Primary Contact: Gary Larson, USGS
Forest and Rangeland Ecosystem Science Center
Large,
deep lakes offer a unique resource for studying the interaction
of the atmosphere, land surface hydrology, and aquatic
ecosystems. heir large volume integrates the complexities of
short-term variability in local climate. Thus, large lakes
provide relatively manageable systems to both monitor and
interpret the effects of global climate change on land surface
processes. Crater Lake offers a pristine and relatively simple
system to detect these changes through long-term study. It also
provides a powerful natural laboratory to study the processes
that link climate physics to the operation of biogeochemical
cycles in aquatic ecosystems.
Crater Lake, the
deepest lake in the United States (590 m), is a
closed-basin caldera lake formed after the explosive
eruption of Mt. Mazama, 6950 years ago. It is the
center piece of Crater Lake National Park and located at
an elevation of 1882 meters in the Cascade mountains of
south-central Oregon.
The steep caldera
walls surrounding the lake result in a very small
watershed such that external flows of nutrients to the
lake are low and dominated by precipitation and dry
deposition from the atmosphere. These conditions
contribute to the lake's low nutrient levels and
exceptional clarity. There are no major inlet streams
and no surface outlet.
The surface elevation of Crater Lake responds quickly to
climate and weather fluctuations because the closed
caldera functions as a giant rain gage. Annual
fluctuations in elevation average about 0.5 m, but
long-term declines and recoveries in lake level have
been recorded since the late 1800's. The maximum drop
(4 m) coincided with the "Dust Bowl" in the Midwest
during the 1930's. Recent climate variations have
resulted in highs and lows that nearly match the
historical extremes.
Changes in climate also affect lake temperatures and
water circulation patterns. In spite of its great
depth, the lake mixes rapidly -- over a timescale of 3-5
years -- and recycled nitrogen carried upwards from the
deep lake represents the largest annual source of this
limiting nutrient.This project continues our long-term
study of the impact of climate on the physics,
hydrology, and biogeochemistry of Crater Lake.
Continuous observations of meteorological
conditions on the lake surface and caldera rim,
as well as detailed water column physical
measurements have been established over the past
decade.
We
will continue these activities, as the data they
produce provide the essential input to our
physical and hydrologic models of the lake. By
coupling these mixing rates to a simple model of
organic matter production, particle settling,
and nutrient regeneration, we have demonstrated
how lake physics controls the lake ecosystem in
a complex way.
Objectives
- Characterize and
model the relationships between vertical mixing and the
input of nitrogen from the deep lake into the euphotic
zone supporting algal production.
- Examine the degree to which the
lake’s edges cause an increase in vertical mixing,
support elevated primary production, and contribute
significant particulate organic matter to the deep
interior through focusing of settling particles.
- Extend and refine the existing data
set of climate forcing and the observed response of the
lake. This will provide the statistical basis needed to
detect long-term change. Better sampling of the widest
range of interannual variations also provides guidance
for specific process studies and a more robust
validation of our physical models of lake mixing.
- Extend our physical model to 3
dimensions in order to represent the observed complexity
of deep mixing that may be most sensitive to climate
change. Experiment with new regional climate models to
forecast changes in mixing, lake level, and
biogeochemical fluxes.
Methods
Our methods include a suite of
meteorological observations collected on a permanent
buoy moored in the middle of the lake, a parallel set of
observations on the Caldera rim, and a set of
observations on a new tower near Park Headquarters.
Physical data within the lake are derived from a
thermistor mooring with continuous records of
temperature taken at 10 minute intervals from 19 depths
in the lake. Regular vertical profiles of water
conductivity, temperature, light transmission, and
chlorophyll fluorescence are taken from the research
boat while the lake is accessible, generally June
through October.
We
measure carbon and nitrogen fluxes in the lake using
moored sediment traps. Three traps located at 200m,
400m, and 10m off the bottom, have been maintained since
1983. This is one of the longest, nearly-continuous
records of particle flux from any site on the globe.
From these samples we determine organic matter produced
in the lake and derived from the caldera walls.
The National Park Service monitoring
program at Crater Lake collects analyses of nutrients,
chlorophyll-a, carbon fixation rates, and plankton
numbers and biomass. These data are being combined into
relational databases for the system.
Modeling
A number of modeling efforts
are underway to explore our understanding of the system and,
eventually, to establish the capability to predict impacts of
global change. These models include careful estimates of the
heat and water fluxes through the surface of the lake based on
our meteorological observations, models of the vertical mixing
of the lake, and models of lake level. Other biogeochemical
models of carbon fluxes and ecosystem dynamics are being coupled
to these physical models to investigate the degree to which the
biological and chemical character of the lake are linked to
climate and thus susceptible to climate change.
Application of Results
Our study of organic matter
fluxes with sediment traps is one of the longest records
in the world (19 years in 2002). The full suite of
physical and meteorological data will exceed a decade
during this project. The project provides local
benefits to the Park staff and visitors, regional
benefits for understanding impacts of global change on
the Pacific Northwest, and helps develop
state-of-the-art tools for the interdisciplinary study
of limnology under the influence of changing climate.
Products
- Articles in
peer-reviewed, scientific journals.
- Project reports to
the BRD and NPS, providing rapid access to data sets and
interpretations.
- Interpretive
materials that will make our results more generally
accessible.
- A valuable long-term
monitoring dataset that supports fundamental,
process-oriented research directed at the linkages
within this system.
Principal Investigators
Bob
Collier, Oregon State University
Gary Larson, USGS Forest and Range
Experiment Station
Other Collaborators and
Contacts
Greg
Crawford, Humboldt State University;
Kelly T. Redmond, Western Regional
Climate Center, Desert Research Institute;
Mark Buktenica, Crater Lake
National Park
