From Laura Klappenbach,
Your Guide to Animals / Wildlife.
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Definition: n. Plankton are microscopic organisms that drift on the oceans' currents. They include organisms such as diatoms, dinoflagillates, krill, and copepods as well as the microscopic larva of crustaceans, sea urchins, and fish. Plankton also include tiny photosynthetic organisms that are so numerous and productive that they are responsible for generating more oxygen than all other plants on Earth combined.

Plankton are categorized into the following groups based on their trophic role (the role they play within their food web):

• phytoplankton - Phytoplankton are the primary producers of the planktonic world. They are photosynthetic plankton and include organisms such as diatoms, dinoflagellates, and cyanobacteria.
• zooplankton - Zooplankton are the consumers of the planktonic world. As such, they feed on other plankton to obtain the energy and nutrients they need to survive. Zooplankton include the larvae of fish, crustaceans.
• bacterioplankton - Bacterioplankton are the recyclers of the planktonic world. They are free-floating bacteria and archaea that serve to break-down and recycle waste material in the seas.

Plankton can also be categorized by whether or not it spends its entire life as a microscopic organism:

• holoplankton - Holoplankton are organisms that are planktonic for the entirety of their life cycle.
• meroplankton - Meroplankton are organisms that are planktonic for only part of their life cycle, for example, only during the larval stage of their development.

Sources:

Burnie, D. and D.E. Wilson. 2001. Animal. London: Dorling Kindersley.

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