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Oceanography

A fishy mix

Ocean life is in almost constant motion, and such activity must surely stir things up. Innovative investigations into this concept of 'biogenic mixing' show a role for jellyfish and their brethren.

Well over 100 years ago, scientists speculated that the ocean was driven by the Sun, the Moon, Earth's rotation and the combined motion of all the fishes' tails (M. G. Briscoe, personal communication). Since then, there has been only sporadic interest in the idea that swimming fish and other organisms help to stir the ocean. But, recently, the concept has experienced a resurgence of interest, and the latest example of thinking on the topic appears in the paper by Katija and Dabiri on page 624 of this issue1. They offer a new angle: “a viscosity-enhanced mechanism for biogenic ocean mixing”.

The broader context here is that the amounts of heat and carbon stored by the ocean dwarf those held by the atmosphere, and that to understand climate change it is essential to understand the processes that affect ocean–atmosphere exchange. The studies involved have led to some counter-intuitive results. First, although ocean properties with length scales of thousands of kilometres matter most to climate, they are sensitive to mixing processes on scales of a few centimetres — think of the way you stir cream into your morning coffee; similarly, tiny whorls mix in the ocean. Second, beneath the surface, starting only about one football field deep, the ocean is a very quiet mixing environment. Roughly speaking, all the energy needed to mix a cubic kilometre of subsurface ocean could be provided by a single hand-held kitchen mixer. Through a remarkable interplay of length and timescales, very weak and small-scale mixing helps to set our climate and affects the burial of atmospheric carbon in the ocean.

It was the venerable Walter Munk2 who, in 1966, attempted to quantify the effect that activity in the ocean biosphere might have on ocean mixing. His result neither strongly dismissed nor supported the idea, but for decades afterwards conventional wisdom held that fish could be ignored in ocean mixing. It often happens in science that a flurry of unconnected activities on a common topic emerge almost simultaneously, however, and such has been the case for biogenic ocean mixing.

In 2004, Huntley and Zhou3 pointed out that the expected levels of turbulence in schools of fish are comparable to those associated with storms. In a subsequent paper, my colleagues and I argued4 that the kinetic energy expended by the biosphere is sizeable compared with global mixing requirements; we further suggested that the true swimmers (fish), when all lumped together, provide about half of the biosphere input, with the balance coming from zooplankton. Exciting, direct confirmation of hugely elevated turbulence levels in vertically migrating shrimp-like animals followed from Kunze and colleagues5.

A major question is how efficiently biogenic turbulence actually mixes the ocean. The answer hinges on length scales. Very small whorls introduced into a fluid will be quickly damped by friction, and thus will not mix the fluid. To illustrate, consider a tall coffee cup with a slight gradient in creaminess from top to bottom; small whorls at the bottom would have little effect on the cream at the top before dying a frictional death. Guidance on the size at which turbulence changes from unimportant to important in mixing is provided by the Ozmidov scale, which takes into account how stratified a fluid is and how strong the turbulence is. Given that many zooplankton are comparable to or smaller than oceanic Ozmidov scales, one view is that biogenic mixing is negligible6. The story is not yet complete, however. It could be that zooplankton schooling introduces larger scales and increases mixing efficiency7. Although an attempt8 to observe such an effect failed to do so, the search continues.

Into this mix comes the paper by Katija and Dabiri1. The authors emphasize that the mere act of swimming implies that some water travels with the swimmer. Whereas viscosity lessens the effect of turbulent mixing, here it is found to increase the total transport. In remarkable videos obtained by scuba divers in shoals of jellyfish (Fig. 1), dye releases clearly show the process (see Supplementary Information1). One wonders what the jellyfish made of all this, but that would be another story.

Figure 1: Shoal of Mastigias jellyfish.
figure1

K. KATIJA

These are the experimental subjects studied by Katija and Dabiri1, also pictured on the cover of this issue. The study site was a saltwater lake — Jellyfish Lake — on the Pacific island of Palau.

The relevance to mixing, however, can be simply described. Suppose a jellyfish is in cold water, and swims vertically to warmer zones. Some amount of cold water will follow (the videos suggest a surprisingly large amount). Once there, mixing of the local fluid properties ensues. From energetics estimates based on the dye's behaviour, the effect seems to be sizeable. This mechanism is implicit in previous energetics estimates, but it has escaped explicit notice until now and lessens doubts, based on Ozmidov scales, about the possible strength of biogenic mixing.

Translation of Katija and Dabiri's results from anecdotes to assessments of possible global impacts remains to be carried out. Should the overall idea of significant biogenic mixing survive detailed scrutiny, climate science will have experienced a paradigm shift. To quote Carl Wunsch9, modellers will “need to start thinking about the fluid dynamics of biology”, to which he added, “that's a tough one” — as, indeed, it is.

References

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Dewar, W. A fishy mix. Nature 460, 581–582 (2009). https://doi.org/10.1038/460581a

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