Plankton - Size Groups

Size Groups

Plankton are also often described in terms of size. Usually the following divisions are used:

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

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.