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Bacteria‐induced mixing in natural waters
Journal article   Open access   Peer reviewed

Bacteria‐induced mixing in natural waters

T Sommer, F Danza, J Berg, A Sengupta, G Constantinescu, T Tokyay, H Bürgmann, Y Dressler, O Sepúlveda Steiner, C. J Schubert, …
Geophysical research letters, Vol.44(18), pp.9424-9432
09/28/2017
DOI: 10.1002/2017GL074868
url
https://doi.org/10.1002/2017GL074868View
Published (Version of record) Open Access

Abstract

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 1.2 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. Plain Language Summary Small aquatic organisms (length < ≈1 cm) 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 10 μm long bacterium Chromatium okenii causes mixing in the Alpine Lake Cadagno (Switzerland). The observed mixed layer is 0.3 to 1.2 m thick and located at around 12 m depth. We suggest that bioconvection may influence the composition of organisms in natural waters and affect large‐scale phenomena like algal blooms. Key Points We report the first observation and quantification of bacteria‐induced mixing in a stratified natural water body The observed mixed layer is 0.3 to 1.2 m thick and located at 12 m depth in an Alpine Swiss Lake Bioconvection may be a common but overseen phenomenon in natural waters and relevant in algal bloom dynamics.
 Bioconvection Biogenic mixing Cadagno Chromatium okenii Microstructure

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