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 limnoiogical 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 of that study are: 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 still unproductive (oligotrophic). Water clarity, as measured with a Secchi disk, is generally in the high 20 to mid 30-meter (m) range. Comparisons of Secchi disk readings obtained in present studies with those from earlier studies suggest that the -3- environmental-limnological conditions that produced the 39-40 m Secchi disk readings in 1937 and 1969 have not occurred during the period from 1978 to 1987. The limited number of August readings suggests that the maximum lake clarity readings appear to have been about 25% lower during 1978-1987 than in 1937 and 1969. Lake color measurements 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 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 vents and biological activity. The highest Secchi disk readings (in 1937 and 1969) were recorded during periods when the lake maximum water 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 normally be less than 39-40 m, especially during periods of fluctuating lake levels. 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, 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 -4- changes which might explain the apparent decline of the August Secchi disk readings. The monitoring program is currently 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 (water budget), 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. Quantitative determination of possible changing lake conditions will be made based on the data from the above-mentioned analyses, plus further studies of lake water color and optical properties, and examination of sediment cores to reconstruct a history of the phytoplankton and crustacean zooplankton communities through time. -5-
Executive Summary Results from limnoiogical 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 of that study are: 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 still unproductive (oligotrophic). Water clarity, as measured with a Secchi disk, is generally in the high 20 to mid 30-meter (m) range. Comparisons of Secchi disk readings obtained in present studies with those from earlier studies suggest that the -3- environmental-limnological conditions that produced the 39-40 m Secchi disk readings in 1937 and 1969 have not occurred during the period from 1978 to 1987. The limited number of August readings suggests that the maximum lake clarity readings appear to have been about 25% lower during 1978-1987 than in 1937 and 1969. Lake color measurements 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 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 vents and biological activity. The highest Secchi disk readings (in 1937 and 1969) were recorded during periods when the lake maximum water 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 normally be less than 39-40 m, especially during periods of fluctuating lake levels. 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, 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 -4- changes which might explain the apparent decline of the August Secchi disk readings. The monitoring program is currently 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 (water budget), 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. Quantitative determination of possible changing lake conditions will be made based on the data from the above-mentioned analyses, plus further studies of lake water color and optical properties, and examination of sediment cores to reconstruct a history of the phytoplankton and crustacean zooplankton communities through time. -5-