The collective vertical migration of centimetre-scale shrimp can generate downward jets that could have a substantial impact on ocean mixing, reports a paper published this week in Nature. This effect could be widespread, as organisms of this size are highly abundant in climatically important regions of the ocean.

Centimetre-scale zooplankton such as krill, despite their high abundance, do not individually create enough turbulence to have a noticeable effect on ocean mixing. Nonetheless, such animals form dense aggregations that span tens of metres in their vertical axis as they undergo daily vertical migration over hundreds of metres.

John Dabiri and colleagues use the brine shrimp Artemia salina as a model to show that collective vertical migration can generate downward jets that could have a substantial effect on ocean mixing. The authors conducted laboratory experiments using shrimp in two stably stratified tanks: one to measure irreversible mixing of the density stratification, and the other to perform several flow visualization techniques. Light stimuli triggered the shrimp to swim. The authors observed that eddies resulted from the flow in the wakes of the individual shrimp coalescing to form a large-scale downward jet during upward swimming, even in densely stratified water.

The results illustrate that the behaviour of small marine animals, though tiny in stature, can considerably alter the physical and biogeochemical structure of the ocean.