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 1981 suggested that the clarity of the lake was declining and that the species composition and vertical distribution of the phytoplankton had changed when compared to studies from 1913 to 1969. Workshops were convened in early 1982 to review the data, and it was concluded that the information base was insufficient to determine if the lake had changed. A limnological study was initiated in 1982 by the National Park Service to increase the data base for the lake. That fall, Congress mandated (Public Law 97-250) a 10-year limnological study of Crater Lake. Program goals were as follows: i) develop a limnological data base for comparison with future conditions; ii) develop a better understanding of physical, chemical, and biological components of the lake system; and iii) establish a long-term monitoring program to examine the lake characteristics through time. An important aspect of the study was to determine if the lake had changed, and if so, identify the cause(s) and recommend mitigation measures if anthropogenic in nature. Results to date have demonstrated that the lake is a complex, dynamic, and unproductive (oligotrophic) system. Water clarity, as measured with a Secchi disk, is generally in the mid-high 20 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 1992. The results suggest that the maximum lake clarity in August was substantially shallower in 1954 and 1978-1992 than in 1937 and 1969. The absence of Secchi disk readings in the range of 34-40 m during the 1978-1992 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 fish, zooplankton and algae, 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 apparent decline of the August Secchi disk readings. The results of these studies will be presented in the final report of the 10-year study. 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 3 hypotheses developed from the extensive limnological data base. The hypotheses will be presented in the final report of the 10-year study. Additional funding will be required to implement the program. 4
Executive Summary Results from limnological studies of Crater Lake between 1978 and 1981 suggested that the clarity of the lake was declining and that the species composition and vertical distribution of the phytoplankton had changed when compared to studies from 1913 to 1969. Workshops were convened in early 1982 to review the data, and it was concluded that the information base was insufficient to determine if the lake had changed. A limnological study was initiated in 1982 by the National Park Service to increase the data base for the lake. That fall, Congress mandated (Public Law 97-250) a 10-year limnological study of Crater Lake. Program goals were as follows: i) develop a limnological data base for comparison with future conditions; ii) develop a better understanding of physical, chemical, and biological components of the lake system; and iii) establish a long-term monitoring program to examine the lake characteristics through time. An important aspect of the study was to determine if the lake had changed, and if so, identify the cause(s) and recommend mitigation measures if anthropogenic in nature. Results to date have demonstrated that the lake is a complex, dynamic, and unproductive (oligotrophic) system. Water clarity, as measured with a Secchi disk, is generally in the mid-high 20 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 1992. The results suggest that the maximum lake clarity in August was substantially shallower in 1954 and 1978-1992 than in 1937 and 1969. The absence of Secchi disk readings in the range of 34-40 m during the 1978-1992 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 fish, zooplankton and algae, 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 apparent decline of the August Secchi disk readings. The results of these studies will be presented in the final report of the 10-year study. 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 3 hypotheses developed from the extensive limnological data base. The hypotheses will be presented in the final report of the 10-year study. Additional funding will be required to implement the program. 4