The tiny, one-eyed villain of SpongeBob SquarePants, Sheldon J. Plankton, takes his name from a very real and globally significant group of aquatic organisms. Plankton represents a vast and diverse collection of life forms found in both marine and freshwater environments. Understanding the scientific basis of the term “plankton” reveals a world far more complex and influential than a cartoon rival to the Krusty Krab.
What Defines Plankton
The term plankton is not a single taxonomic group, but rather a description of a lifestyle defined by movement. Plankton comes from the Greek word planktos, meaning “drifter” or “wanderer.” This classification applies to any aquatic organism that is nonmotile or too small to swim effectively against ocean currents and tides.
This definition encompasses a huge range of life, from microscopic bacteria and single-celled algae to the juvenile stages of fish, mollusks, and crustaceans. Even large organisms like certain jellyfish are classified as plankton because their limited swimming ability cannot overcome major ocean currents. Plankton is contrasted with nekton (strong swimmers like fish) and benthos (organisms living on the seafloor).
The size of plankton varies dramatically, spanning eight orders of magnitude from the smallest viruses to the largest jellyfish. Some organisms, known as meroplankton, are only considered plankton temporarily, such as the larval stages of crabs or sea stars. Other organisms, called holoplankton, spend their entire existence drifting in the water column.
Phytoplankton Versus Zooplankton
Scientists divide this drifting community into two primary functional categories based on how they obtain energy: phytoplankton and zooplankton. The prefix phyto signifies the plant-like nature of the first group, which are the producers of the aquatic world. Phytoplankton are autotrophs, using chlorophyll to perform photosynthesis and converting sunlight and carbon dioxide into chemical energy.
These organisms, which include diatoms and dinoflagellates, form the base of the marine food web, much like terrestrial plants. Because they rely on sunlight, phytoplankton are found predominantly in the upper, sunlit layers of the water column, known as the photic zone. Their productivity is directly influenced by the availability of light and nutrients like nitrate and phosphate.
In contrast, zooplankton are the animal-like consumers, classified as heterotrophs because they must consume other organisms for energy. This group includes protozoans, small crustaceans like copepods and krill, and the larval forms of many larger animals. Zooplankton primarily feed by grazing on phytoplankton, making them the ocean’s primary consumers.
Zooplankton also prey on smaller zooplankton, placing them at various levels of the food chain above the producers. The fictional character Plankton fits the ecological role of a zooplankton, as he is a microscopic consumer. These consumers link the energy captured by phytoplankton to higher trophic levels, serving as a food source for fish and baleen whales.
Plankton’s Role in Global Ecology
Beyond their function as the foundation of the marine food web, plankton play a massive role in regulating the Earth’s atmosphere and climate. Phytoplankton are responsible for generating a significant portion of the planet’s breathable air. Through photosynthesis, these microscopic drifters produce an estimated 50% of all the oxygen on Earth.
For every two breaths a person takes, one can be traced back to the activity of these tiny oceanic plants. Plankton are also involved in regulating the global carbon cycle, consuming carbon dioxide on a scale comparable to all the world’s forests. When phytoplankton absorb carbon during photosynthesis, they incorporate it into their bodies. This process, known as the biological carbon pump, transfers carbon from the atmosphere into the deep ocean.
When these organisms die or are consumed by zooplankton, their carbon-rich remains and waste products sink into the ocean depths. This process effectively sequesters carbon away from the atmosphere, helping to mediate global climate conditions. Even small changes in plankton productivity, influenced by ocean temperature and nutrient availability, can profoundly impact atmospheric carbon dioxide concentrations. Plankton communities are a central component of global biogeochemical cycles.