plankton

Concise Oxford English Dictionary © 2008 Oxford University Press:
plankton /ˈplaŋ(k)t(ə)n, -tɒn/

▶noun the small and microscopic organisms drifting or floating in the sea or fresh water, consisting chiefly of diatoms, protozoans, small crustaceans, and the eggs and larval stages of larger animals.

– derivatives
planktic adjective,
planktonic adjective.

– origin C19: from Ger., from Gk planktos ‘wandering’, from the base of plazein ‘wander’.