Plankton on the Battlefield

How small can an animal be, and still shape a battlefield? Very small, it turns out, as long as you’re zooplankton.

In 1942, the US Navy came to a group of oceanographers working in the University of California Division of War Research with an odd question. Navy sonar operators were getting echoes from “false bottoms,” with their signals bouncing back from depths of 1500 feet where the true depth of the ocean was several thousand. The navy was flummoxed, looking for ways to explain the phenomenon, and turned to the oceanographers Charles Eyring, R.J. Christensen, and Russell Raitt.

They turned to Martin Johnson of the Scripps Institute, a marine biologist who had already had success helping the navy recognize that interference with their sonar signals was the work of snapping shrimp (which are, apparently, shockingly loud when you put hundreds of them together).

Johnson was struck by the idea that the source of the reflection might be organic. In the deep ocean, zooplankton and the fishes which feed on them follow a diurnal cycle – staying deep during the day and climbing close to the surface at night. Clustered together, these fishes and plankton were dense enough to generate a sonar return that confused the operator.

In 1945, Johnson had the chance to prove his theory, watching as the false bottom (soon to be known as the “deep scattering layer”) rose towards the surface overnight and sank again in the morning. Once again, he had been able to help explain the environment in which the navy was fighting.

The use of oceanography during the Second World War became the basis for a massive expansion of government-funded ocean research during the Cold War, and Johnson’s discoveries helped ensure that marine biology and the study of the deep scattering layer would be part of that. After all, even tiny plankton could affect the operation of the US Navy’s most advanced sensors.

h/t Gary E. Weir, An Ocean in Common: American Naval Officers, Scientists, and the Ocean Environment (Texas A&M University Press, 2001); Robert L. Fisher, Edward D. Goldberg, and Charles S. Cox (eds.), Coming of Age: Scripps Institution of Oceanography : A Centennial Volume, 1903–2003 (Scripps Institution of Oceanography, 2003).

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