In the popular cartoon “SpongeBob SquarePants,” Plankton is a tiny troublemaker who fantasizes about someday taking over the world. Well, according to the scientists who study these microscopic migrators, they may already have. For this vast family of plant and animal drifters—most too small to see with the naked eye—comprises up to 75 percent of the primary productivity biomass in aquatic ecosystems.
The term plankton is a collective name for oceanic drifters, most of them too fragile and weak to swim against the prevailing currents on which they float.
Plankton are divided into two groups: phytoplankton and zooplankton. Algae and plant-like organisms called phytoplankton are primary producers and the foundation of the ocean's food web.
Just as plants do on land, phytoplankton get their energy from the sun through photosynthesis. As such, they’re limited to those depths to which the sun’s rays can penetrate the ocean’s surface. This is called the photic zone.
Not so with the heartier zooplankton, a category of animal plankton that includes larval forms of crabs, shrimps, mollusks, jellies, worms and thousands of other marine creatures—some of them grazing on phytoplankton like tiny cows on a floating grassland, others preying on fellow plankton with surprising ferocity.
Among those that spend their juvenile phase as marine zooplankton is our iconic Chesapeake blue crab, whose life cycle begins in a tiny form called a zoea. Though plankton cannot swim freely, they do migrate. Interestingly, most do so vertically, moving up and down in the water column in a daily rhythm governed by the sun and the moon.
Like most mammals, humans are diurnal, meaning we’re most active in daylight. Zooplankton, however, are as nocturnal as hours, making their daily migration to surface waters in the evening to forage for food in the cover of darkness, then scurrying to deeper, safer, darker waters before morning’s light. This daily vertical migration strategy, called DVM for short, helps them elude predators who need light to hunt.
How did scientists make this discovery? During World War II, with the advent of sonar, Navy operators trying to map the ocean floor noticed that their pings were showing an indecipherable “false bottom” on their charts. On careful inspection, they discovered that this second seafloor was in fact millions of small creatures that rose as the sun set and then fell as the sun rose. They were so numerous that they scattered the sonar waves and ever since have been referred to as the “deep scattering layer.”
These daily up-and-down movements of the smallest creatures in the ocean may affect the largest. Scientists at Woods Hole Oceanographic Institution are studying how the DVM of shrimp-like zooplankton, called copepods, has an impact on right-whale behavior. It's no small significance, as copepods are the favorite food of these gentle giants, one of which can eat up to 2,500 pounds of copepods per day.
Zooplankton are so small that they can thicken seawater to the consistency of molasses. Since they lack brains, how do they know when to get a move on? Scientists think their behaviors may be triggered by light levels, chemical signals or even sound.
Though small and unassuming, plankton are capable of a great butterfly effect, as massive schools of fish and pods of whales follow their movements across entire oceans.