Figure 3. Crater Lake spectral diffuse attenuation from spectral irradiance in Figure 1, plus an average for 10–18 m depth calculated from profiles by PRR-800 and PUV-2500 instruments from Biospherical Instruments, Inc. (BSI). LI-1800uw Kd was calculated for each depth interval and then averaged for a range of depths (error bars are +/-S.E. of mean for depth ranges included in mean). BSI Kd was calculated for 10–18 m from polynomial regressions of Ed,Z vs depth to reduce noise from surface reflections. Note that the minimum Kd was at 412 nm. Values for wavelengths longer than 560 nm are excluded below 40 m because of Raman scatter artifact described in Figure 2B. Shorter UV wavelengths are excluded at depths below 55 m because of instrument detection limits. The lowest Kd,320 value was 0.057 m-1 (PUV-2500, 10–18m).