Persistence of bioconvection‐induced mixed layers in a stratified lake

In situ observations of biophysical interactions in natural waters typically focus on physical mechanisms influencing biological activity. Yet, activity can also drive processes aquatic environments. A community photoautotrophic, motile and heavy bacteria—Chromatium okenii, which requires light, sulfide, anoxic conditions to perform anoxygenic photosynthesis, accumulates below the chemocline meromictic Lake Cadagno (Switzerland). Upward vertical migration drives bioconvection, modifies environment bacteria-populated water create a deep homogeneous mixed layer up 1 m thickness. Continuous convection within diapycnal diffusivity from its adjacent stratified surroundings determine ecologically relevant gradients. The daytime that induce convective instabilities is well-established. It consists bacteria swimming upward towards light accumulating at upper part layer, leading locally-unstable density excess. However, nocturnal has not yet been analyzed. An intensive 48-h survey was conducted August 2018 using standard microstructure profilers, as well moored high-resolution current meter coupled with temperature turbidity sensors deployed across depth. This revealed persistent during nighttime hours. Using shape model, velocity turbulent dissipation estimates, we conclude photoautotrophic continue their night. thereby “dark bioconvection” maintains subsurface bacterial throughout diel cycle.