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 1995 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 associated with increasing lake productivity. Water clarity, measured with a Secchi disk, is generally greater than 24 m but less than 35 m. A comparison of Secchi disk readings through time suggests that the environmental and limnological conditions that produced 39-40 m readings in the month of August in the years 1937, 1969, and 1994 are rare. Readings this deep were not detected during observations made in 1954, 1978 through 1993, and 1995. The absence of Secchi disk readings in the range of 39-40 m during most years can probably be explained by variation in the densities of light-scattering particles in the upper portion of the water column. Small increases in the densities of these particles can have an effect on Secchi disk readings. The particle density can be affected by many factors, such as natural environmental changes, additions of anthropogenic material from atmospheric and on-site sources, and perhaps internal lake processes such as hydrothermal and biological activity. 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. ii
Executive Summary Results from limnological studies of Crater Lake between 1978 and 1995 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 associated with increasing lake productivity. Water clarity, measured with a Secchi disk, is generally greater than 24 m but less than 35 m. A comparison of Secchi disk readings through time suggests that the environmental and limnological conditions that produced 39-40 m readings in the month of August in the years 1937, 1969, and 1994 are rare. Readings this deep were not detected during observations made in 1954, 1978 through 1993, and 1995. The absence of Secchi disk readings in the range of 39-40 m during most years can probably be explained by variation in the densities of light-scattering particles in the upper portion of the water column. Small increases in the densities of these particles can have an effect on Secchi disk readings. The particle density can be affected by many factors, such as natural environmental changes, additions of anthropogenic material from atmospheric and on-site sources, and perhaps internal lake processes such as hydrothermal and biological activity. 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. ii