Small swimmers may be responsible for much mixing of the ocean's waters, a significant factor previously ignored by ocean models
Scientists had previously thought that tiny, shrimplike copepods, or even animals the size of jellyfish, had little influence on ocean mixing. Any turbulence created by these small creatures would quickly dissipate, quashed by the viscosity of the water, which is thick like honey at the copepod scale. But a combination of fieldwork, theoretical modeling, and energy calculations reveals that generating a wake is not the only way to stir the waters. Swimmers also drag fluid with them as they move, and this effect is especially enhanced in the viscous setting of the small scale, the research found.
When a swimmer moves through water of different temperatures (as krill do each night when they migrate en masse from deeper waters to the surface) the cold water that is entrained is mixed with warmer water nearer the surface. The researchers note that for many years scientists considered an animal’s wake the main biological kinetic energy input to the oceans. But turbulence from a wake was not considered that significant, especially for small creatures or super-efficient swimmers like dolphins, which leave little wake. But even though wake is diminished at small scales, the amount of water carried with the swimmer is significant, the study shows. Add in the huge numbers of these animals, and the volume becomes significant.
In addition to doing theoretical modeling, the researchers studied the fluid dynamics of mixing by the jellyfish Mastigias. By squirting a fluorescent dye in front of the swimming jellyfish, the researchers tracked the flow of water moving along with the jellies. A laser device recorded the jellies’ velocities. Calculations suggest that the amount of ocean power input from all sea creatures may be as much as a trillion watts, comparable to the power input from winds and tides. That calculation does not include the contribution of fecal pellets and other debris that drift down through the water column.
Thus, even small movements may add up to a serious role in the cycling of heat and carbon in the oceans, important factors influencing global climate. Once again, models will need to be modified to take into account this significant, but previously ignored factor.