Bacteria-induced mixing in natural waters

Sommer, T and Danza, Francesco and Berg, Jasmin and Sengupta, A and Costantinescu, G and Tokyay, T and Bürgmann, H and Dressler, Y and Sepúlveda Steiner, O.R. and Schubert, Carsten and Tonolla, Mauro and Wüest, A (2017) Bacteria-induced mixing in natural waters. Geophysical Research Letters. (In Press)

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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 simulations, we demonstrate that the bacterium Chromatium okenii is capable of mixing 0.3- to 1.2-m-thick water layers at around 12-m-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.

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