Plankton

Assemblage of aquatic organisms, usually microscopic, that drift passively or, if they swim, are generaly unable to move against the prevailing currents. These include bacteria and microalgae (phytoplankton), microscopic crustacea, cnidarians, larvae of fish and other animals (zooplankton). They constitute the base of the aquatic food chains. ^[1].

This is the common definition for Plankton, other definitions can be discussed in the article