The day the plankton rose
How a solar eclipse helped solve an oceanographic mystery
During WWII, widespread use of sonar helped naval oceanographers discover the ‘deep scattering layer’—a strong reflection on their sonar screens that rose from the depths at night and sank back down with morning light (submarine captains quickly began to use this phenomenon to hide their own movements).
Subsequent research revealed that this layer was made up of countless marine organisms migrating toward the ocean surface as evening fell—to feed under the cover of darkness—then swimming back to the inky depths at dawn to escape their own predators during daylight hours.
But one question persisted: what did these vertical migrators use to time their daily journey, the amount of available light, or some type of internal clock? Biologists had a similar question about sea creatures that make their own light—is marine bioluminescence due to external cues or a circadian rhythm?
In 1963, a team of scientists at the Woods Hole Oceanographic Institution thought up a novel way to test these competing hypotheses—to monitor what would happen on July 23 when a solar eclipse moved over New England and out into Atlantic waters from about 3:30 to 5:30 pm. If marine organisms responded to the temporary darkness of the moon’s shadow by moving up and lighting up, it would support the available-light hypothesis. No response would support the idea of a circadian clock.
“It’s a really neat example of using a natural event to help solve a question in oceanography,” says VIMS professor Deb Steinberg, who continues to study vertical migration and the animals of the deep scattering layer as part of her own research.
Steinberg’s focus is to better fathom the role vertical migrators play in climate change. “Humans release billions of tons of carbon dioxide to the air each year through the burning of fossil fuels, about a third of which goes into the ocean” she says. 
“My research attempts to quantify how much of that carbon is pumped into the deep sea by feeding and vertical migration of zooplankton and other marine organisms.” Carbon that is exported to the deep sea via this "biological pump” contributes nothing to current global warming.
But on that cool late summer day 54 years ago— the temperature only reached 73°—global warming was on no one’s sonar screen. Instead, the WHOI scientists—Drs. Richard Backus, Robert Clark, and Asa Wing—watched as vertical migrators 12,000 feet beneath their feet responded to the moon’s passing shadow by beginning to rise towards their vessel as it bobbed on the surface off Cape Cod.
They also recorded a response from bioluminescent algae in a tank they had placed near shore—the creatures began to produce their usually nocturnal glow in the middle of a Massachusetts afternoon.
In their own words, “We can say that both the scattering layer organisms and the bioluminescent organisms responded to the eclipsing Sun much as they normally respond to the setting Sun. Their behaviour from mid-eclipse to eclipse end resembled dawn behaviour. The response of these organisms to the change from decreasing light to increasing light near mid eclipse was rapid. Thus, it appears that the exogenous factor of changing light largely controls the behaviour examined in these organisms, overriding such endogenous rhythms as may exist.”
To read the study in its entirety, see
Backus, R.H., Clark, R.C. and Wing, A.S., Behaviour of Certain Marine Organisms during the Solar Eclipse of July 20, 1963: Nature 205: 989-991.