[OANNES Foro] Vertically migrating swimmers generate aggregation-scale eddies in a stratified column

[Mario Cabrejos]

Nature volume 556, pages497-500 (2018)

doi:10.1038/s41586-018-0044-z

Published: 18 April 2018


 


Vertically migrating swimmers generate aggregation-scale eddies in a
stratified column


 
<https://www.nature.com/articles/s41586-018-0044-z?spMailingID=56435783&spUs
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Isabel A. Houghton,
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Jeffrey R. Koseff,
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Stephen G. Monismith &
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John O. Dabiri


Abstract


Biologically generated turbulence has been proposed as an important
contributor to nutrient transport and ocean mixing
<https://www.nature.com/articles/s41586-018-0044-z?spMailingID=56435783&spUs
erID=MjA1NTE2ODQxMAS2&spJobID=1382845054&spReportId=MTM4Mjg0NTA1NAS2#ref-CR1
> 1,
<https://www.nature.com/articles/s41586-018-0044-z?spMailingID=56435783&spUs
erID=MjA1NTE2ODQxMAS2&spJobID=1382845054&spReportId=MTM4Mjg0NTA1NAS2#ref-CR2
> 2, <https://www.nature.com/articles/s41586-018-0044-z#ref-CR3> 3. However,
to produce non-negligible transport and mixing, such turbulence must produce
eddies at scales comparable to the length scales of stratification in the
ocean. It has previously been argued that biologically generated turbulence
is limited to the scale of the individual animals involved
<https://www.nature.com/articles/s41586-018-0044-z#ref-CR4> 4, which would
make turbulence created by highly abundant centimetre-scale zooplankton such
as krill irrelevant to ocean mixing. Their small size notwithstanding,
zooplankton form dense aggregations tens of metres in vertical extent as
they undergo diurnal vertical migration over hundreds of metres
<https://www.nature.com/articles/s41586-018-0044-z#ref-CR3> 3,
<https://www.nature.com/articles/s41586-018-0044-z#ref-CR5> 5,
<https://www.nature.com/articles/s41586-018-0044-z#ref-CR6> 6. This
behaviour potentially introduces additional length scales-such as the scale
of the aggregation-that are of relevance to animal interactions with the
surrounding water column. Here we show that the collective vertical
migration of centimetre-scale swimmers-as represented by the brine shrimp
Artemia salina-generates aggregation-scale eddies that mix a stable density
stratification, resulting in an effective turbulent diffusivity up to three
orders of magnitude larger than the molecular diffusivity of salt. These
observed large-scale mixing eddies are the result of flow in the wakes of
the individual organisms coalescing to form a large-scale downward jet
during upward swimming, even in the presence of a strong density
stratification relative to typical values observed in the ocean. The results
illustrate the potential for marine zooplankton to considerably alter the
physical and biogeochemical structure of the water column, with potentially
widespread effects owing to their high abundance in climatically important
regions of the ocean
<https://www.nature.com/articles/s41586-018-0044-z#ref-CR7> 7.

 



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