ill., maps; Report title; Includes appendices: Crater Lake Liminological Studies 1984 and 1985; Includes bibliographical references; "Robert E. Benton, Superintendent Crater Lake National Park." - T.p.
I. Executive Summary Independent studies of Crater Lake during the late 1970's and early 1980's suggested that the lake had decreased in transparency and that the species composition and distribution of the phytoplankton had changed relative to results from short-term studies conducted between 1913 and 1969. Careful review found that the transparency and phytoplankton data were inadequate for the bases of any definitive conclusions on whether long term changes had occurred in lake water quality. Nonetheless, in order to build on the existing baseline data, the National Park Service initiated a limnological study of Crater Lake in the summer of 1982. That fall, Congress mandated (Public Law 97-250) a 10-year study of the lake to be conducted by the Secretary of the Interior. The goals of the program were to: (1) develop a limnological data base; (2) develop an understanding of lake organization and structure; and (3) establish a long-term monitoring program to examine lake characteristics through time. The studies will investigate the lake for changing conditions and if found, studies to identify the cause(s) will be initiated. Field work, begun in 1982, has been limited to the period between late June-early July to the middle of September owing to logistical difficulties during the rest of the year. The results to date have indicated that the lake is very oligotrophic and that water clarity is high. Secchi disc depth readings are generally in the high 20 to low 30 meter range. In 1985, the Secchi reading of 37.1 meters in early July was near record level. Readings during August and September, 1985, were at levels similar to those recorded in 1982 through 1984. Statistical comparison of existing data indicates that the assumption of a 25% loss of clarity since the late 1960's may be premature. Subtle changes, either natural and/or anthropogenic, may be causing significant seasonal variations in water clarity as measured by the Secchi disc. The monitoring program has indicated that lake clarity fluctuates seasonally due to factors such as particulate matter carried in from rainfall and/or snow avalanches. It is, however, still too early in the project to definitively determine if lake clarity has changed over time. With the monitoring program in place, the direction of the program has been moved to emphasize the interrelationships among environmental, terrestrial, anthropogenic and aquatic aspects of the ecosystem. A conceptual model has been developed from which specific research objectives have been established. Using the conceptual model, areas needing further study have become apparent and understanding of the ecosystem has been increased. The monitoring program will be refined and special studies designed to determine if changes have indeed occurred and the nature and apparent cause(s) of these changes.
I. Executive Summary Independent studies of Crater Lake during the late 1970's and early 1980's suggested that the lake had decreased in transparency and that the species composition and distribution of the phytoplankton had changed relative to results from short-term studies conducted between 1913 and 1969. Careful review found that the transparency and phytoplankton data were inadequate for the bases of any definitive conclusions on whether long term changes had occurred in lake water quality. Nonetheless, in order to build on the existing baseline data, the National Park Service initiated a limnological study of Crater Lake in the summer of 1982. That fall, Congress mandated (Public Law 97-250) a 10-year study of the lake to be conducted by the Secretary of the Interior. The goals of the program were to: (1) develop a limnological data base; (2) develop an understanding of lake organization and structure; and (3) establish a long-term monitoring program to examine lake characteristics through time. The studies will investigate the lake for changing conditions and if found, studies to identify the cause(s) will be initiated. Field work, begun in 1982, has been limited to the period between late June-early July to the middle of September owing to logistical difficulties during the rest of the year. The results to date have indicated that the lake is very oligotrophic and that water clarity is high. Secchi disc depth readings are generally in the high 20 to low 30 meter range. In 1985, the Secchi reading of 37.1 meters in early July was near record level. Readings during August and September, 1985, were at levels similar to those recorded in 1982 through 1984. Statistical comparison of existing data indicates that the assumption of a 25% loss of clarity since the late 1960's may be premature. Subtle changes, either natural and/or anthropogenic, may be causing significant seasonal variations in water clarity as measured by the Secchi disc. The monitoring program has indicated that lake clarity fluctuates seasonally due to factors such as particulate matter carried in from rainfall and/or snow avalanches. It is, however, still too early in the project to definitively determine if lake clarity has changed over time. With the monitoring program in place, the direction of the program has been moved to emphasize the interrelationships among environmental, terrestrial, anthropogenic and aquatic aspects of the ecosystem. A conceptual model has been developed from which specific research objectives have been established. Using the conceptual model, areas needing further study have become apparent and understanding of the ecosystem has been increased. The monitoring program will be refined and special studies designed to determine if changes have indeed occurred and the nature and apparent cause(s) of these changes.