Plankton are diverse organisms that drift through aquatic environments like oceans, lakes, and rivers. They are defined by their lifestyle, as they are largely unable to swim against currents. Plankton exhibit enormous size variability, from organisms barely larger than molecules to those visible to the naked eye.
The Scale of Plankton
Scientists categorize plankton into various size classes to better study their ecological functions. These classifications primarily use micrometers (µm) and millimeters (mm) as units of measure. A micrometer represents one-millionth of a meter. Understanding these size categories is foundational for appreciating the diversity within plankton communities.
Plankton size categories include:
Femtoplankton: Less than 0.2 µm, predominantly viruses.
Picoplankton: 0.2 to 2 µm, encompassing many bacteria and the smallest photosynthetic organisms.
Nanoplankton: 2 to 20 µm, including various single-celled flagellates and smaller diatoms.
Microplankton: 20 to 200 µm, comprising a mixture of larger phytoplankton and protozoan zooplankton.
Mesoplankton: 0.2 to 20 millimeters.
Macroplankton: 2 to 20 centimeters.
Megaplankton: Exceed 20 centimeters.
Size Ranges of Different Plankton Types
Different plankton types occupy distinct, though sometimes overlapping, size ranges. Phytoplankton, the plant-like organisms that perform photosynthesis, typically inhabit smaller size classes. Picophytoplankton, such as the cyanobacteria Prochlorococcus and Synechococcus, are less than 2 µm. Nanophytoplankton (2 to 20 µm) include many flagellates and smaller diatom species. Microphytoplankton (20 to 200 µm) often consist of larger diatoms and dinoflagellates.
Zooplankton, animal-like organisms that consume others, exhibit a much broader size range. Microzooplankton (20 to 200 µm) include single-celled protozoans like ciliates and foraminiferans. Mesozooplankton, such as copepods and rotifers, are found from 0.2 to 20 millimeters. Larger zooplankton, like krill and some jellyfish, are macroplankton (2 to 20 centimeters). The largest zooplankton, such as the Lion’s Mane Jellyfish, can reach megaplankton sizes, exceeding 20 centimeters.
Bacterioplankton are bacteria that drift in the water column, typically falling within the picoplankton size range (0.2 to 2 µm). Virioplankton, the viruses present in aquatic environments, are the smallest known plankton. Their diameters commonly range from 0.03 to 0.7 µm, with some giant viruses reaching the larger end of this spectrum.
Visualizing Plankton Size
Understanding the minuscule sizes of many plankton can be challenging without relatable comparisons. For instance, a human hair is approximately 70 to 100 micrometers in diameter. Many microplankton can be anywhere from a fraction to several times the width of a human hair. A single grain of sand, typically between 60 micrometers and 2 millimeters, can be comparable in size to a large microplankton or a smaller mesoplankton.
Scientists employ various tools and techniques to observe and study these tiny organisms. Plankton nets, with their fine mesh, are used to collect larger plankton, typically microplankton and above, by filtering them from vast volumes of water. For smaller plankton, different types of microscopes are essential. Light microscopes are suitable for viewing microplankton and some larger nanoplankton.
To visualize picoplankton, nanoplankton, and especially virioplankton, more powerful electron microscopes are required. Advanced imaging systems, such as flow cytometers, FlowCam, and Imaging FlowCytobot, provide automated ways to analyze and count plankton. Despite their often-microscopic dimensions, the sheer abundance and collective activity of plankton make them profoundly significant to global ecosystems.