plankton in American English
the usually microscopic animal and plant life found floating or drifting in the ocean or in bodies of fresh water, used as food by nearly all aquatic animals
noun: the aggregate of passively floating, drifting, or somewhat motile organisms occurring in a body of water, primarily comprising microscopic algae and protozoa
plankton in British English
noun: the organisms inhabiting the surface layer of a sea or lake, consisting of small drifting plants and animals, such as diatoms