Crater Lake Limnological Studies Final Report
Executive Summary
The National Park Service began a study of Crater Lake in 1982 because of indications that lake clarity might be declining. Later the same year, Congress passed Public Law 97-250, which authorized and directed the Secretary of the Interior to promptly initiate a 10-year program to assess the status of the water quality of the lake. Little was known about the ecology of the lake in 1982. Consequently, the National Park Service adopted the following major goals for the study of this unique lake: (1) develop a data base to compare present and future conditions of the lake, (2) develop an understanding of lake components and processes, (3) develop a long-term program for monitoring changes, (4) determine if the lake had experienced recent changes, and if so, (5) identify causes and recommend mitigation procedures if the changes were related to human activity.
Looking at the data in its entirety, researchers concluded at the end of the study 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 lake’s food web, 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 available data.
Original concerns about changes in lake clarity were prompted by measurements of clarity with a Secchi disk. Measurements of lake clarity with a 20- cm (8 in) Secchi disk with black and white quadrants were included in the 10-year study in order to gain data for comparison with historical data. Analysis of the Secchi disk data revealed that clarity was now generally greater than 25 m (82 ft) but less than 35 m (115 ft). The shallowest reading during this study (21.9 m; 72 ft) was recorded in August of 1982, and the deepest reading (39.2 m; 129 ft) was recorded in June of 1988. This deepest reading was 0.8 m (3 ft) short of the maximum Secchi reading on record for the lake using the 20-cm disk. August Secchi disk readings in the range of 39-40 m (128-131 ft), a range that encompassed the maximum Secchi depths recorded in August of 1937 and 1969, were not observed between 1982 and 1992. In addition to measurements with a Secchi disk, changes in the composition and depth of penetration of surface light were measured with a photometer, and changes in the spatial distribution of particles in the water column were measured with a transmissometer. Natural variabilities were apparent using all three measurements of clarity, but as a whole, the data did not support the hypothesis that clarity of Crater Lake had undergone long-term change.
Extensive measurements were taken of temperature, alkalinity, conductivity, pH, and other chemical and physical properties of water in the lake. No long-term changes in these measures of water quality were evident during the duration of the study nor through a comparison of current and historical data. Concentrations of phosphorus and nitrate, two very important nutrients for growth of algae, were low. An estimated 90%of the nitrogen and 30% of the phosphorus brought into the lake each year came from the atmosphere. The remaining fractions entered the lake from other sources associated with the caldera, including springs flowing from the walls of the caldera into the lake. Contribution of nitrates to the lake from the springs was specifically studied because of concerns about a sewage drain field for visitor facilities located just outside the caldera wall. One spring located on the caldera wall near the sewage drain field exhibited relatively high nitrate concentrations but contributed less than 1% of the total annual input of new nitrate into the lake. Although an analysis of the water chemistry of the spring could not confirm the source of the nitrates, the drain field was removed in 1991 as a precautionary measure.
Hydrothermal fluids, discovered on the lake bottom, contributed to the lake’s relativelyhigh salt content and were, in part, responsible for the long-term stability in water chemistry of the lake. The hydrothermal inputs were determined to be highly significant in maintaining the lake’s natural biological, chemical, and physical processes.
Phytoplankton and zooplankton communities were sparse, diverse, and complex. Seasonal and annual changes in chlorophyll primary production, phytoplankton abundance, and zooplankton abundance were observed. The abundance of the largest species of zooplankton was markedly cyclic and appeared to respond to changes in lake productivity and fish predation. Kokanee salmon and rainbow trout continue to persist in the lake beyond the last stocking in 1941. Kokanee salmon were cyclic in abundance, lived both near the shore and in open deep water, and fed on zooplankton and small bottom-dwelling insects. Rainbow trout lived along the edges of the lake and fed on terrestrial insects, large-bodied bottom fauna, and kokanee. Fish clearly exhibited the potential both to alter the food webs within open-water and near-shore habitats and to affect nutrient cycling within the lake.
In general, the Crater Lake ecosystem was extremely responsive and sensitive to environmental change and was judged to be pristine, except for the consequences of fish introductions. The study documented many of the components and processes important to lake clarity and the lake system as a whole. The study also identified many questions needing further study. Long-term change could not be fully evaluated because very little historical data were available to compare with the detailed data base assembled during this 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. Global climate change, air pollution, on-site auto and boat use, and non-native fish present the greatest potential human-related threats to the pristine nature of Crater Lake. Additional studies would refine knowledge of the components and dynamic processes of the lake system as well as separate changing lake conditions caused by natural phenomena from those caused by human-related activities.
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