Crater Lake Clarity: it doesn't get any better than this
By Scott F. Girdner, Crater Lake
Vol. 22, No. 1, Fall 2003
The remarkable color and
clarity of Crater Lake has dazzled visitors for generations.
When the park commemorated its 100th anniversary in summer
2002, a new generation of visitors from around the world
celebrated the wonders of Crater Lake for the first time.
Their first view was one of surprise and amazement at the
lake’s beauty and majesty. Previous generations had the same
reaction when initially seeing the lake. Geologist Clarence
Dutton wrote in 1886, “As the visitor reaches the brink of
the cliff, he suddenly sees below him an expanse of
ultramarine blue, of a richness and intensity which he has
never seen before, and will not be likely to see again.”
Likewise, Author Zane Grey wrote in 1919, “I expected
something remarkable, but was not prepared for a scene of
such wonder and beauty....Nowhere else had I ever seen such
a shade of blue.... How exquisite, rare, unreal!”
Reports of early explorations of Crater Lake
indicated the remarkable color and clarity resulted from a
deficiency of suspended particles in the water, and unlike
most lakes, few phytoplankton (microscopic plants) in the
top 50 feet (15 m). Recent studies at Crater Lake by
Emmanuel Boss (Oregon State University) and Bruce Hargreaves
(Lehigh University), along with information collected
through the park’s lake monitoring program (fig. 1), have
shown just how amazing Crater Lake’s clarity is. The waters
of Crater Lake are actually resetting the standards for the
optical properties of “pure water” and may have the highest
clarity of any lake in the world.
At the same time, this information
underscores the delicate balance in the Crater Lake
ecosystem and the potential for future pollution to degrade
the world-renowned clarity of the lake.
Joseph Diller attempted the first
measurement of the lake’s clarity in 1896 by lowering a
white 10-inch-diameter dinner plate until it disappeared.
Diller and other early explorers must have been amazed that
the dinner plate was still visible at nearly 100 feet (30 m)
deep! Since that time, investigators have taken hundreds of
readings using a standard black and white Secchi disk to
measure water clarity; the deepest reading, taken in June
1997, was a staggering 139 feet (42 m).
Why is Crater Lake so clear?
To appreciate the lake’s remarkable color
and clarity, one must understand the relationship between
ultraviolet (UV) light and the presence of dissolved organic
matter in water. In most lake and ocean systems, dissolved
organic matter, resulting from plants and animals within the
watershed, limits the penetration of UV light. Only a very
small amount of organic matter dissolved in water is needed
to absorb the UV rays. Because the UV light is absorbed near
the lake surface, aquatic organisms within the lake are
mostly protected from the harmful UV rays. In Crater Lake,
however, the natural concentration of organic matter is
minimal, low enough to allow significant UV light
penetration into the lake.
Crater Lake has very little dissolved
organic material near the surface compared to even the
clearest ocean environments: less than half the absorption
of the clear waters of the Gulf of Aqaba, the Red Sea, and
the Tongue of the Ocean in the Bahamas. The level of organic
material in Crater Lake is so low that UV light penetrates
as deep as 320 feet (100 m). A recent analysis of published
UV measurements (Hargreaves 2003) indicates that in some
months the surface waters of Crater Lake have greater UV
transparency than any other known lake or ocean in the
world! (Lake Vanda, in Antarctica, was a close second.) The
results of these studies along with data collected during
the park’s monitoring program indicate the top 50 feet (15
m) of Crater Lake has greater UV transparency than the
values published for pure water.
The effects of UV penetration on the world’s
lakes and oceans are significant because of recent changes
in Earth’s ozone layer, the impact of climate change on
dissolved organic matter, and the corresponding fluctuations
in underwater UV levels. Therefore, this research at Crater
Lake is important. The work of Hargreaves and Boss will
contribute to establishing new UV absorption limits for pure
water and will also help in determining the ecological
factors that control the underwater penetration of UV light
in lake and ocean ecosystems.
Figure 1. Scientists aboard the park research vessel
Neuston recover a sediment trap from Crater Lake.
Studies such as this have revealed extremely low
concentrations of dissolved organic compounds,
contributing to lake clarity.
We do not fully understand the direct
effects of UV light penetration on the ecology of Crater
Lake. Investigators speculate that a combination of
limited nutrients and the deep penetration of harmful UV
light influence the virtual absence of phytoplankton
during most of the year in the upper 50 feet (15 m) of
Crater Lake. Because of the limited phytoplankton in
near-surface waters, very few particles exist to
interfere with the penetration of visible light, and
consequently the clarity of Crater Lake is still as
clear as it was 100 years ago. The near absence of
phytoplankton in surface waters also helps to explain
the lake’s beautiful color, which comes from rays of
sunlight that are not absorbed by particles but rather
are scattered upwards into the eyes of generations of
visitors. Further studies on the effects of UV light
penetration and the Limnological Monitoring Program at
Crater Lake will continue to assess the long-term trends
to ensure preservation of this irreplaceable gem.
Hargreaves, B. R. 2003.Water column
optics and penetration of UVR. Pages 59–105 in E.W.
Helbling and H. E. Zagarese, editors. UV effects in
aquatic organisms and ecosystems. Comprehensive series
in photochemical and photobiological sciences. Royal
Society of Chemistry, Cambridge, United Kingdom.
About the author
Scott F. Girdner is an Aquatic Biologist
with Crater Lake National Park. He can be reached at
About the investigators
Bruce Hargreaves is an Associate
Professor in the Earth and Environmental Sciences
Department at Lehigh University, Pennsylvania. He can be
contacted at 610-758-3683 or firstname.lastname@example.org.
Emmanuel Boss is an Assistant Professor
in the School of Marine Sciences at the University of
Maine. He can be contacted at 207-581-4378.