There is no single agreed classification of plankton by size and the student will find the same terms applied to slightly different size categories in older texts. The definitions given below are based on those given in Omori and Ikeda (1984) which are both practical and easy to understand.
Megaloplankton - Gelatinous plankton such as medusae (jellyfish) and salps, >20 mm.
Micronekton - These fall into the same size category as megaloplankton but the term is usually applied to strongly swimming animals such as euphausids, mysids and fish larvae, 20-200 mm. Macroplankton - Large planktonts visible to the unaided eye, such as pteropods, copepods, euphausiids and chaetognaths, 2-20 mm. Mesoplankton - The principal components of zooplankton fall into this and the macroplankton category. Mesoplankton includes cladocerans, copepods, and larvaceans, 200 [m-2 mm. Net plankton - The four larger size categories described above are collectively called net plankton because they can be effectively caught using nets. Microplankton - Includes most phytoplankton species, foraminiferans, ciliates, rotifers and copepod nauplii, 20-200 [m. Nanoplankton - Organisms such as fungi, small flagellates and small diatoms, 2-20 [m.
Ultraplankton or picoplankton - Mainly bacteria and cyanobacteria, < 2 [m. Water bottle plankton - The three smaller categories (micro-, nano- and ultraplankton) which cannot effectively be caught by nets.
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