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.

Changes in the diel vertical migration cycle as a result of the 2017 Total Solar Eclipse. The daily solar radiation cycle (top panel) is altered as the moon passed in front of the sun on 8/21 1600 UTC. Zooplankton, as seen through the Bio-acoustic Sonar data (bottom panel) similarly altered their daily migration cycle as they began to move from deep waters (light blue band) interpreting the dark conditions as nighttime. Once the moon passed by the sun, the zooplankton reversed direction heading back to deeper waters.

Access the Data

Disclaimer: data used in this example and provided in the .csv file were downloaded from the OOI on June 22, 2020. The file format and/or contents could have changed if downloaded directly from OOI Net after this date.

Processing the Data

Pull Data Using Python Code. Code demonstrates how to download solar radiation data from a Surface Mooring Bulk Meteorology instrument via the OOI Machine-to-Machine interface, download raw echo sounder data from the OOI raw data server, process the raw data files using echopype, and plot both the solar radiation and echo sounder data.

This tutorial was adapted from the notebook provided in the echopype package, which explains how to download and process raw echosounder data from the OOI raw data server. This package and notebook were originally written by Wu-Jung Lee. See Wu-Jung’s additional blog post about the eclipse and ground-truthing the OOI echo sounders.

Endurance Array Oregon Offshore Cabled Shallow Profiler Mooring (CE04OSPS)

Location: On the Continental Slope off the coast of Oregon in the NE Pacific
Lat/Lon: 44.3741°N, 124.956°W
Water Column Depth: 588m
Platform: 200m Platform (PC01B)
Instruments: Bio-acoustic Sonar, Coastal (ZPLSC-B) – pointed upwards, measures to near surface

Endurance Array Oregon Offshore Surface Mooring (CE04OSSM)

Location: On the Continental Slope off the coast of Oregon in the NE Pacific
Lat/Lon: 44.3811°N, 124.956°W
Water Column Depth: 588m
Platform: Surface Buoy
Instruments: Bulk Meteorology Instrument Package (METBK-A) – attached to buoy tower 3m above sea surface

OOI Science Theme

Coastal Ocean Dynamics and Ecosystems
Climate Variability, Ocean Circulation, and Ecosystems

Essentials of Oceanography Textbook Sections

12.2 How are Marine Organisms Classified?
12.4 How are marine organisms adapted to the physical conditions of the ocean?

For more details, check out the full “Textbook Crosswalk”

Next Gen Science Standard Connections

HS- LS2-6. Evaluate claims, evidence, and reasoning that the complex interactions in ecosystems maintain relatively consistent numbers and types of organisms in stable conditions, but changing conditions may result in a new ecosystem. In this data example, a perturbation in the environmental (total solar eclipse) caused the zooplankton to migrate toward the surface during the daytime making them vulnerable to daytime predators.

HS-LS2-8. Evaluate evidence for the role of group behavior on individual and species’ chances to survive and reproduce. The zooplankton community highlighted in this OOI Nugget provide an example of organisms migrating vertically through the water column as a group in a daily cycle to feed in the productive surface waters while avoiding predators.

Related Publications

Barth, J.A., et al. 2018. Warm blobs, low-oxygen events, and an eclipse: The Ocean Observatories Initiative Endurance Array captures them all. Oceanography 31(1):90–97. https://doi.org/10.5670/oceanog.2018.114.

Benoit-Bird, K.J. and G.L. Lawson. 2016. Ecological Insights from Pelagic Habitats Acquired Using Active Acoustic Techniques. Annual Review of Marine Science 8:463-490. http://doi.org/10.1146/annurev-marine-122414-034001.

Additional Resources

NASA – Total Solar Eclipse 2017 webpage; (Downloadable Resources)
August 21 Eclipse-Related Data from the Endurance Array

More from Dr. Wu-Jung Lee:

Los Angeles Times Articles: