Limnol. Oceanogr., 44(2), 1999, 282–294

Secchi depth has been measured in Lake Tahoe an average of every 12 d since July 1967. Because of the unusual clarity of the lake, Secchi depth measurement is responsive to small changes in light-attenuating particles, and the record exhibits strong variability at the seasonal, interannual, and decadal scales. Using recently developed methods of applied time-series analysis, the mechanisms of change were delineated at each scale. The seasonal pattern is a bimodal one, with two minima at approximately June and December. The June minimum is due mostly to cumulative discharge of suspended sediments following melting of the snowpack. The December minimum is probably a result of mixed-layer deepening as the thermocline passes through layers of phytoplankton and other light-attenuating particles that reach a maximum below the summer mixed layer. The interannual scale exhibits two modes of variability, one during the weakly stratified autumn–winter period and the other during the more stratified spring– summer period. The first mode is a result of variable depth of mixing in this unusually deep lake, while the second results from year-to-year changes in spring runoff. A decadal trend also exists (20.25 m yr21), resulting from accumulation of materials in the water column. It is not yet understood, however, how much of this change is due to phytoplankton or recent phytoplankton-derived materials and how much is due to other materials such as mineral suspensoids. Based on the available measurements and physical considerations, both categories may play a significant role. Optical clarity is important in making judgments about water quality. Clarity is the first and perhaps the only water quality parameter that many casual observers encounter. Moreover, clarity is often used by the layperson as a basis for judging potability as well as the safety of water contact (Smith et al. 1995). In both freshwater and marine oligotrophic water bodies, optical clarity can also be an important aesthetic characteristic. Certain lakes have an exceptionally high optical clarity. Several different reasons can account for low concentrations of dissolved and particulate materials (Hakansson 1995). The same characteristics that lend uniqueness to these lakes in the form of transparency and color, however, also render them unusually susceptible to small changes in dissolved and suspended matter. The Secchi disc is commonly used to measure these changes in visual clarity. The Secchi depth, ZS, or the depth at which the disc disappears from the view of a surface observer, is inversely proportional to the beam attenuation coefficient for luminance, c, plus the attenuation coefficient for illuminance, K: