The term “planktonic” describes a diverse collection of organisms, known as plankton, that live in aquatic environments and are incapable of swimming against a current. As drifters, their movement is dictated by tides and currents. This group is defined by this shared characteristic rather than taxonomic classification, encompassing everything from microscopic bacteria to large jellyfish. The name originates from the Greek word “planktos,” meaning “wanderer” or “drifter.”
The Drifting Lifestyle
While many planktonic organisms possess some method of self-propulsion, like the whip-like flagella of dinoflagellates, their locomotive abilities are too weak to overcome the force of currents. This makes their lifestyle fundamentally different from other marine life. Think of a planktonic organism as a balloon carried by the wind, its path determined by the prevailing breezes rather than its own power.
In contrast, nektonic organisms, or “nekton,” are the active swimmers of the aquatic realm. This category includes most adult fish, squid, and marine mammals, all of which can navigate and move against strong currents. Another distinct group is the “benthos,” which comprises organisms living on, in, or near the bottom of a body of water. Benthic creatures include crabs, corals, and sea stars, which crawl, attach, or burrow into the seabed.
Major Groups of Planktonic Organisms
The world of plankton is incredibly diverse and is broadly categorized into functional groups based on their ecological roles. The most well-known are phytoplankton, the “plants” of the plankton world. These are microscopic, single-celled organisms, such as diatoms and dinoflagellates, that perform photosynthesis, using sunlight to create their own food. They are typically found in the upper layers of the water where sunlight can penetrate.
Feeding on the phytoplankton are the zooplankton, which are the animal-like members of the plankton community. This group includes a wide range of organisms from microscopic protozoans to larger animals like krill and jellyfish. Zooplankton are a primary link in the food chain, consuming phytoplankton and, in turn, being consumed by larger animals. Some zooplankton species undertake the largest migration on Earth daily, moving from deep waters during the day to the surface at night to feed.
A further distinction is made between organisms that spend their entire lives as plankton, known as holoplankton, and those that are only planktonic for a portion of their life cycle, called meroplankton. Holoplankton include organisms like copepods and krill, which are permanent drifters. Meroplankton are the temporary residents, typically the larval stages of fish, crabs, barnacles, and other invertebrates that will later adopt a nektonic or benthic lifestyle. A third major group, bacterioplankton, consists of free-floating bacteria that are instrumental in breaking down organic matter and recycling nutrients back into the ecosystem.
Global Role of Planktonic Life
Despite their small size, planktonic organisms are foundational to life on Earth. Phytoplankton form the base of nearly all aquatic food webs, from the smallest fish to the largest whales. Baleen whales, for example, can consume tons of krill, a type of large zooplankton, every day. Without this primary food source, the complex web of life in the oceans and freshwater systems would collapse.
Beyond their role in the food web, phytoplankton are responsible for producing a significant portion of the oxygen in our atmosphere. Through the process of photosynthesis, they absorb carbon dioxide and release oxygen, generating an estimated 50% of the world’s supply. This process not only sustains aquatic life but is also a major contributor to the air we breathe. The immense collective impact of these tiny drifters underscores their importance to the entire planet.