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

Abstract

Biologically generated turbulence has been proposed as an important contributor to nutrient transport and ocean mixing1,2,3. 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 involved4, 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 metres3,5,6. 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 ocean7.

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Fig. 1: Laboratory experiment for controllable vertical migrations.
Fig. 2: Density profile temporal evolution.
Fig. 3: Effective diffusivity due to vertical migration.
Fig. 4: Flow visualization from animal to aggregation scales.

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Acknowledgements

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).

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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.

Corresponding author

Correspondence to John O. Dabiri.

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The authors declare no competing interests.

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Supplementary information

Video 1: Vertical migration video A 10x playback speed video of vertical migration in the 2-meter-tall tank, started approximately 2 minutes after activation of the blue laser

Life Sciences Reporting Summary

Video 2: Schlieren imaging video: single animal The schlieren imaging technique is used to visualize density gradients around a freely swimming animal in the 10 cm x 5 cm x 15 cm tank. The field of view is approximately 1.5 cm across and video playback is real time. The single swimmer produces a notable rearward jet, despite the very high buoyancy frequency of N ≈1.3 s−1 which causes the displaced fluid to quickly re-stratify

Video 3: Schlieren imaging video: migration The schlieren imaging technique is used to visualize density perturbations near a vertical migration. The field of view is approximately 5 cm across and video playback is at 10x speed. The field of view is centered approximately 8 cm to the left of the center of the migration, where the animal migration appears less coordinated

Video 4: Tracer particles within swarm video A red laser sheet was used to illuminate 10 micrometer neutrally buoyant particles in a plane through the center of the migration. The field of view is approximately 5 cm vertically. Video playback is at 4x speed

Video 5: Large-scale tracer particle overlay video A set of 250 consecutive images, corresponding to 4.2 seconds of video, was used to create each new frame and shifted 30 images forward in time for each subsequent frame. This resulted in a video progressing at 7.5x speed, illustrating the large-scale eddies propagating laterally outward from the central migration. The field of view is 40 cm vertically and 22 cm horizontally

Video 6: Planar laser induced fluorescence video A 10x playback speed video of planar laser induced fluorescence obtained in the 1.2-meter-tall tank with an interface buoyancy frequency of N int = 0.05 s−1. Video was obtained after a 120-minute mixing experiment, so the tank was stably stratified over the entire vertical extent of the tank (i.e. profile in Fig. 2b ’final’)

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Houghton, I.A., Koseff, J.R., Monismith, S.G. et al. Vertically migrating swimmers generate aggregation-scale eddies in a stratified column. Nature 556, 497–500 (2018). https://doi.org/10.1038/s41586-018-0044-z

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