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.

 

Project Contacts

Principle Investigator: Gary Larson, Aquatic Ecologist
USGS Forest & Rangeland Ecosystem Science Center
777 NW 9th St., Suite 400
Corvallis, OR 97330
Phone: 541-750-1032
gary_l._larson@usgs.gov

Collaborators:

Mark Buktenica, Biologist
National Park Service
Crater Lake National Park
Crater Lake, OR 97604
Phone: 541-594-3077

Scott Girdner, Biologist
National Park Service
Crater Lake National Park
Crater Lake, OR 97604
Phone: 541-594-3078

C. David McIntire, Aquatic Ecologist
Department of Botany & Plant Pathology
Oregon State University
Corvallis, OR 97331
Phone: 541-757-1811

Robert Collier, Marine Geochemist
College of Oceanic and Atmospheric Sciences
Oregon State University
Corvallis, OR 97331
Phone: 541-737-4367

Robert Hoffman, Aquatic Ecologist
College of Oceanic and Atmospheric Sciences
Oregon State University
Corvallis, OR 97331
Phone: 541-758-7782

Physical and Chemical Data

Lake level has exhibited considerable long-term variation since the late 1890's. From 1910 to 1942 the lake dropped about 4 m in elevation, but returned to the 1910 level by the late 1950's. The lake fluctuated by 1 m above and below a bench mark level of 1882 m observed from 1958 to 1985. Between 1986 and 1994, the level dropped to about 3.5 m below the long-term average elevation. Since then the lake has risen to nearly the bench mark level.

The surface of Crater Lake seldom freezes over, owing to the heat content of the massive lake volume and wind mixing. The only known occurrences of ice and snow cover occurred in 1948 and 1985. In 1986 the lake was nearly covered by ice at various times in January and March.

In winter and spring the water mass in Crater Lake circulates to a depth of between 200 and 250 m by wind action and cooling. The deep lake is mixed in winter and early spring each year when relatively cold water near the surface sinks and exchanges positions with water in the deep basins of the lake. In late spring or early summer the temperature of the lake near the surface increases and a thermocline forms between July and September. The depth of the epilimnion is between 5 m and 20 m and is usually deepest in fall because of cooler air temperatures and an increase in mixing generated by storms. Maximum near-surface temperatures typically ranged from 14 to 19 degrees C from June or early July to mid-September. The metalimnion extends to a depth of about 100 m; thus, most of the water volume is a cold hypolimnion. Annually water temperatures do not vary by more than 1 degrees C below a depth of 80 m. The temperature at the bottom of the water column is about 3.5 degrees C.

The average pH of the entire water column of Crater lake is 7.5. Average near-surface pH ranges from about 7.6 to 7.7, whereas the pH decreases with increased lake depth and is about 7.3 at a depth of 550 m. The average total alkalinity and specific conductance are 27 mg/l and 115 mhos/cm, respectively. Both variables increase slightly with increasing lake depth. The average concentration of dissolved oxygen is relatively uniform in the water column in spring, but decreases to about 90% saturation at 550 m during the period of thermal stratification. Nitrate-nitrogen, ammonia-nitrogen, Kjeldahl-nitrogen, total phosphorus and orthophosphate-phosphorus occur in low concentrations. Nitrate-nitrogen is virtually undetectable in the upper 200 m of the water column; however, the concentration increases to maximum with increased depth. Kjeldahl-nitrogen and ammonia-nitrogen decrease in concentration with increased lake depth, whereas orthophosphate-phosphorus increases. Total phosphorus is nearly uniform in concentration throughout the water column.

Secchi disk clarity readings are generally in the mid-high 20s and low 30s each year from June through September. The highest (deepest) readings usually occurred in late June and July and the lowest in August. Nonetheless, there were some differences in the temporal patterns during the 17 years (Figure 4). For example, in 1997 the readings from late July through August were the deepest recorded during the study period, whereas Secchi disk readings in late July 1995 were the shallowest readings from 1982-1999.

Figure 4: Secchi Disk Measurements in Crater Lake by month from 1982 to 1999.

Phytoplankton Data

During the period from 1983 to 1998 a total of 157 phytoplankton taxa were identified, including 55 diatoms, 53 chrysophytes, 1 xanthophyte, 21 chlorophytes, 12 dinoflagellates, 6 cryptomonads, 7 cyanobacteria, and 2 unknown taxa. In winter the flora was uniformly distributed to the depth of mixing. Stephanodiscus hantzschii, Ankistrodesmus spiralis and a small unidentified chrysophyte were the dominate taxa during this period of the year. During the period of thermal stratification, phytoplankton are spatially segregated within the water column to a depth of 200 m (Figure 5). Nitzschia gracilis was the dominant taxa in the upper 40 m of the water column. Ankistrodesmus spiralis, Dinobryon sertularia, Tribonema sp., Rhodamonas lacustris and Gymnodinium inversum were the dominant taxa from 60 to 100 m. From 120 to 200 m the dominant taxa were the same as during winter.

Concentrations of total chlorophyll were maximum usually at 120 m during periods of thermal stratification from 1984 to 1999 (Figure 6). Peak concentrations of total chlorophyll were always less than 2 g/l however. Total chlorophyll integrated to a depth of 200 m exhibited cyclic changes between 1979 to 1990. Primary production was low between 1986 and 1990, using the carbon-14 assimilation method. Maximum primary production occurred between 40 and 80 m during periods of thermal stratification from 1986 to 1990, (Figure 7) although relatively high production values occasionally were observed in near-surface samples. Primary production in August exhibited a similar cyclic pattern as did total chlorophyll.

Figure 5: Depth profiles of selected phytoplankton taxa. Data presented as averages per depth. Bars refer to 1 standard deviation.

Figure 6: Depth profiles of chlorophyll for selected months. Data presented as averages per depth. Bars represent 1 standard deviation.

Figure 7: Depth profiles of primary production for selected months. Data presented as averages per depth. Bars represent 1 standard deviation.

Phytoplankton Photos

Zooplankton Data

Since 1985, two crustacean taxa and 11 rotifer taxa where collected in Crater Lake. In winter, most of the taxa were distributed from the lake surface to the depth of mixing (200-250 m). During periods of thermal stratification the taxa were spatially segregated within the water column (Figure 8). Polyarthra was the dominant taxon in the upper 40 m of the lake, but it occurred in low density. Between 40 m and 80 the dominant taxa were Bosmina, Polyarthra, Kellicottia and Asplanchna. From 80 to 120 m the dominant taxa were Daphnia, Keratella, Synchaeta, Filinia and Polyarthra. The dominant taxa from 120 to 200 m were Philodina, Conochilus, Keratella and Collotheca.

Some taxa were not present every year from 1985 to 1990. Philodina was present between 1985 and 1988, Conochilus was present in 1985, and Asplanchna was present in 1990. Daphnia was not present in quantitative subsamples in 1985, and was in low density in 1986 (Figure 9). The population reached its maximum abundance in 1988, declined to low density by 1990, and was absent in 1993. The taxon returned in abundance in 1998-99.

Figure 8: Depth profiles of selected zooplankton taxa. Data presented as averages per depth. Bars represent 1 standard deviation.

Figure 9: Densities of Daphnia pulicaria integrated to 200 m from 1985 to 1999.

Zooplankton Photos

Fish Data

Crater Lake was naturally barren of fish. Several salmonid species were introduced into the lake between 1888 and 1941. Kokanee salmon and rainbow trout were the only species of fish collected from the lake between 1986 and 1999. Kokanee salmon were cyclic in abundance. Rainbow trout appeared to be less cyclic in abundance than were kokanee salmon.

Kokanee salmon primarily live in the pelagic zone of the lake from the surface to a depth of about 100 m. They prey on small emerging benthic macroinvertebrate pupae and larvae and terrestrial insects landing on the lake surface. They also prey on Daphnia. Rainbow trout live in the nearshore area of the lake. They prey on large bodied terrestrial insects from the lake surface and benthic macroinvertebrates. Rainbow trout also prey on kokanee salmon.

Project Summary

Crater Lake is a dynamic and complex system as illustrated by long-term fluctuations of water level, clarity, chlorophyll, primary production, zooplankton and kokanee salmon, and the spatial segregation of the water column by phytoplankton and zooplankton. Long-term changes in lake level results from shifts in the water budget. Changes in the amount of chlorophyll and primary production appear to be related to deep-water mixing of the water column during winter and spring. This upwelling phenomenon moves nutrient-rich waters in the deep lake to the upper 200 to 250 m of the water column. Daphnia abundances appear linked with periods of increased primary productivity; however, predation by kokanee salmon probably impacts their abundance and may be the reason for its reduced abundance in 1990 and disappearance in quantitative samples by 1993.

Chemical and physical properties of Crater Lake that are most consistent with typical oligotrophic characteristics of lakes include high transparency, an orthograde nitrate-N depth profile, and low concentrations of nitrate-N in the epilimnion. Specific conductance in Crater Lake often exceeds those of eutrophic, mesotrophic and oligotrophic lakes in the Cascade Mountains of Oregon. The relatively high conductivities of Crater Lake and two other Oregon caldera lakes (East Lake and Paulina Lake) are associated with inputs from hydrothermal fluids. In comparison to the range of conductivities of caldera lakes worldwide, however, the conductivities of caldera lakes in Oregon are low. Furthermore, the relatively high concentrations of total phosphorus in Crater lake is in the range usually associated with mesotrophic lakes. Therefore, some of the chemical properties of Crater lake do not conform to the entire range of criteria usually associated with oligotrophic status.

Crater Lake is a unique lake from an international perspective, and it is highly valued both nationally and locally. Responsibility for management of such a system is a priority for the National Park Service. Furthermore, the long-term data set that now exists for the lake has great scientific value for understanding processes that are common to all aquatic systems. Few pristine lake have received such extensive and intensive studies. The National Park Service recognizes that maintaining the pristine conditions of the lake will require regulation of human activities within the context of existing information and regulations, while simultaneously supporting the collection of additional information. Long-term monitoring of selected features of the lake system coupled with special short-term studies are needed for additional information for management and scientific purposes.

Sources

Larson, Gary, C. David McIntire, and Ruth W. Jacobs, eds. 1993. Crater Lake: Limnological Studies, Final Report. Technical Report NPS/PNROSU/NRTR-93-03. Cooperative Park Studies Unit, college of Forestry, Oregon State University, Corvallis, OR.

Larson, Gary L. 1996. Development of a 10-year Limnological Study of Crater Lake, Crater Lake National Park, Oregon, USA. Lake and Reservoir Management. 12(2): 221-229.

Larson, Gary, C. David McIntire, Michael Hurley, Mark W. Buktenica. 1996. Temperature, water chemistry, and optical properties of Crater Lake. Lake and Reservoir Management. 12(2): 230-247.

McIntire, C. David, Gary L. Larson, Robert E. Truitt, Mary K. Debacon. 1996. Taxonomic structure and productivity of phytoplankton assemblages in Crater Lake, Oregon, USA. Lake and Reservoir Management. 12(2):259-280.

Larson, Gary, C. David McIntire, Robert E. Truitt, Mark W. Buktenica, Elena Karnaugh-Thomas. 1996. Zooplankton assemblages in Crater Lake, Oregon, USA. Lake and Reservoir Management. 12(2): 281-297.

Mark W. Buktenica and Gary L. Larson. 1996 Ecology of Kokanee salmon and rainbow trout in Crater Lake, Oregon, USA. Lake and Reservoir Management. 12(2): 298-310.