Daily Vertical Migration Gets Eclipsed!

On August 21, 2017 day became night for over 2 and a half minutes. For those of us fortunate enough to find ourselves in the path of totality, the sight of the moon blocking sun in the sky, the eerie darkness, the sound of insects beginning their evening song, is an experience we will never forget. For zooplankton, the experience was less mesmerizing, more confusing; to quote the LA Times, “they got totally punk’d.”

Zooplankton spend their daylight hours deeper in the water column to avoid visual predators. As the sun sets and the water column darkens, they make their way to the food-rich surface. During the eclipse, as the moon blocked the sun’s light, the zooplankton began their nightly vertical migration through the water column. Once the moon moved passed the sun and light in the water column increase, the zooplankton realized their mistake and swam back down. Note that since the eclipse was such a short event, the zooplankton did not make it all the way to the surface.

The zooplankton observed in this nugget are in a shallow sound scattering layer. Further out in the open ocean, this type of daily vertical migration happens deeper in the water column in a thick layer of several different types of small marine organisms, including zooplankton, called the Deep Scattering Layer (DSL). The DSL, first discovered by acoustic tools on submarines during World War II, is a layer of marine organisms that migrate into deep waters (~1000m) during the day to hide from visual predators and then rise to the sea surface to feed at night under the cover of darkness.

These zooplankton movements were captured by an upward-looking bio-acoustic sonar deployed at 200m on the Offshore Cabled Shallow Profiler Mooring deployed on the OOI Coastal Endurance Array Oregon Line. A bio-acoustic sonar works by emitting sound waves into the water column. These sound waves bounce off organisms and travel back to the sensor in a phenomena known as “backscatter”, which is then measured by the sensor. The more objects for sound waves to bounce off of, the higher the backscatter; this is similar to how a “fish finder” on a boat works. A bio-acoustic sonar can also emit sound waves at different wavelengths, which gives scientists an idea of the different sizes and types of organisms in the water column. As these sensors are measuring sound waves and the OOI does not provide processed data for these sensors, the data in this nugget are more complex than the other OOI Nuggets and cannot be exported to a CSV file. We are grateful to Dr. Wu-Jung Lee for providing an open-source Python code she developed specifically to process these complicated data which allowed us to share them with you.