Size Groups
Plankton are also often described in terms of size. Usually the following divisions are used:
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Group Size range
(ESD)Examples Megaplankton > 20 mm metazoans; e.g. jellyfish; ctenophores; salps and pyrosomes (pelagic Tunicata); Cephalopoda Macroplankton 2→20 mm metazoans; e.g. Pteropods; Chaetognaths; Euphausiacea (krill); Medusae; ctenophores; salps, doliolids and pyrosomes (pelagic Tunicata); Cephalopoda Mesoplankton 0.2→2 mm metazoans; e.g. copepods; Medusae; Cladocera; Ostracoda; Chaetognaths; Pteropods; Tunicata; Heteropoda Microplankton 20→200 µm large eukaryotic protists; most phytoplankton; Protozoa (Foraminifera); ciliates; Rotifera; juvenile metazoans - Crustacea (copepod nauplii) Nanoplankton 2→20 µm small eukaryotic protists; Small Diatoms; Small Flagellates; Pyrrophyta; Chrysophyta; Chlorophyta; Xanthophyta Picoplankton 0.2→2 µm small eukaryotic protists; bacteria; Chrysophyta Femtoplankton < 0.2 µm marine viruses
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However, some of these terms may be used with very different boundaries, especially on the larger end of the scale. The existence and importance of nano- and even smaller plankton was only discovered during the 1980s, but they are thought to make up the largest proportion of all plankton in number and diversity.
The microplankton and smaller groups are microorganisms and operate at low Reynolds numbers, where the viscosity of water is much more important than its mass or inertia.
Read more about this topic: Plankton
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