Cloud shadows cue mini-migrations
Zooplankton swim up and down repeatedly due to subtle changes in daylight intensity

Date:
August 20, 2021
Source:
Virginia Institute of Marine Science
Summary:
A new study shows that zooplankton swim up and down repeatedly
within the ocean's twilight zone due to subtle changes in daylight
intensity, with implications for deep-sea ecology and the Earth's
carbon cycle.



FULL STORY ==========================================================================
Cued by the setting sun, droves of zooplankton and small fish each night migrate from the ocean's depths to its surface to feed while evading
predators under the cover of darkness. At dawn, they swim hundreds of
feet back down into the dimmer, safer waters of the ocean's "twilight
zone." A new study shows that some zooplankton also swim up and down
repeatedly within this daytime sanctuary, responding to cloud shadows
so subtle they escape the notice of shipboard oceanographers.


==========================================================================
The study's lead author, Dr. Melissa Omand of the University of Rhode
Island's Graduate School of Oceanography, says "Our finding poses
some really good questions about whether there's an evolutionary or
ecological advantage to this daytime behavior." The newly discovered high-frequency "mini-migrations" would also appear to significantly
increase the metabolic requirements of zooplankton, and likewise their
capacity to reduce the build-up of the greenhouse gas carbon dioxide in
Earth's atmosphere.

Joining Omand on the study, the cover story of this month's Proceedings
of the National Academy of Sciences, are Drs. Deborah Steinberg
and Karen Stamieszkin of William & Mary's Virginia Institute of
Marine Science. Their discovery comes from data collected in the
northeastern Pacific Ocean during NASA's EXPORTS field campaign in
2018. EXPORTS, for EXport Processes in the Ocean from RemoTe Sensing,
is a multi-institutional, 5-year project involving more than 40 principal scientists from 17 organizations in 11 countries.

Steinberg, CSX professor and chair of Biological Sciences at VIMS, is one
of the EXPORTS project's lead scientists. She has been conducting field
studies of zooplankton vertical migration for the last three decades,
most recently during EXPORTS' second and final field campaign, a May
2021 cruise to the North Atlantic.

The daily trek between the ocean depths and surface has been called the
largest migration on Earth, because of both the vast number of migrators
and how far these tiny creatures travel roundtrip. "For creatures this
small -- many the size of a rice grain -- a daily migration of 900 feet
is like you and me walking 25 miles every day to and from breakfast,"
says Steinberg.

"We've known about daily vertical migration -- an adaptation for avoiding visual predators -- for more than a hundred years," she adds, "but we
had no idea this high-frequency migration was also occurring. It just
goes to show how little we still know about the ecology and behavior
of organisms in the deep sea." The team collected their data using
a radiometer to measure surface sunlight and a sonar-like device that
can detect zooplankton in the water. Comparing these two data streams
showed that when thickening 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 thinned,
they would swim back down. According to a model produced by Omand,
the zooplankton were responding to changes in brightness of only 10 or
20%?an imperceptible difference to the shipboard scientists.



========================================================================== "It's amazing how sensitive to light these tiny animals are," says
Steinberg.

"It was overcast for almost our entire 6-week cruise, but we
discovered some zooplankton are somehow able to detect and respond to
very subtle changes in light intensity due only to changes in cloud
thickness. Settings with passing clouds and otherwise clear skies are
likely to induce even more pronounced mini-migrations." "It's such a cool thing to have a window into the daytime lives of these little animals,"
says Omand. "Hopefully our research sheds light on the cues these animals
are using and why they do what they do." Implications for Earth's carbon
cycle The daily migrators play a key role in Earth's carbon cycle by
eating surface- dwelling phytoplankton, then transporting to depth the
carbon these microscopic plants have removed from the water through photosynthesis (this removal then allows the surface ocean to soak up
more CO2 from the air). The CO2removed from the atmosphere and exported
to the deep sea as carbon via this "biological pump" contributes nothing
to current global warming.

The newly discovered mini-migrations have an unknown but possibly
significant effect on global carbon transport via the biological
pump. The average distance for each leg of the mini-migrations is
only about 50 feet, but summed through the day, the repeated jaunts
add up to more than 600 feet, more than 30% of the average nightly
migration distance. Steinberg says the implications of this extra energy expenditure are clear. "The amount of carbon that migrating zooplankton
need to meet their energetic demands, and thus the amount they ingest
and can transport to depth, may be higher than previously predicted." Quantifying the role of the mini-migrations in Earth's carbon budget will require further research. 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.

Steinberg credits the team's discovery to the interdisciplinary nature
of the EXPORTS program. "Programs like EXPORTS are important," she
says, "because they allow scientists from widely varied disciplines --
in our case, a physical oceanographer and zooplankton ecologists --
to combine and interpret their field observations. Melissa brought the expertise to detect the high-frequency migration, while Karen and I
helped put it in an ecological context and recognize its implications." ========================================================================== Story Source: Materials provided by
Virginia_Institute_of_Marine_Science. Original written by David
Malmquist. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Melissa M. Omand, Deborah K. Steinberg, Karen Stamieszkin. Cloud
shadows
drive vertical migrations of deep-dwelling marine life. Proceedings
of the National Academy of Sciences, 2021; 118 (32): e2022977118
DOI: 10.1073/pnas.2022977118 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/08/210820111101.htm

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