Secchi disk and photometer estimates of light regimes in Alaskan lakes: Effects of yellow color and turbidity

Variations in underwater light regimes among 58 Alaskan lakes were indexed by Secchi disk (SD) transparency and by vertical attenuation coefficients ( K d ) and euphotic zone depths (EZD) derived from using a submarine photometer (SP) sensitive to photosynthetically active radiation (PAR). Lake‐specific ratios between turbidity (light scattering) and color (light absorption) explained 52% of the variation ( P < 0.0001) in K d × SD values, which ranged as a continuum between 0.52 and 3.83. A clear ‐ water median value of 1.86 (range, 1.13–3.26) is elevated by color to a median of 2.70 (range, 1.81–3.83), whereas turbidity reduces the median value of 0.93 (range, 0.52–2.56). EZD: SD, PAR at the SD depth, and PAR reflection (backscatter) also changed with the turbidity‐to‐color ratio. The nearly 10‐fold ranges between K d × SD (0.59–4.09) and EZD: SD (0.89–8.67) values taken from 35 studies on lakes, estuaries, and oceans could be explained by color and turbidity differences. Background attenuation from small amounts of color (<10 Pt units) and turbidity (<5 NTU) uncoupled SD and SP measurements from changes in Chl a, limiting their use as an index of trophic state. Changes in K d × SD can serve, however, as a useful index of system loading by turbid particulate material or organic color.