1982 - 2002; ill., maps; Title covers calendar years 1990-2002; Bibliographic description is based on 1990 annual report; CA 9000-8-0006 Subagreement 8; Includes bibliographic references; Issues lack volume numbering
Executive Summary Results from limnological studies of Crater Lake between 1978 and 1994 have demonstrated that the lake is a complex, dynamic, unproductive (oligotrophic) system. Long-term changes in water clarity, chlorophyll, primary production, zooplankton abundance and species assemblages, and kokanee salmon biomass were observed. In no case, however, did any component of the lake system exhibit unidirectional or exponential changes which are associated with increasing lake productivity. Water clarity, measured with a Secchi disk, is generally in the mid to high-20-meter to mid-30-meter (m) range. Comparisons of Secchi disk readings suggest that the environmental-limnological conditions that produced 39-40 m Secchi disk readings in the month of August in 1937 and 1969 did not occur during the period from 1978 to 1993, but did occur in 1994. The results suggest that the maximum lake clarity in August was substantially shallower in 1954 and 1978-1993 than in 1937, 1969, and 1994. The absence of Secchi disk readings in the range of 34-40 m during the 1978-1993 time interval can probably be explained by small increases in the densities of light-scattering particles in the water column. The particle density can be affected by many factors, e.g., natural environmental changes, loading of anthropogenic material from atmospheric and on-site sources, and perhaps internal lake processes such as hydrothermal and biological activity. Studies have been underway to examine the extent to which changing lake conditions can be explained by natural and human-caused reasons such as relationships between the climate and changing lake levels, interspecific relationships of biological components, and human-caused increases of nutrients from the atmosphere and on-site sources. The results from these studies will be used to evaluate whether the lake has undergone any extensive physical, chemical, and biological changes which might explain the variation of the August Secchi disk readings. The monitoring program has emphasized the interrelationships among environmental, terrestrial, anthropogenic, and aquatic components of the lake system. Conceptual models were developed that guided the research. Future work will continue to emphasize an ecosystem approach and additional development of conceptual models. However, the studies will test hypotheses developed from the extensive limnological data base. n
Executive Summary Results from limnological studies of Crater Lake between 1978 and 1994 have demonstrated that the lake is a complex, dynamic, unproductive (oligotrophic) system. Long-term changes in water clarity, chlorophyll, primary production, zooplankton abundance and species assemblages, and kokanee salmon biomass were observed. In no case, however, did any component of the lake system exhibit unidirectional or exponential changes which are associated with increasing lake productivity. Water clarity, measured with a Secchi disk, is generally in the mid to high-20-meter to mid-30-meter (m) range. Comparisons of Secchi disk readings suggest that the environmental-limnological conditions that produced 39-40 m Secchi disk readings in the month of August in 1937 and 1969 did not occur during the period from 1978 to 1993, but did occur in 1994. The results suggest that the maximum lake clarity in August was substantially shallower in 1954 and 1978-1993 than in 1937, 1969, and 1994. The absence of Secchi disk readings in the range of 34-40 m during the 1978-1993 time interval can probably be explained by small increases in the densities of light-scattering particles in the water column. The particle density can be affected by many factors, e.g., natural environmental changes, loading of anthropogenic material from atmospheric and on-site sources, and perhaps internal lake processes such as hydrothermal and biological activity. Studies have been underway to examine the extent to which changing lake conditions can be explained by natural and human-caused reasons such as relationships between the climate and changing lake levels, interspecific relationships of biological components, and human-caused increases of nutrients from the atmosphere and on-site sources. The results from these studies will be used to evaluate whether the lake has undergone any extensive physical, chemical, and biological changes which might explain the variation of the August Secchi disk readings. The monitoring program has emphasized the interrelationships among environmental, terrestrial, anthropogenic, and aquatic components of the lake system. Conceptual models were developed that guided the research. Future work will continue to emphasize an ecosystem approach and additional development of conceptual models. However, the studies will test hypotheses developed from the extensive limnological data base. n