Sustained viability of aphotic phytoplankton in Lake Tahoe (California‐Nevada)

Abstract

Biomass, chlorophyll a, and photosynthetic potential of phytoplankton were measured year‐round throughout a 450‐m water column of ultraoligotrophic Lake Tahoe. Phytoplankton biomass was uniform from the surface to the maximum depth while the lake mixed completely in March, but was lower in aphotic waters during thermal stratification. Aphotic chlorophyll a concentrations decreased progressively in fall and reached a minimum of 17% of mean euphotic concentrations in January, before the mixing depth exceeded the euphotic depth. Photosynthetic rates measured in an incubator under constant light conditions were significantly correlated with pigment concentrations in both euphotic and aphotic phytoplankton and demonstrated viability of the aphotic phytoplankton throughout the year.High surface : volume ratios of the small species dominating the phytoplankton assemblage suggest low sinking velocities. Extremely slow decomposition is postulated to explain the vertical pigment concentration gradients in the aphotic zone.Re‐entry of viable aphotic phytoplankton into the euphotic zone during deep mixing of the lake can diminish the dilution of euphotic phytoplankton, thereby increasing the magnitude of the subsequent vernal bloom.