plankton vs organism

plankton and organism both are nouns.

Word	Noun	Adjective	Verb	Adverb
plankton	Yes	No	No	No
organism	Yes	No	No	No

As nouns, organism is a hypernym of plankton; that is, organism is a word with a broader meaning than plankton:

plankton: the aggregate of small plant and animal organisms that float or drift in great numbers in fresh or salt water
organism: a living thing that has (or can develop) the ability to act or function independently

Other hypernyms of plankton include being.

plankton (noun)	organism (noun)
the aggregate of small plant and animal organisms that float or drift in great numbers in fresh or salt water	a living thing that has (or can develop) the ability to act or function independently
	a system considered analogous in structure or function to a living body