2909 17-Mile Drive © 2000 by Leo Poppoff

Pebble Beach, CA 93953

\(831\) 375-3302

September 28, 2000

Basin Watch

by

Leo Poppoff

ULTRAVIOLET LIGHT IN LAKE TAHOE

Five years ago, I was lucky enough to be cruising Lake Tahoe aboard the
John LeConte, the Tahoe Research Group's research vessel. My fellow
passengers were two scientists from Finland. Pirjo Huovinen and Markus
Soimasuo, with the Department of Biological and Environmental Science at
the University of Jyvaskyla, were studying the effects of ultraviolet
light (UV) on aquatic life in Lake Tahoe.

Since my research, once upon a time, was about UV, Freon, and
stratospheric ozone, I was intrigued. And, as a plus, I got to steer the
boat while Bob Richards, the skipper, helped the researchers prepare
their samples. Huovinen and Soimasuo went on to study UV in other lakes
and oceans around the world, and I often wondered what they learned.
Well, this year, I found out. Pirjo Huovinen published "Ultraviolet
Radiation in Aquatic Environments," and thoughtfully sent me a copy. It
includes the Lake Tahoe study.

A couple of years later, U.S. Geological Survey scientists discovered a
class of chemical compounds called polycyclic aromatic hydrocarbons
(PAHs) floating around in the lake. Then, J.T. Oris, of Miami
University, learned that some of those PAHs in Lake Tahoe became toxic
when they were irradiated with UV-A. The longer wavelength UV-A
radiation is generally thought to be nonhazardous. Yet, those
photo-toxic chemicals could be fatal to small aquatic critters. So, UV
studies by the scientists from Finland are even more pertinent to Tahoe.

While they were here, Huovinen and Soimasuo investigated the effect of
UV on our algae. They were particularly interested in a narrow band of
the spectrum known as UV-B, which is biologically harmful. Shorter
wavelengths of UV (UV-C) are more hazardous, but fortunately, they never
reach the ground. Some UV-B, however, does reach the ground. In
addition, it's predicted that UV-B radiation at ground level might
increase as stratospheric ozone is depleted.

Water in lakes and streams generally contain more organic material than
is found in ocean water. Consequently, UV-B penetrates much deeper into
ocean water than into fresh water. Generally, fresh water penetration
(to 90 percent attenuation) is less than a meter, while in the Sargasso
Sea and near Antarctica, penetration has been measured at 20 to 25
meters.

Because of Tahoe's clear water, it was believed that UV-B penetration
would rival that of the ocean. Huovinen and Soimasuo calculated that 90
percent of UV-B radiation is attenuated at depths of 10 meters in Lake
Tahoe's water. UV-A was calculated to penetrate as deep as 30 meters.

Samples of Lake Tahoe water (including algae) were taken at depths from
the surface to 50 meters. They were put in glass flasks, along with
carbon-14, and lowered back down to incubate for a while. One Pyrex
flask at each depth was covered with opaque plastic as a dark control.
Another Pyrex flask was covered with Mylar film, which together with
Pyrex, filters UV-B out of the sunlight reaching the flask. The third
flask was quartz glass, which passes UV-B; so algae were exposed to all
the sunlight (including UV-B) that penetrated to that depth.

The object was to see if UV-B inhibited the production of algae. The
intake of carbon-14 by algae (which can be measured) is related to algal
productivity in the flasks. In their June experiment, Huovinen and
Soimasuo found that algal productivity was inhibited to some extent down
to about 20 meters. In their two July experiments, they found
productivity inhibition down to 50 meters. However, the most significant
reductions in algal productivity occurred in the top two meters in the
lake.

The researchers noted that productivity is inhibited down to some 25
meters in the ocean, while in Lake Tahoe, the magnitude of UV-B effects
on algae dropped off quickly below two meters. Comparing their results
with measurements on lakes elsewhere led Huovinen to conclude that the
algae in Lake Tahoe had acclimated to UV-B exposure.

In later work at Bodega Bay, Huovinen and Soimasuo found that UV-B could
also inhibit the development of kelp, which are giant algae. In another
study, they found that retene, one of the PAHs, became toxic to daphnia,
tiny aquatic critters, when irradiated with UV-B. The experiment wasn't
done at Tahoe, but it suggests that both UV-A and UV-B can
photosensitize some of the PAHs in Lake Tahoe, perhaps making our
potential problem with PAHs worse.

The objective of research done by Huovinen and Soimasuo is to determine
what potential increases in UV-B radiation, caused by stratospheric
ozone depletion, might do to aquatic life. What they found is that
aquatic critters might be at risk, at Lake Tahoe and elsewhere, unless
they learn to live at greater depths.