Plankton encompasses aquatic organisms defined by their drifting lifestyle within water currents. Unable to swim against the flow of oceans, lakes, and rivers, they are passive travelers. This diverse group includes microscopic bacteria, various forms of algae, and animals.
The Spectrum of Plankton Sizes
Scientists categorize plankton into distinct size classes, spanning an immense scale from the minuscule to surprisingly large organisms.
The smallest category is Picoplankton, measuring 0.2 to 2 micrometers, comparable to a single bacterium and invisible to the naked eye. Nanoplankton range from 2 to 20 micrometers, still microscopic but larger, similar to a red blood cell.
Microplankton follow, with sizes between 20 and 200 micrometers. Many single-celled algae and protozoa fall into this category, roughly the size of a grain of salt.
Mesoplankton are considerably larger, measuring from 0.2 millimeters to 20 millimeters. This range encompasses organisms visible to the unaided eye, akin to the size of a rice grain or even a small insect.
Macroplankton are larger, between 2 and 20 centimeters. This size class includes creatures comparable to a small fish or a playing card.
The largest plankton fall into the Megaplankton category, exceeding 20 centimeters. Some organisms in this group can rival the size of a beach ball or even a small car.
Examples Across the Size Range
Specific organisms illustrate the broad size spectrum of plankton, from the tiniest producers to large drifting animals.
Prochlorococcus is a picoplankton example, measuring 0.5 to 0.7 micrometers. This cyanobacterium is the smallest known photosynthetic organism and is abundant in oceans.
Diatoms are microplankton, with some species reaching up to 1 millimeter. These single-celled algae are encased in silica shells and can form chains.
Dinoflagellates, another microplankton group, range from 5 to 2,000 micrometers. Many possess two flagella, whip-like tails that allow for limited movement.
Copepods are mesoplankton, ranging from 0.2 to 20 millimeters. These small crustaceans are widespread in marine and freshwater environments and are a food source for many aquatic animals.
Krill represent macroplankton, with most adults measuring 1 to 2 centimeters, though some species grow up to 15 centimeters. Antarctic krill form vast swarms and are a food source for baleen whales, seals, and penguins.
The Lion’s Mane Jellyfish (Cyanea capillata) exemplifies megaplankton. Its bell can reach 2 meters in diameter, and its tentacles extend over 30 meters, making it one of the longest animals. These large jellyfish drift with ocean currents, consuming smaller plankton and fish.
The Ecological Importance of Size
Plankton size influences their role and interactions within aquatic ecosystems, determining their position in the food web.
Small plankton, particularly picoplankton and nanoplankton, form the base of food webs as primary producers, converting sunlight into energy. Larger plankton consume these, transferring energy up trophic levels to predators like fish and marine mammals.
Size also plays a role in the biological carbon pump, moving carbon from the surface ocean to deeper waters. Larger, denser plankton like diatoms and krill transport carbon to the deep ocean when they die and sink. This sequesters carbon from the atmosphere, influencing global climate regulation.
Smaller plankton cells have a higher surface-area-to-volume ratio, aiding nutrient uptake. This allows them to absorb dissolved nutrients more efficiently, especially in nutrient-sparse environments. This efficiency enables small cells to thrive where larger cells might struggle, influencing plankton community productivity and structure.