Marine animals cause a stir

An all-but forgotten contribution of Sir Charles Darwin — physicist grandson of the Charles Darwin — has resolved a long-standing debate about ocean mixing. He suggested, in 1953, a mechanism that enables swimming animals to contribute significantly to the mixing of water in the ocean. The debate about the biogenic mixing in the oceans today focuses on comparisons between animal wake turbulence and ocean turbulence. ‘Darwinian’ mixing is different, and occurs when a solid body travelling in a fluid sets a portion of the surrounding fluid into motion so that it propagates along with the body. Kakani Katija and John Dabiri used field measurements of jellyfish swimming in a remote island lake, combined with a new theoretical model, to demonstrate that the contribution of living organisms to ocean mixing via this mechanism is substantial — of the same order of magnitude as winds and tides.