KINGSTON, R.I., — August 2, 2021 — Every evening, small fish and microscopic animals called zooplankton journey to the ocean surface, where they feast on microscopic plants under the moonlight before returning to the depths at dawn. With data collected during a NASA campaign in 2018, a team of scientists led by a University of Rhode Island oceanographer has now shown that some zooplankton swim up and down in response to shifts in light due to cloud cover as well.

The nightly trek from the ocean depths to the surface has been called the largest migration on Earth, because of both the number of animals who make the nightly trek and how far these tiny creatures travel roundtrip. NASA has observed this global migration with a space-based laser on the CALIPSO satellite; scientists have also documented these migrations during events such as eclipses, full moons and storms.

“The amount that they swim is pretty remarkable given their body length,” said Melissa Omand, an associate professor at URI’s Graduate School of Oceanography. “It’s like me in Rhode Island going to Boston and back every day,” she said, a distance of roughly 80 miles.

Zooplankton don’t swim the full distance every time a cloud passes overhead, though, instead swimming about 50 feet on average. However, throughout the day, these “mini-migrations” add up to about 30% of the average nightly migration distance, the team report in a study published in the journal Proceedings of the National Academy of Sciences. The findings could have implications for the metabolic requirements of zooplankton, which are key players in the marine food web and the transfer of carbon in the ocean.

The discovery comes from data collected during NASA’s Export Processes in the Ocean from Remote Sensing (EXPORTS) field campaign in 2018. Omand was aboard one of the research vessels to measure ocean currents using an instrument called an acoustic doppler current profiler, or ADCP. The instrument sends out pings of sound that bounce off suspended particles or zooplankton in the water column. Some of those pings are reflected back to the instrument, while others are scattered.

When Omand went below deck to analyze the data on her computer, she noticed something intriguing. There were “wiggles” in the data, signifying that something was moving up and down in the water column. Based on the frequency of the sound waves, 150 kHz, and the marine animals captured in nets for other EXPORTS experiments, that something was most likely zooplankton. She also noticed that those wiggles lined up with the changes in sunlight measured by the radiometer―a device that measures the intensity of sunlight―mounted on the ship.

To Omand, this implied that the zooplankton were swimming up and down as the light changed due to clouds passing overhead. She made a simple computer model that confirmed her suspicions: the zooplankton were following isolumes, or areas in the ocean with the same amount of light throughout. For example, when cloud cover prevented sunlight from reaching as deep in the ocean, the zooplankton would swim toward the surface to stay in water with their preferred brightness. When the clouds passed, they would swim back down. According to the model, the zooplankton were responding to changes of only 10 or 20%―an imperceptible difference to Omand and the rest of the crew standing on the ship deck.

“This finding poses some really good questions about whether there’s an evolutionary or ecological advantage to this daytime behavior,” said Omand. She notes, however, that this is just one series of observations in one spot in the northeastern Pacific Ocean. In addition, the ADCP data gives the approximate size of the zooplankton but cannot pinpoint specific species. More information is needed to fully understand why zooplankton exert energy swimming up and down all day in response to small changes in light, and if this behavior is common among different species and throughout oceans worldwide.

“But it’s such a cool thing to have a window into the daytime lives of these little animals,” Omand said, “and hopefully this sheds light on the cues these animals are using and why they do what they do.”