1982-2002; ill., maps; Title covers calendar years 1985-1987; CA 9000-3-0003 Subagreement 12; 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 on the lake. That fall, Congress mandated (Public Law 97-250) a 10-year limnological study of Crater Lake. The goals were to (1) develop a limnological data base for comparison with future conditions; (2) develop a better understanding of physical, chemical, and biological components of the lake system; and (3) 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. Field work from 1982 to 1985 was restricted to the period from late June/early July to the middle of September because the research boats could not be stored in the caldera during other portions of the year. A boat house was built on Wizard Island in 1985, and in 1986 sampling was extended to March and May. Results to date have demonstrated that the lake is still unproductive. Water clarity, as measured with a Secchi disk, is generally in the high 20 to low 30 meter (m) range. Comparisons of Secchi disk readings obtained in present studies with those from earlier studies suggest that the environmental-limnological conditions that resulted in 39-40 m Secchi disk readings in 1937 and 1969 have not occurred from 1978 to 1986. The limited number of August readings suggests that the maximum lake clarity readings have been about 25% lower during 1978-1986 than in 1937 and 1969. Data from lake color studies suggest a slight decrease in the optical properties of the lake. The apparent decline in Secchi disk readings can probably be explained by small increases in the densities of light scattering particles in the water column. The particle density is affected by natural environmental changes, loading of anthropogenic material from atmospheric and onsite sources, and perhaps internal lake processes such as hydrothermal vents and biological activity. The highest Secchi disk readings (in 1937 and 1969) were recorded during periods when the lake maximum level was not changing much on an annual basis. Reduced lake level fluctuations may correspond to reduced particle loading. Secchi disk clarity of the lake may naturally be less than 39-40 m during periods of fluctuating lake levels. -2- Studies are underway or planned 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, interrelationships 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 the extensive physical, chemical, and biological changes as might be suspected from the decline of the Secchi disk readings. With the monitoring program underway, the program is emphasizing the interrelationships among environmental, terrestrial, anthropogenic, and aquatic components of the lake system. Conceptual models have been developed and are being used to guide the research. Future work will include an assessment of the relationships between climate and lake level fluctuations, estimates of nutrient loading, recycling, and sedimentation, evaluations of the existence of hydrothermal vents and their effects on the lake system, and documentation of interrelationships among the physical, chemical, and biological components of the lake. Assessment of changing lake conditions will be addressed from analyses of the above-mentioned programs, repeating earlier work on lake color and optical properties, and an examination of sediment cores to reconstruct a history of the phytoplankton and crustacean zooplankton communities through time. -3-
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 on the lake. That fall, Congress mandated (Public Law 97-250) a 10-year limnological study of Crater Lake. The goals were to (1) develop a limnological data base for comparison with future conditions; (2) develop a better understanding of physical, chemical, and biological components of the lake system; and (3) 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. Field work from 1982 to 1985 was restricted to the period from late June/early July to the middle of September because the research boats could not be stored in the caldera during other portions of the year. A boat house was built on Wizard Island in 1985, and in 1986 sampling was extended to March and May. Results to date have demonstrated that the lake is still unproductive. Water clarity, as measured with a Secchi disk, is generally in the high 20 to low 30 meter (m) range. Comparisons of Secchi disk readings obtained in present studies with those from earlier studies suggest that the environmental-limnological conditions that resulted in 39-40 m Secchi disk readings in 1937 and 1969 have not occurred from 1978 to 1986. The limited number of August readings suggests that the maximum lake clarity readings have been about 25% lower during 1978-1986 than in 1937 and 1969. Data from lake color studies suggest a slight decrease in the optical properties of the lake. The apparent decline in Secchi disk readings can probably be explained by small increases in the densities of light scattering particles in the water column. The particle density is affected by natural environmental changes, loading of anthropogenic material from atmospheric and onsite sources, and perhaps internal lake processes such as hydrothermal vents and biological activity. The highest Secchi disk readings (in 1937 and 1969) were recorded during periods when the lake maximum level was not changing much on an annual basis. Reduced lake level fluctuations may correspond to reduced particle loading. Secchi disk clarity of the lake may naturally be less than 39-40 m during periods of fluctuating lake levels. -2- Studies are underway or planned 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, interrelationships 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 the extensive physical, chemical, and biological changes as might be suspected from the decline of the Secchi disk readings. With the monitoring program underway, the program is emphasizing the interrelationships among environmental, terrestrial, anthropogenic, and aquatic components of the lake system. Conceptual models have been developed and are being used to guide the research. Future work will include an assessment of the relationships between climate and lake level fluctuations, estimates of nutrient loading, recycling, and sedimentation, evaluations of the existence of hydrothermal vents and their effects on the lake system, and documentation of interrelationships among the physical, chemical, and biological components of the lake. Assessment of changing lake conditions will be addressed from analyses of the above-mentioned programs, repeating earlier work on lake color and optical properties, and an examination of sediment cores to reconstruct a history of the phytoplankton and crustacean zooplankton communities through time. -3-