Project Summary

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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.