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Swimming organisms can enhance mixing in their natural environments by creating eddies in their wake and by dragging water along. However, these mixing mechanisms are inefficient for microorganisms, because swimming-induced variations in velocity, temperature, and dissolved substances are evened out before they can be advected. In bioconvection, however, microorganisms induce water movement not by propulsion directly but by locally changing the fluid density, which drives convection. Observations of bioconvection have so far mainly been limited to laboratory settings. We report the first observation and quantification of bioconvection within a stratified natural water body. Using in situ measurements, laboratory experiments, and numerical simulations, we demonstrate that the bacterium Chromatium okenii is capable of mixing 0.3 to 1.2m thick water layers at around 12m water depth in the Alpine Lake Cadagno (Switzerland). As many species are capable of driving bioconvection, this phenomenon potentially plays a role in species distributions and influences large-scale phenomena like algal blooms.

Plain Language Summary Small aquatic organisms (length < approximate to 1cm) do not efficiently mix water by swimming, because they are too small and swim too slowly to create whirls that lead to mixing. In bioconvection, however, small organisms (that are denser than water and, on average, swim upward) can mix water. When such organisms accumulate locally in a layer, the density of the water increases. This layer of heavier water on top of lighter water sinks and mixes with the surrounding water. Continuous upward swimming provides the energy to maintain the water motion. Bioconvective mixing has been observed for a wide range of species, but so far mainly in the laboratory. We report the first observation of bioconvection in a natural water body and show that the only 10m long bacterium Chromatium okenii causes mixing in the Alpine Lake Cadagno (Switzerland). The observed mixed layer is 0.3 to 1.2m thick and located at around 12m depth. We suggest that bioconvection may influence the composition of organisms in natural waters and affect large-scale phenomena like algal blooms.