Cloud shadows signal mini-migrations | Guillaume and Marie
by David Malmquist, VIMS
September 27, 2021
Zooplankton rises and falls repeatedly due to subtle changes in the intensity of daylight
Driven by the setting sun, herds of zooplankton and small fish migrate every night from the depths of the ocean to its surface to feed while avoiding predators under the cover of darkness. At dawn, they swim for hundreds of meters in the darker, safer waters of the “twilight zone” of the ocean. A new study shows that some zooplankton also swim up and down repeatedly in this daytime sanctuary, responding to cloud shadows so subtle they escape the attention of oceanographers aboard ships.
The main author of the study, Melissa Omand of the Graduate School of Oceanography at the University of Rhode Island, says, “Our discovery asks very good questions about whether there is an evolutionary or ecological benefit to this diurnal behavior. The recently discovered high-frequency “mini-migrations” also appear to significantly increase the metabolic requirements of zooplankton, as well as their ability to reduce the build-up of carbon dioxide, a greenhouse gas, in the Earth’s atmosphere.
Join Omand on the study are Deborah Steinberg and Karen stamieszkin from William & Mary’s Virginia Institute of Marine Science. Their discovery, this month’s cover story Proceedings of the National Academy of Sciences, comes from data collected in the northeastern Pacific Ocean during NASA EXPORTS field campaign in 2018. EXPORTS, for RemoTe Sensing’s EXport Processes in the Ocean is a 5-year multi-institutional project involving over 40 senior scientists from 17 organizations in 11 countries.
Steinberg, CSX professor and holder of the chair of biological sciences at VIMS, is one of the principal scientists of the EXPORTS project. She has conducted field studies on the vertical migration of zooplankton over the past three decades, most recently during the second and final field campaign of EXPORTS, a May 2021 North Atlantic cruise.
The daily trek between the depths and the surface of the ocean has been called the greatest migration on Earth, due both to the large number of migrants and the distance traveled by these tiny creatures.
“For creatures this small – many the size of a grain of rice – a 900-foot daily migration is like you and I walking 25 miles each day to and from breakfast,” says Steinberg.
“We’ve known about daily vertical migration – an adaptation to avoid visual predators – for over a hundred years,” she adds, “but we had no idea that this high-frequency migration was also happening. This shows how little we still know about the ecology and behavior of deep-sea organisms. “
The team collected their data using a radiometer to measure surface sunlight and a sonar-type device capable of detecting zooplankton in the water. Comparison of these two data streams showed that when thickening cloud cover prevented sunlight from reaching the depths of the ocean, zooplankton swam towards the surface to stay in the water at their preferred brightness. . When the clouds cleared, they swam back down. According to a model produced by Omand, zooplankton responded to changes in brightness of only 10% or 20%, a difference imperceptible to the scientists on board.
“It’s amazing how sensitive these little animals are to light,” says Steinberg. “The skies were overcast for most of our six week cruise, but we found that some zooplankton are somehow able to detect and respond to very subtle changes in light intensity due only to changes in cloud thickness. Environments with isolated clouds crossing an otherwise clear sky are likely to induce even more pronounced mini-migrations. “
“It’s so cool to have a window into the day life of these little animals,” says Omand. “I hope that our research will shed light on the clues these animals use and why they are doing what they are doing.”
Implications for the Earth’s carbon cycle
Daily migrants play a key role in the Earth’s carbon cycle by eating surface phytoplankton and then transporting the carbon these microscopic plants have removed from the water through photosynthesis deep down (this removal then allows the ocean surface area to absorb more CO2 air). CO2 extracted from the atmosphere and exported to the deep sea in the form of carbon via this “biological pump” does not contribute to current global warming.
The recently discovered mini-migrations have an unknown but possibly significant effect on global carbon transport via the biological pump. The average distance for each stage of the mini-migrations is only about 50 feet, but added up throughout the day, the repeated escapades add up to over 600 feet, which is over 30% of the average night-migration distance. . Steinberg says the implications of this extra energy expenditure are clear.
“The amount of carbon that migrating zooplankton need to meet their energy needs, and therefore the amount that they ingest and can carry at depth, may be higher than expected,” said Steinberg.
Quantifying the role of mini-migrations in the Earth’s carbon footprint will require further research. More information is needed to fully understand why zooplankton exert energy by swimming up and down all day in response to small changes in light, and whether this behavior is common in different species and in all of the world’s oceans. whole world.
Steinberg attributes the team’s discovery to the interdisciplinary nature of the EXPORTS program.
“Programs like EXPORTS are important,” she says, “because they allow scientists from a wide variety of disciplines – in our case a physical oceanographer and zooplankton ecologists – to combine and interpret their observations in the field. Melissa brought the expertise to detect high frequency migration, while Karen and I helped place it in an ecological context and recognize its implications.