Lake Tahoe Color: Why Algae Affects Its Cobalt Blue Water

Lake Tahoe is an ultra-oligotrophic lake with exceptionally low biological productivity compared with nutrient-rich lakes, and UC Davis researchers describe its open water as deep cobalt blue. To move beyond a visual description, researchers from UC Davis, NASA’s Jet Propulsion Laboratory and Université Laval developed a quantitative Blue Water Index, or Bw, from hyperspectral measurements of light leaving the lake.

The resulting study compared color with two very different measurements: phytoplankton concentration, estimated with in-water chlorophyll-a fluorescence, and Secchi depth, the traditional measure of water transparency. The Blue Water Index showed a strong inverse relationship with phytoplankton concentration, but it did not show a significant relationship with Secchi depth. That is the core reason Lake Tahoe can become more intensely blue without becoming clearer at the same time.

The hyperspectral record used in the published study showed that Blue Water Index values were highest in summer. The seasonal pattern reflects more than sunlight alone. Lake Tahoe’s physical structure changes through the year as mixing, stratification, upwelling and nutrient movement alter the conditions in which phytoplankton grow.

UC Davis describes spring upwelling as common from late May through mid-June, when wind can lift deeper, nutrient-rich water toward the surface while the lake is still weakly stratified. Before roughly mid-May, the lake is typically more vertically mixed; by late June, stronger stratification makes these upwelling events less common until fall storms return around October. Those physical transitions help explain why a single photograph or a single month cannot represent Tahoe’s color behavior for the entire year.

Lake Tahoe’s famous transparency is measured with a Secchi disk, while its blueness study used hyperspectral reflectance. Because the measurements respond to different parts of the lake’s optical environment, they should not be treated as interchangeable.

This last point matters because the older shorthand—“sediment controls clarity, algae controls blueness”—is useful but incomplete. The 2026 State of the Lake reporting notes that unusually high concentrations of tiny suspended particles were observed during summer 2025 and that microscopic algae, picoplankton and the small diatom Cyclotella may play a larger role in clarity than previously understood. The distinction between color and clarity remains valid, but modern monitoring is showing that biology can influence both.