through
the food web seems to be an important process. It's not well understood,
but it does show how complex lake processes really are.