Introduction

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Crater Lake is located in Crater Lake National Park in the southern Cascade Mountains of Oregon, USA. It is the deepest lake in the United States and the 7th deepest lake in the world. The lake basin (caldera) was formed by catastrophic collapse of the sides of Mount Mazama following a violent eruption about 6,800 years ago. The present caldera has steep walls and is between 8 and 10 km in diameter. The lake occupies 78% of its own drainage basin. Crater Lake is the deepest caldera lake in the world, as well as one of the highest in elevation and largest in surface area. No surface outlet exists, but over 40 permanent and ephemeral inlet streams drain into the lake. The lake has a surface area of 53.2 km², a maximum depth of 589 m, and a mean depth of 325 m. Steep caldera walls surround the lake, resulting in a ratio of lake area to watershed area (flat map) of about 3.6. A secondary intracaldera volcanic cone forms Wizard Island, (see figure 1 below) the largest island in the lake. Surface inflow is restricted to intracaldera springs and small streams. There is no surface outflow.

Aquatic studies at Crater Lake from 1896 to the mid-1950s consisted mostly of short-term evaluations of physical, chemical, and biological features. Although these studies were fragmentary in nature, it was obvious that the lake was ultraoligotrophic (nutrient poor), exceptionally deep (589 m), and extremely clear. Studies undertaken from 1959 to 1969 were more detailed than the earlier studies and provided additional information on morphometry, optical properties, sediments, fluctuations of the water level, water budget, and general limnological characteristics, as well as initial documentation of chlorophyll concentrations, primary production, phytoplankton, and zooplankton. These studies reaffirmed the ultraoligotrophic status of the lake. Results from studies conducted from 1978 to 1981 indicated a possible decline in lake clarity and possible changes in the species composition and vertical distribution of the phytoplankton community. Verification that these changes had occurred was not possible because the amount of historic information was too small, and sampling techniques and methods varied over time. Nonetheless, the suggestion of possible changes in the lake led to a Congressionally mandated 10-year monitoring and research program, beginning in the fall of 1982, to investigate the overall water quality of Crater Lake.

The goals of the 10-year monitoring and research program were to: 1) develop a reliable quantitative limnological data base for future comparison; 2) develop an understanding of the physical, chemical, and biological features of the lake; and 3) establish a long-term monitoring program to examine the characteristics of the lake through time. If changes in the lake condition were detected, studies were to be designed to identify the causes, and mitigation measures were to be recommended.

At the end of the 10-year study, researchers concluded that Crater Lake was a complex and dynamic system with considerable seasonal and annual variability. Although fish, which were introduced into the lake between 1888 and 1941, affected the food web in the lake, no other changes caused by human activities could be specifically identified or separated from those caused by natural phenomena. Although the possibility of long-term changes in the lake could not be dismissed, researchers regarded such changes to be too subtle for detection over a time scale represented by the data.

The 10-year study documented many of the components and processes important to lake clarity and the lake system as a whole. Long-term change could not be fully evaluated because very little historical data were available to compare with the detailed data base assembled during the 10-year study. This situation underscored the need for a long-term monitoring program to evaluate future change against the benchmark set in the 10-year study. Implementation of the proposed long-term monitoring program at Crater Lake required additional funding. Such funding was available starting in the 1994 field season.

The sampling program for the long-term study follows the protocols established during the 10-year study, except that the complete suite of nutrient and trace element samples are collected only in August. Spring water samples are collected to continue monitoring for any signs of changing levels of nitrate after the rim sewage facility was disconnected in 1991 and to assess any possible contamination from construction activities at Rim Village.

The program can be summarized in two broad objectives. First, baseline data will be collected to characterize the limnological conditions of the lake from 1982/1983 to 1999. Second, lake structure and organization will be defined in order to develop reliable relationships among physical, chemical, and biological components of the ecosystem.

Although the two broad objectives are useful for general discussion and program direction, project selection requires the initial development of conceptual models as shown in Figure 2 and Figure 3. The first model illustrates the general components and the broad relationships between components within the ecosystem, such as the interrelationships among climatological, terrestrial, anthropogenic perturbations, and lake characteristics. The focus of the second model is on the within-lake aspects of the ecosystem, which is only part of the caldera ecosystem shown in Figure 2. Components of the long-term monitoring program are shown in Table 1.

Figure 2: Conceptual Model of the Crater Lake Ecosystem

Figure 3: Details of the lake aspects of the conceptual model shown in figure 2.

Table 1. Components of the Crater Lake Baseline Limnological Monitoring Program, 1999

1. Lake

A. Temperature: Conductivity, temperature, and depth probe (CTD)

B. Optical

1. Secchi disk (20 cm)
2. Transmissometer (to 550 m)
3. Spectroradiometer (to 200 m)

Determine pH, total alkalinity, specific conductance, dissolved oxygen, total phosphorus, orthophosphate-P, nitrate-N, total Kjeldahl-N, ammonia-N, silica, and trace elements at all or selected depths from the following depth sequence: 0, 5, 10, 20, 60, 100, 200, 300, 400, 500, and 550 m.

D. Biological

1. Total chlorophyll E

Estimate the in vitro total chlorophyll at the following depth sequence:
5m intervals from 0 to 10  m
10 m intervals from 10 to 40 m
20 m intervals from 40 to 200 m
25 m intervals from 200 to 300 m

2. Primary production (carbon-14 light/dark bottle)

Estimate primary production at the chlorophyll sampling depths to 180 m.

3. Phytoplankton

Determine species, densities, and biovolumes at all chlorophyll sampling depths.

4. Zooplankton

Determine species, densities, and biomasses. Samples taken with a vertical haul .5 m diameter number 25 (64 m) closing net.

5. Fish

Determine species, abundance, biomass, distribution, age, sex, growth and food habits. Fish samples collected with gill nets and by angling. Pelagic distribution and abundance of fish using an echosounder.

2. Springs

Physical and chemical water quality

Record temperature and take samples for pH, conductivity, alkalinity, nutrients, and trace elements.