Ultraviolet Radiation and Bio-optics in Crater Lake, Oregon, 2005
Current interest in ultraviolet radiation (UVR) in Crater Lake, Oregon, follows naturally from its well-known transparency (Larson, 2002) and the greater incident UVR and lake transparency found at high elevations (Laurion et al., 2000; Sommaruga, 2001). Crater Lake is famous for its depth (594 m, Bacon et al., 2002), visual clarity, and deep blue color. It is subalpine in elevation at 1800 m yet it rarely freezes during winter; it has an average radius of 4.1 km and is enclosed by a volcano’s caldera, whose steep walls shelter it from strong winds and limit hydraulic exchange largely to snowmelt, direct precipitation, evaporation, and out-seepage (Redmond, 1999). Optical studies of water transparency in Crater Lake began in 1896 when a white dinner plate was lowered into the water until it disappeared at 30 m (Diller, 1897, cited in Larson et al., 1996a). Black and white Secchi disk measurements were made sporadically since then until the late 1970s, when regular measurements began. An underwater photometer equipped with colored filters and matching deck cell was used to characterize water transparency in 1940 (Utterback et al., 1942) and again in the late 1960s (Larson et al., 1996a; Larson, 2002). Concerns about pollution and degraded water quality in the 1970s led to an improved sewage disposal at the tourist facilities in 1975 and the complete removal of sewage in 1991 (Larson 2002), and to a federally funded monitoring program beginning in 1983 (Larson et al., 1996a; Larson, 2002). Routine optical measurements with modern instruments began in 1987 with a beam transmissometer (25 cm, 660 nm beam) attached to an automated CTD profiler (conductivity, temperature, depth); a UV scanning radiometer was added in 1996, and a chlorophyll fluorescence sensor was added in 1999.
Crater Lake is not only visually clear; it is also remarkably transparent to solar UVR (280–400 nm). In the 1930s Crater Lake water was examined to determine scattering of UVR in a laboratory comparison with purified water and deep ocean water (Pettit, 1936), only a decade after the first measurements of solar ultraviolet radiation (UVR) in the atmosphere using a photoelectric sensor (Coblentz & Stair, 1936), and more than a decade before the penetration of solar UV-B radiation (280–320 nm) into neutral waters was reported using a photoelectric sensor (Johnson [Jerlov], 1946; Jerlov 1950; Højerslev 1994). Crater Lake underwater UVR was first investigated in the 1960s. Tests of newly designed scanning underwater radiometers were the basis for a series of summer measurements in Crater Lake during 1964–69 (Tyler 1965; Smith & Tyler, 1967; Tyler & Smith, 1970; Smith et al., 1973). Spectral measurements from 1969 in Crater Lake were also compared with similar measurements in Lake Tahoe, another clear deep lake (Smith et al., 1973). These authors and Pettit (1936) observed similarities between the upper 20 m of Crater Lake water and highly purified water and noted the importance of scattering of short wavelengths to the color of Crater Lake. In an effort to characterize spectral absorption and attenuation by pure water from data published to date, Smith and colleagues used the 1967 Crater Lake optical measurements (360–700 nm) as the basis for equating its transparency to the clearest waters of the Sargasso Sea (Smith & Tyler, 1976) and established absorption and attenuation spectra (200–800 nm) for pure freshwater and seawater that would be used for many years (Smith & Baker, 1981). Morel & Prieur (1977) used Crater Lake reflectance data in an assessment of scattering and absorption in both pure and natural waters to infer the content of ocean water by remote sensing.