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Published April 26, 2018 | Supplemental Material
Journal Article Open

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


Biologically generated turbulence has been proposed as an important contributor to nutrient transport and ocean mixing. 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, 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. 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.

Additional Information

© 2018 Springer Nature Publishing. Received: 10 May 2014; Accepted: 12 March 2018; Published online: 18 April 2018. We gratefully acknowledge M. M. Wilhelmus for the original development of laboratory controlled migrations and for advice on development of the current facility; E. Meiburg for technical advice; and R. Strickler for assistance in development of the schlieren imaging system and use of his facilities for obtaining Fig. 4a. We also gratefully acknowledge funding from the US National Science Foundation (Grant 1510607). Author Contributions: I.A.H., J.O.D., J.R.K. and S.G.M. conceived the irreversible mixing experiments; I.A.H. and J.O.D. conceived the flow visualization experiments; I.A.H. conducted all experiments; all authors contributed to data analysis; I.A.H. and J.O.D. wrote the initial draft of the manuscript, and all authors contributed to manuscript revisions. Reporting summary: Further information on experimental design is available in the Nature Research Reporting Summary linked to this paper. Data availability: Density profile data are available at https://github.com/ihoughton/Houghton2018_data. All other data are available from the corresponding author upon reasonable request. The authors declare no competing interests.

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