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).

Figure 4. Kd versus depth for selected wavelengths in Crater Lake (20 August 2001) showing minimum values at surface and increasing values with depth to the deep chlorophyll maximum (DCM). Four wavebands and three instruments are compared (smoothed with 8 m running averages above 60 m). Narrow-band signals show parallel changes with depth while the PAR waveband increases near the surface (where highly-attenuated red wavelengths are detected). BSI instruments (PUV-2500, solid circles; PRR-800, solid triangles) give similar results to LICOR LI-1800uw (open circles) except for greater noise in the latter near the surface because of long scan times. The 320 nm curve for LI-1800uw data was limited to about 75 m because of instrument sensitivity but the PUV-2500 was only lowered to 63 m. The PRR-800 radiometer was not equipped with a 320 nm sensor during this comparison and the PUV-2500 PAR signal is not included for greater clarity. The optically-mixed depths correspond to the thermally mixed epilimnion (0–10 m, not shown). The lowest Kd,320 averages were 0.057 m-1 (PUV-2500, 10–18m) and 0.058 m-1 (LI-1800uw, 0–20 m).

Figure 5. Average Kd for three wavebands versus depth in Crater Lake (June–September 1996–2002, LI-1800uw data, N=15 dates, bars indicate +/- S.E.) showing the summer pattern with minimum Kd at the surface and maximum Kd at the deep chlorophyll maximum (DCM). Kd,blue was calculated for the irradiance waveband 400–500 nm while Kd,320 and Kd,380 were calculated using the average from 2 nm above to 2 nm below the central wavelength. The average value for Kd,320 near the surface (0–5 m) was 0.062 m-1.

Figure 6. Crater Lake Kd ratios from data in Figure 5 (June–September 1996–2002, LI-1800uw). Least squares regression for Kd,320:Kd,380 versus depth (m), y = -0.0031x + 2.32, r2=0.68. Average 0–10 m ratios: Kd,320:Kd,blue, 2.28 (SE=0.007), Kd,380:Kd,blue, 0.98 (SE=0.002), Kd,320:Kd,380 = 2.32 (SE = 0.012). Average 0–40 m ratios: Kd,320:Kd,blue = 2.52 (SE=0.058); Kd,380:Kd,blue = 1.11 (SE=0.029); Kd,320:Kd,380 = 2.27 (SE = 0.016).