Origins and scale dependence of temporal variability in the transparency of Lake Tahoe, California–Nevada

Abstract

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 springsummer 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 (Ϫ0.25 m yr Ϫ1 ), 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.

Effects of climate change on thermal properties of lakes and reservoirs, and possible implications

“…This impact is greatest in years following heavy stream runoff, and is prolonged by an absence of deep-water mixing events (Jassby et al 1999). Following mixing, the fine particles suspended at the upper layer are dispersed throughout the water column.…”

Climate change in the Tahoe basin: regional trends, impacts and drivers

“…The former is influenced by the timing of snowmelt and storm runoff, and by the suspended sediment concentration, and the latter by the seasonal cycle of warming, stratification and mixing. Additional modeling may help to sort out the interaction between stream runoff, lake thermal structure and water clarity, but it seems likely that increased inflow of fine sediment (from more intense rainfall, drought and wildfires) will interact with increasing thermal stability to prolong the period of reduced water clarity that follows periods of heavy runoff (Jassby et al 1999). …”

Lake Tahoe watershed assessment: volume I

“…In earlier synoptic studies of lake primary productivity, Goldman found the Index Station to represent whole lake conditions (Goldman 1974). Scientific data shows that Secchi depth is directly related to the amount of suspended matter in the water (Jassby et al 1999). This suspended matter is composed of both biotic materials and suspended inorganic silt or sediment Extensive research on the spatial distribution of free-floating algae indicates a marked correspondence between the highest algal growth rates and the most extensive shoreline development.…”

“…Most commonly, clarity is expressed as a Secchi depth: the depth at which an eight-inch white disk is no longer visible from the surface as it is lowered into a waterbody. Regular measurements at the UC Davis/Tahoe Research Group Index Station began in 1967 and have been made on average every 12.2 days since then (Jassby et al 1999). In earlier synoptic studies of lake primary productivity, Goldman found the Index Station to represent whole lake conditions (Goldman 1974).…”

“…Much of the strategy for lake/watershed management has come about in response to longterm decline in clarity and increase in algal growth rate since the 1960s (Goldman 1988;Jassby et al 1999). For decades, planning, regulatory, and implementation actions have focused on controlling nutrient and sediment inputs to the lake.…”