Figure 1B. Attenuation depths (10% and 1%) from spectral irradiance in Figure 1A. For PAR, Z10%=50m, Z1%=100 m, Z0.1%=140 m. For UV-B (320 nm), Z10%=37 m, Z1%=62 m. Polynomial regressions fitted to the points: Z10% = 4.9534E-07?WL4 – 7.6076E-04?WL3 + 4.3388E-01?WL2 – 1.0855E+02?WL1 + 1.0082E+04; Z1% = 8.2806E-07?WL4 – 1.2583E-03?WL3 + 7.0974E-01?WL2 – 1.7559E+02?WL1 + 1.6128E+04.

Figure 2A. Crater Lake upwelling irradiance, 20 August 2001 (Biospherical Instruments PRR-800 reflectance profiler lowered inverted, units ?W cm-2 nm-1). Depth (m) is indicated in the legend. Peak near 400 nm is source of deep blue color when viewed from above. Red peak is fluorescence from phytoplankton (683 nm).

Figure 2B. Crater Lake radiance reflectance, 20 August 2001 (Biospherical Instruments PRR_800, data binned at 1m intervals). Near the surface the spectrum is similar to upwelling irradiance. At greater depths the reflectance in UV and blue wavelengths declines because absorbance coefficients increase with depth faster than backscatter coefficients. At longer wavelengths the signal rises with increasing depth because Raman scatter and phytoplankton fluorescence (Chlorophyll-a at 683 nm and for 120 & 150 m depths, possibly phycoerythrin at 589 nm) increase rapidly in the upwelling radiance signal relative to downwelling irradiance.