Thermal,chemical, and optical properties of Crater Lake,Oregon |
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Authors: | Gary L Larson Robert L Hoffman David C McIntire Mark W Buktenica Scott F Girdner |
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Institution: | (1) USGS Forest and Rangeland Ecosystem Science Center, 3200 SW Jefferson Way, Corvallis, OR 97331, USA;(2) USGS Forest and Rangeland Ecosystem Science Center, 777 NW 9th Street, Suite 400, Corvallis, OR 97330, USA;(3) Department of Botany and Plant Pathology, Oregon State University, Corvallis, OR 97331, USA;(4) Crater Lake National Park, Crater Lake, OR 97604, USA |
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Abstract: | Crater Lake covers the floor of the Mount Mazama caldera that formed 7700 years ago. The lake has a surface area of 53 km2 and a maximum depth of 594 m. There is no outlet stream and surface inflow is limited to small streams and springs. Owing
to its great volume and heat, the lake is not covered by snow and ice in winter unlike other lakes in the Cascade Range. The
lake is isothermal in winter except for a slight increase in temperature in the deep lake from hyperadiabatic processes and
inflow of hydrothermal fluids. During winter and spring the water column mixes to a depth of about 200–250 m from wind energy
and convection. Circulation of the deep lake occurs periodically in winter and spring when cold, near-surface waters sink
to the lake bottom; a process that results in the upwelling of nutrients, especially nitrate-N, into the upper strata of the
lake. Thermal stratification occurs in late summer and fall. The maximum thickness of the epilimnion is about 20 m and the
metalimnion extends to a depth of about 100 m. Thus, most of the lake volume is a cold hypolimnion. The year-round near-bottom
temperature is about 3.5°C. Overall, hydrothermal fluids define and temporally maintain the basic water quality characteristics
of the lake (e.g., pH, alkalinity and conductivity). Total phosphorus and orthophosphate-P concentrations are fairly uniform
throughout the water column, where as total Kjeldahl-N and ammonia-N are highest in concentration in the upper lake. Concentrations
of nitrate-N increase with depth below 200 m. No long-term changes in water quality have been detected. Secchi disk (20-cm)
clarity varied seasonally and annually, but was typically highest in June and lowest in August. During the current study,
August Secchi disk clarity readings averaged about 30 m. The maximum individual clarity reading was 41.5 m in June 1997. The
lowest reading was 18.1 m in July 1995. From 1896 (white-dinner plate) to 2003, the average August Secchi disk reading was
about 30 m. No long-term changes in the Secchi disk clarity were observed. Average turbidity of the water column (2–550 m)
between June and September from 1991 to 2000 as measured by a transmissometer ranged between 88.8% and 90.7%. The depth of
1% of the incident solar radiation during thermal stratification varied annually between 80 m and 100 m. Both of these measurements
provided additional evidence about the exceptional clarity of Crater Lake. |
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Keywords: | Crater Lake Water quality Secchi disk Optical properties |
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