Crater Lake Limnological Studies Final Report
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.