WHAT DO WE KNOW ABOUT IT?
Crater Lake covers the floor of a steep-sided
caldera formed 7,700 years ago during the violent volcanic eruption of Mount
Mazama followed by collapse of the mountaintop. The average lake level is 1,883
meters above sea level, but for periods of 5 to 10 years lake elevation can vary
as much as 5 meters. The lake has a surface area of 54 square kilometers and a
maximum depth of 594 meters, making it the seventh deepest lake in the world.
The scalloped edge of the caldera is about 8 kilometers in diameter, varying in
elevation from 159 to 586 meters above the lake surface. Essentially, the lake
rests in a “leaky pot,” losing water only through seepage and evaporation. The
lake’s surface elevation rises and drops relative to the amount of snow and rain
that falls on its surface.
FIGURE 1. Mean Secchi disk depths in August.
The lake is among the clearest in the world and
its waters appear deep blue in color. When sunlight penetrates into the lake,
the red and green portions of the light are preferentially absorbed by water and
suspended particles. The blue light, which penetrates more deeply, is eventually
scattered by water molecules and returns to the lake surface and our eyes. The
blueness of the water is greatest when the sun is bright and the particle
density is very low. A 20 centimeter black and white disk, known as a Secchi
disk, is used to measure the water clarity. In summer, the disk can be seen to a
depth of about 30.5 meters, but clarity readings range from under 20 meters to
over 40 meters. Changes in water clarity are related to the amount of algae (or
phytoplankton) suspended in the lake, as well as particles eroded from the lake
shoreline and carried by runoff from numerous small streamlets that enter the
lake. Changes in lake water clarity are a natural part of the lake system (Figure
Crater Lake seldom freezes in winter because the
great mass of water, although very cold, retains enough heat to keep the surface
free of ice. Only during exceptionally cold, windless periods does a thin layer
of clear ice cover the lake. This has only happened three times in the last 100
years. During winter, strong winds mix the upper lake waters down to a depth of
over 200 meters; however, the deep parts of the lake exchange with the surface
only periodically when cold, dense water sinks to the bottom. This process,
which appears to be focused along the edges of the lake, forces deep water to
“upwell” into the wind-mixed portion of the lake. This physical process occurs
annually, but its magnitude varies from year to year due to different weather
patterns. Upwelling water is important to the ecology of the lake because it
contains nutrients that are important for photosynthesis by algae. The nutrients
build up in the deep lake from the decomposition of organic particles sinking to
the lake floor; however, the overall concentrations of nutrients such as
nitrogen and phosphorus are very low in the lake which is why algal growth is
low in the first place. In summer, thermal stratification of the lake results in
a warm, less dense surface layer called the epilimnion that floats on top of the
deep, cool, dense layer called the hypolimnion (Figure 2). The
transitional zone is called the metalimnion.
The maximum surface temperatures in August are
generally near 18°C. The bottom of the lake has a year-round temperature of
about 3.6°C, which is warmed very slightly by the input of geothermal fluids.
These fluids also contribute dissolved salts to the lake, creating a much higher
salt content than would be expected if direct precipitation were the only source
The algae in the lake are diverse in species but
sparse in abundance. Over 150 species have been detected and algal abundance has
varied considerably through the years (Figure 3). In summer, when the
lake is thermally stratified, different algae species live in different layers
of the water column down to a depth of about 200 meters (Figure 4).
Because of the extreme clarity of the lake, the maximum amount of photosynthesis
is able to occur at 40-80 meters. Phytoplankton that are adapted to very low
light levels can produce a chlorophyll maximum at 120-140 meters (Figure 5).
The animal plankton (zooplankton) in Crater Lake
include nine rotifer species (microscopic multicellular animals 0.20-1.52
millimeters in length) and two crustacean species (0.45-3.12 millimeters in
length). The abundance of zooplankton varies greatly through time (Figure 6).
Some species are present every year, whereas others occur infrequently. During
summer when the lake is thermally stratified, different species live in
different portions of the water column down to a depth of about 200 meters.
Two species of fish inhabit Crater Lake. Both
were stocked into this naturally-fishless lake. These include kokanee salmon
(landlocked sockeye salmon) and rainbow trout. The latter inhabit the nearshore
area of the lake and feed on large-bodied terrestrial insects that fall onto the
lake surface, aquatic invertebrates living on the bottom of the lake, and
kokanee salmon. Kokanee feed on small-bodied terrestrial insects and bottom
fauna, and crustacean zooplankton. Kokanee live in the open waters of the lake
to a depth of about 150 meters and, to a lesser extent, along the shoreline of
the lake. Fish may have altered the species composition of the lake and have
assumed the role of top predator over zooplankton. This may have had cascading
effects throughout the food web.
WHY CONTINUE TO STUDY CRATER LAKE?
Crater lake is a world-class laboratory for
studying lakes because of its pristine condition. Because it is preserved in a
national park it is expected that there will be minimal future onsite impacts
from human activities. The lake provides scientists and park managers with a
gauge for assessing changing environmental conditions external to the Park.
Long-term monitoring of Crater Lake has been used to develop a baseline of
information about the natural dynamics and complexity of the lake. This baseline
will serve as a reference when studying the impacts of global climate change and
human activities, such as agriculture and urban growth, on other lakes.
Scientists working with the U.S. Geological Survey, the National Park Service,
and Oregon State University have systematically studied Crater Lake for the last
two decades. Long-term monitoring of this lake is a priority of Crater Lake
National Park and will continue far into the future.
The long-term lake program involves studying many
components of Crater Lake. Currently, the relationships between many of these
model components are being investigated using mathematical models. Based on this
research, new studies will be undertaken to refine our understanding of how
these components interact. Once the models are completed, researchers will have
a better understanding of how the lake “works” and how the lake may respond to
future environmental changes.
THIS DOCUMENT WAS CREATED IN COLLABORATION WITH OREGON STATE
UNIVERSITY AND CRATER LAKE NATIONAL PARK.