Crater Lake Limnological Studies Final Report
Abstract
Limnological studies of Crater Lake were initiated by the National Park Service in 1982 in response to an apparent decline in lake clarity and possible changes in characteristics of the algal community. Congress passed Public Law 97- 250 in the fall of 1982, which authorized and directed the Secretary of the Interior to conduct a 10-year limnological study of Crater Lake and to immediately implement such actions as may be necessary to retain the lake’s natural pristine water quality. The broad project goals adopted for the study included: (1) develop a limnological data base to be used for comparisons of future conditions of the lake; (2) develop a better understanding of physical, chemical and biological components of the lake system; (3) develop a long-term monitoring program; (4) determine if the lake had experienced recent changes, and if changes were present and human related; (5) identify the causes and recommend ways of mitigating the changes.
An ecosystem approach was used to develop the program. Conceptual models of the lake ecosystem were developed and used to guide research and analyses. Studies included quantity and chemistry of precipitation, lake-level fluctuations, solar radiation, chemistry of intra-caldera springs, lake clarity, lake color, lake chemistry, particle flux, chlorophyll, primary production, phytoplankton, zooplankton, bottom fauna and flora, and fish. An extensive data base was assembled for each aspect of the study.
Crater Lake was found to be a complex, dynamic, and oligotrophic (nutrient- poor) system. The volume of the lake responded quickly to changes in precipitation because the basin has no surface outlet. Water leaves the lake through seepage and evaporation. Although the lake level normally fluctuates about 0.5 m annually, the lake surface dropped about 3 m in elevation between 1984 and 1992. The lake was relatively high in dissolved salts, total alkalinity, and conductivity; pH ranged between 7 and 8. Hydrothermal fluids from the lake bottom contributed to the relatively high salt content of the lake. Phosphorus and nitrate were low in concentration, although the concentration of the latter increased substantially below a depth of 200 m. On an annual basis, atmospheric bulk deposition accounted for about 90% of the nitrogen and 30% of the phosphorus input to the lake. Recycling of nutrients was important to the internal nutrient budget of the lake.
Wind-driven circulation mixed the lake in winter and spring to a depth of about 200 m. Some deep-water mixing was indicated by high concentrations of dissolved oxygen at the lake bottom. The lake was thermally stratified in summer and fall. The interface between the warmed surface waters and the cold waters of the deep lake was at a depth of about 80 m.
Secchi disk clarity generally was in the high-20-m to mid-30-m range. The depth of 1% of the incident surface light generally was between 80 and 100 m. Seasonal changes in Secchi disk readings and the depth of 1% incident light were observed. In summer, a layer of near-surface turbidity was associated with changes in Secchi disk clarity. Lake color measurements indicated that the near-surface water was very blue.
Water chemistry of the caldera inlet springs exhibited a wide range of chemical concentrations and total ionic compositions over short distances around the perimeter of the lake. Calcium, magnesium, and sodium were the major cations; bicarbonate was the major anion. Contribution of nitrates to the lake from the springs was specifically studied because of concerns about a sewage drain field for visitor facilities located just outside the caldera wall. One spring located on the caldera wall near the drain field system exhibited relatively high nitrate concentrations but contributed less than 1% of the total annual input of new nitrate into the lake. Although an analysis of the water chemistry of the spring could not confirm the source of the nitrates, the drain field was removed in 1991 as a precautionary measure.
Chlorophyll, phytoplankton, and zooplankton were uniformly distributed in winter and spring from the lake surface to the depth of mixing (maximum depth about 200 m), and maximum primary production occurred between 40 and 60 m. A deep-water chlorophyll maximum developed between 100 and 140 m in summer and fall, and maximum primary production typically occurred between 40 and 80 m. About 96% of total primary production was associated with nutrients recycled in the euphotic zone. A sparse but complex phytoplankton community partitioned the water column to a depth of 200 m. A high density of phytoplankton typically developed in the warm near-surface waters. Cyclic seasonal and annual changes in chlorophyll, primary production, and phytoplankton density were observed. Periods of upwelling of nutrient-rich waters from the deep lake were thought to influence the productivity of the lake.
In summer and fall the zooplankton community, which was comprised of eight rotifer species and two species of crustaceans, partitioned the water column to a depth of 200 m. Zooplankton abundance in the upper 20 m of the water column was very low. Highest densities of zooplankton were located in the depth interval of 80 to 180 m. Closely related or competing species were found in different portions of the water column. The largest crustacean species was cyclic in abundance, and its abundance was related to lake productivity and fish predation. When it was abundant, rotifer abundances declined, and changes in the distribution of the other crustacean species were observed.
Two species of fish, rainbow trout and kokanee salmon, continued to persist in the lake. Both species were stocked many years ago, continued to reproduce in the lake, and had long-term effects on the lake system. Kokanee salmon mostly were pelagic and fed primarily on crustacean zooplankton and small-bodied bottom fauna Abundance of kokanee was cyclic owing to the numerical dominance of one year class. Rainbow trout were found along the littoral zone of the lake and fed on terrestrial insects at the lake surface, large-bodied bottom fauna, and kokanee.
Benthic macroinvertebrate richness was moderate in Crater Lake and comparable to the richness found in other large, cold, oligotrophic lakes in the northern hemisphere. Densities of epibenthic macroinvertebrates on rocky substrates in the littoral zone were relatively high. Most taxa in the littoral zone were types common to streams and rivers in montane areas of western North America Snails were common to a depth of 100 m. Oligocheata worms and chironomid midges were common in the deep lake.
A new species of aquatic mite, Algophagopsis sp., was found in the lake. Crater Lake remains the only known local for this species. The mite was abundant on rock surfaces in association with aquatic lichen and Nostoc in the main lake, on filamentous algae in Emerald pool located on Wizard Island, and on the deep-water moss, Drepanocladus aduncus, with the deepest collection from 118 m.
Beds of macrophytes were found on some of the sand-gravel benches around the perimeter of the lake. Drepanocladus aduncus was present in dense beds in the lake in the depth interval of 30 to 120 m. Several species of diatoms were associated with the moss. Periphyton was collected from many sites around the margin of the lake, as well as from depths of 120 m or more.
Comparisons of limnological data collected prior to the study with data collected during the study did not reveal any major long-term changes in the near-surface water quality of the lake. Hydrothermal inputs were responsible for the stable concentrations of dissolved salts through time. The analysis of Secchi disk records collected between two time intervals, 1913-1969 and 1978-1991, suggested that the data sets were fairly comparable. However, this finding was insufficient to summarily dismiss the possibility of subtle long-term change to the lake. Changes in nutrient input from the atmosphere and potential local sources of nutrients may have some long-term roles to play in the productivity and clarity of Crater Lake. It remained difficult to separate the natural variability of the Secchi disk readings from any changes that may have resulted from human-related activities. Disk readings in the range of 3940 m, which were recorded in August of 1937 and 1969, were not repeated in readings taken in August from 1978 through 1991. However, readings of 37 m and 39 m were recorded in July of 1985 and June of 1988, respectively. The absence of extremely deep Secchi disk readings during this study may have been a sign of change, but a 335 m reading in August 1954, the only bona-fide August Secchi disk reading between 1937 and the late 1960’s, illustrated the problem of separating the natural dynamics of lake clarity from any long-term decreases in clarity.
In general, the Crater Lake ecosystem was extremely responsive and sensitive to environmental change and was judged to be pristine, except for the consequences of fish introductions. The study documented many of the components and processes important to lake clarity and the lake system as a whole. Knowledge of the relative importance of these components and processes was high in many instances, although the level of knowledge of any one of the complex features tended to be low to moderate. The study also identified many questions needing further study. Long-term change could not be fully evaluated because very little historical data was available to compare with the detailed data base assembled during this study. This situation underscored the need for a long-term monitoring program to evaluate future change against the benchmark set in the 10-year study. Global climate change, air pollution, on-site auto and boat use, and non-native fish present the greatest potential human-related threats to the pristine nature of Crater Lake. Additional studies would refine knowledge of the components and dynamic processes of the lake system as well as separate changing lake conditions caused by natural phenomena from those caused by human-related activities.
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