1982-2002; ill, maps; Report title; CA 9000-3-0003 Subagreement 8; Includes appendices: Crater Lake Liminological Studies 1988 and 1989; "Submitted by Robert E. Benton, Superintendent Crater Lake National Park." - T.p.; Includes bibliographical references; Issues lack volume numbering
I. Executive Summary Limnological studies of Crater Lake were initiated by the National Park Service in 1982 in response to an apparent decline in lake clarity and changes in the phytoplankton community characteristics. In the fall of 1982, Congress mandated, through Public Law 97-250, a 10-year study of the lake. The goals of the study included: (1) development of a limnological data base for comparison with future conditions; (2) development of a better understanding among physical, chemical and biological components of the lake system; and (3) development of a long term monitoring program. An important aspect of the goals was to determine if the lake has experienced recent changes, and if present and human related in nature, identify the cause(s) and recommend mitigations. Project development has been directed toward an ecosystem approach. Studies include precipitation quantity and chemistry, lake level fluctuations, solar radiation, intracaldera spring chemistry, lake clarity, lake color, particle flux, water quality, chlorophyll, primary production, phytoplankton, zooplankton, bottom fauna, and fish. An extensive data base has been assembled for each aspect of the study. Results to date indicate that the oligotrophic (nutrient poor) lake system is dynamic and complex. The volume of the lake responds quickly to changes in precipitation because the closed lake basin (no surface outlet) serves as a leaky rain gauge. Although the level of the lake fluctuates seasonally, overall it has dropped about 2.0 m (6.6 ft.) since 1984. The lake becomes thermally stratified between July and September. The thermocline (interface between warmed surface waters and cold water in the deep lake) extends to a depth of about 80 m (262 ft.) Although the lake does not completely mix in winter, some deep mixing must occur as indicated by high concentrations of dissolved oxygen at the lake bottom. Secchi disk clarity is generally in the high 20 to mid 30 m (66-100 ft.) range. The depth of 1% of the incident light at the lake surface ranges from 80-100 m (262-328 ft.). Maximum photosynthesis typically occurs between 4 0 and 8 0 m (131-262 ft.). A deep-water chlorophyll maximum occurs between 100 and 120 m (328-394 ft.). In summer, a dense population of a species of diatom typically develops in the epilimnion (the warm layer above the thermocline). The rest of the sparse and complex phytoplankton community exists to a depth of about 200 m (656 ft.)- Most zooplankton species exist between 20 and 120 m (66-394 ft.). Rotifer and crustacean species are present. Rotifers are in high abundance. The largest crustacean species appears to be cyclic in abundance. Two species of fish exist in the lake. Both were stocked many years ago and now reproduce in the lake. Kokanee salmon are mostly pelagic and feed primarily on terrestrial insects at the lake surface and bottom fauna. Kokanee densities are cyclic due to the numerical dominance of one year class. Changes in rainbow trout densities are less variable due to the multiple age structure of the population. Comparisons with older data collected from Crater Lake, though sparse, indicate that there have been no major changes in the near-surface lake water quality for pH, conductivity and selected elements. Secchi disk clarity in August in recent years is about 25% shallower than in 1937 and 1969. Such a decrease in Secchi disk clarity can probably be explained by a small increase in the abundance of particles in the water column. Whether these changes in particle abundance are human related or naturally occurring is still unknown. Fluctuations in the surface elevation of the lake since 1982 have been similar to those in the past. The cyclic nature of the largest crustacean zooplankton species is consistent with results from studies conducted in the late 1960s. Population fluctuations of the kokanee salmon are consistent with general observations made at the lake for the last 25-30 years. Conceptual models of the lake ecosystem have been developed, and these are being used to guide the research and analyses. Studies are either underway or planned to examine the extent to which changing lake conditions can be accounted for by natural and human caused reasons. These studies include relationships between climate and fluctuating lake levels (water budget), chemical and nutrient budgets, interactions among nutrients, algae, zooplankton and fish, human-caused increases of nutrients from the atmosphere and on-site sources, paleolimnology, comparisons of modern vs. older sampling techniques and data sets, estimates of nutrient recycling and sedimentation, and evaluations of the potential role of hydrothermal systems to the ecology of the lake. The results will be used to evaluate whether the lake has experienced any extensive physical, chemical and biological changes in recent years, especially those which could affect lake clarity. II. Management Actions The Crater Lake Program consists of limnological research/monitoring and management activities directed at the caldera system. Primary management activities occurring during the last two years are summarized below. 1. Data management - All data are stored on computer files and hard copies. The data areas are located at the park and the Cooperative Park Studies Unit at Oregon State University. 2. Research/monitoring program - The two research boats are stored on Wizard Island in a boat house during winter. The
I. Executive Summary Limnological studies of Crater Lake were initiated by the National Park Service in 1982 in response to an apparent decline in lake clarity and changes in the phytoplankton community characteristics. In the fall of 1982, Congress mandated, through Public Law 97-250, a 10-year study of the lake. The goals of the study included: (1) development of a limnological data base for comparison with future conditions; (2) development of a better understanding among physical, chemical and biological components of the lake system; and (3) development of a long term monitoring program. An important aspect of the goals was to determine if the lake has experienced recent changes, and if present and human related in nature, identify the cause(s) and recommend mitigations. Project development has been directed toward an ecosystem approach. Studies include precipitation quantity and chemistry, lake level fluctuations, solar radiation, intracaldera spring chemistry, lake clarity, lake color, particle flux, water quality, chlorophyll, primary production, phytoplankton, zooplankton, bottom fauna, and fish. An extensive data base has been assembled for each aspect of the study. Results to date indicate that the oligotrophic (nutrient poor) lake system is dynamic and complex. The volume of the lake responds quickly to changes in precipitation because the closed lake basin (no surface outlet) serves as a leaky rain gauge. Although the level of the lake fluctuates seasonally, overall it has dropped about 2.0 m (6.6 ft.) since 1984. The lake becomes thermally stratified between July and September. The thermocline (interface between warmed surface waters and cold water in the deep lake) extends to a depth of about 80 m (262 ft.) Although the lake does not completely mix in winter, some deep mixing must occur as indicated by high concentrations of dissolved oxygen at the lake bottom. Secchi disk clarity is generally in the high 20 to mid 30 m (66-100 ft.) range. The depth of 1% of the incident light at the lake surface ranges from 80-100 m (262-328 ft.). Maximum photosynthesis typically occurs between 4 0 and 8 0 m (131-262 ft.). A deep-water chlorophyll maximum occurs between 100 and 120 m (328-394 ft.). In summer, a dense population of a species of diatom typically develops in the epilimnion (the warm layer above the thermocline). The rest of the sparse and complex phytoplankton community exists to a depth of about 200 m (656 ft.)- Most zooplankton species exist between 20 and 120 m (66-394 ft.). Rotifer and crustacean species are present. Rotifers are in high abundance. The largest crustacean species appears to be cyclic in abundance. Two species of fish exist in the lake. Both were stocked many years ago and now reproduce in the lake. Kokanee salmon are mostly pelagic and feed primarily on terrestrial insects at the lake surface and bottom fauna. Kokanee densities are cyclic due to the numerical dominance of one year class. Changes in rainbow trout densities are less variable due to the multiple age structure of the population. Comparisons with older data collected from Crater Lake, though sparse, indicate that there have been no major changes in the near-surface lake water quality for pH, conductivity and selected elements. Secchi disk clarity in August in recent years is about 25% shallower than in 1937 and 1969. Such a decrease in Secchi disk clarity can probably be explained by a small increase in the abundance of particles in the water column. Whether these changes in particle abundance are human related or naturally occurring is still unknown. Fluctuations in the surface elevation of the lake since 1982 have been similar to those in the past. The cyclic nature of the largest crustacean zooplankton species is consistent with results from studies conducted in the late 1960s. Population fluctuations of the kokanee salmon are consistent with general observations made at the lake for the last 25-30 years. Conceptual models of the lake ecosystem have been developed, and these are being used to guide the research and analyses. Studies are either underway or planned to examine the extent to which changing lake conditions can be accounted for by natural and human caused reasons. These studies include relationships between climate and fluctuating lake levels (water budget), chemical and nutrient budgets, interactions among nutrients, algae, zooplankton and fish, human-caused increases of nutrients from the atmosphere and on-site sources, paleolimnology, comparisons of modern vs. older sampling techniques and data sets, estimates of nutrient recycling and sedimentation, and evaluations of the potential role of hydrothermal systems to the ecology of the lake. The results will be used to evaluate whether the lake has experienced any extensive physical, chemical and biological changes in recent years, especially those which could affect lake clarity. II. Management Actions The Crater Lake Program consists of limnological research/monitoring and management activities directed at the caldera system. Primary management activities occurring during the last two years are summarized below. 1. Data management - All data are stored on computer files and hard copies. The data areas are located at the park and the Cooperative Park Studies Unit at Oregon State University. 2. Research/monitoring program - The two research boats are stored on Wizard Island in a boat house during winter. The