Here in Maui we witness one of the greatest migrations in the world—that of the humpback whale. However, what our human eyes can’t see is another one of the world’s largest mass migrations happening every dawn and dusk in waters around the world.  Microscopic zooplankton move vertically through the water column to seek prey and avoid predators. This is called diel vertical migration (DVM).

Zooplankton caught in the leeward waters of Maui. Specimens shown under the microscope.

 Under the cover of darkness each night, zooplankton migrate to the surface of the ocean to feed on phytoplankton (“plant” plankton). In the darkness, predators can’t see them as well which makes feeding safer. Once the sun rises, the zooplankton retreat deeper into sea to avoid being seen by hungry fish. Because zooplankton are so small, this journey is as difficult for them as it would be for a person to swim through an ocean of molasses.
Here in Hawaii and throughout the tropics, plankton is much less abundant. This is why our water is so crystal clear and why whales must migrate up to Alaska’s plankton-rich water to feed. However, there are still small amounts of plankton in these waters, and the cycles of DVM affect it each day and night.
DVM is very important to a healthy ocean. After zooplankton fill up on carbon-rich phytoplankton and migrate deeper into the water column, they excrete that same carbon. This helps a process called the “biological pump” that captures carbon dioxide from the air and moves it into the deep sea where some provides nutrients for other organisms, and some becomes part of the sandy bottom.  If the biological pump didn’t exist, almost twice as much carbon dioxide would be in our atmosphere. DVM is a small variable in this large process that keeps our planet healthy.
 
For further information, see the links below.

By Simona Clausnitzer