Determining long-term water quality change in the presence of climate variability: Lake Tahoe (USA)

We developed a time series model of Secchi depth for Lake Tahoe incorporating a mechanistic understanding of interannual variability with sufficient simplicity to allow data-based parameter estimation. Secchi depth is still occasionally over 40 m at Lake Tahoe, but mean annual Secchi depth has declined nearly 10 m since 1967, prompting a large-scale restoration program. Year-to-year variability is extremely high, obscuring restoration actions and compliance with water quality standards. The model focused on Secchi depth during summer, when the lake is least transparent and most heavily used. Interannual variability for the summer season is driven largely by precipitation differences. The model offers a means for determining compliance with water quality standards when precipitation anomalies may persist for years. As demonstrated by means of an ex-post forecast, increasing Secchi depths during 1999-2001 were simply climate-driven and do not represent a recovery of the lake. The long-term trend for summer is most likely due to the accumulation of allochthonous mineral suspensoids.