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) x 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) x 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) x SD can serve, however, as a useful index of system loading by turbid particulate material or organic color.