The term “plankton” originates from the Greek word for “drifter” or “wanderer.” They are defined not by their species, but by their ecological niche: they exist in a free-floating state, carried by the currents and tides. While many are microscopic, this group also includes larger creatures, like jellyfish, whose swimming ability is not strong enough to move against the water’s flow. This single characteristic unites a variety of life that forms the foundation of aquatic ecosystems.
The Two Major Groups of Plankton
Plankton are broadly separated into two main functional groups. The first, phytoplankton, are the “plants” of the aquatic world. These single-celled organisms, such as diatoms and dinoflagellates, get their energy through photosynthesis. Because they require sunlight, they inhabit the upper layers of the water column, converting light energy and nutrients into organic matter.
The second major category is zooplankton, the “animals” of the plankton world. These organisms are heterotrophs, meaning they must consume other organisms to get energy, often grazing on phytoplankton. This group is divided into two types based on their life cycle. Holoplankton, such as copepods and krill, spend their entire lives as drifting organisms. In contrast, meroplankton are temporary members, consisting of the larval stages of larger animals like fish, crabs, and sea stars.
The Underwater Engine of Life
Phytoplankton form the base of almost every aquatic food web, functioning as the “pastures of the sea.” They are consumed by herbivorous zooplankton, which are then eaten by carnivorous zooplankton, small fish, and other invertebrates. This chain of energy transfer continues up to the largest animals in the ocean; for example, blue and humpback whales rely on large quantities of krill, a type of zooplankton, for sustenance.
Beyond their role in the food web, phytoplankton are responsible for producing a significant amount of the Earth’s oxygen. Through photosynthesis, these organisms release oxygen as a byproduct. It is estimated that marine phytoplankton are responsible for 50-80% of the oxygen in the atmosphere. They absorb large quantities of carbon dioxide from the atmosphere, playing a part in the global carbon cycle.
Spectacular Plankton Phenomena
Bioluminescence is a captivating display in the ocean often caused by plankton. Certain types of dinoflagellates produce a chemical light when they are mechanically disturbed by waves, boats, or swimmers. This “sea sparkle” is believed to be a defense mechanism. The sudden flash of light can startle a predator or act as a “burglar alarm” that attracts a larger predator to the creature that disturbed the dinoflagellate.
Another behavior is known as diel vertical migration, the largest synchronized movement of biomass on the planet. Each day, countless zooplankton migrate from the deep ocean waters toward the surface as dusk falls. They spend the night near the surface feeding on phytoplankton in relative safety from visual predators. Before dawn, they descend back into the depths to hide during daylight hours, a journey that can cover hundreds of meters.