Thermal, chemical, and optical properties of Crater Lake, Oregon

Abstract

Crater Lake covers the floor of the Mount Mazama caldera that formed 7700 years ago. The lake has a surface area of 53 km 2 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

Chemical and thermal stratification in lakes

“…For example, in Lake Toba in Indonesia, thermal and chemical stratification conditions are totally different from each other (International Lake Environment Committee 1990). Thermal and chemical stratification are being studied in lakes and wetlands (Rimmer et al 2005;Talling 2006;Michael et al 2006;Larson et al 2007;Elçi 2008). In the case of lakes, considerable data obtained by surveying vertical profiles are essential, as widely used stratification indices are calculated by detailed data.…”

Long-term limnological research and monitoring at Crater Lake, Oregon

“…The present work suggests that the lake remains in pristine conditions and, within the limits of the methods used, we are not able to identify any changes in general water quality responding to human activities over the period of observation, although potential impacts on the ecosystem from introduced fishes remain uncertain (Larson et al, 2007b). The lake is usually isothermal in winter except for a slight increase in temperature in the lower depths of the lake from ongoing geothermal inputs (see also Crawford and Collier, 2007).…”

“…Secchi disk clarity readings varied from 18.1 m (July 1995) to 41.5 m (June 1997), with an average reading of about 30 m. Light transmission readings (25 cm beam transmissometer at 660 nm) throughout the water column between June and September typically range between 88.8 and 90.7%, relative to air (pure water at 91.3%). The depth of 1% of incident solar radiation during thermal stratification varies annually between 80 an 100 m. No long-term changes in the optical measurements were observed (see Larson et al, 2007bHargreaves et al, 2007b.…”

“…As part of their discussion of the time series of water quality properties, Larson et al (2007b) summarized the substantial body of Secchi disk clarity measurements. This included the large number of measurements taken since 1983 and all the related data collected over the previous century.…”

Exceptional visual clarity and optical purity in a sub‐alpine lake

“…Spring freshwaters, such as Pupu Springs (Te Waikoropupu; Davies-Colley and Smith 1995), are often of high optical purity because of efficient removal of light-scattering fine particles within aquifers. A very few volcanic crater lakes are frequently cited for their optical purity, notably Crater Lake, Oregon, USA (Larson 1972;Smith et al 1973;Larson et al 2007). More typically, nutrients promote phytoplankton growth in lakes, while rivers are usually less clear again because of suspension of particles by continual water movement (Davies-Colley et al 2003;Julian et al 2008).…”