plankton

n. microscopic organisms which live in water and serve as a basic food source for aquatic animals

Plankton are any drifting organism that inhabits the pelagic zone of oceans, seas, or bodies of fresh water. It is a description of life-style rather than a genetic classification. They are widely considered to be some of the most important organisms on Earth, due to the food supply they provide to most aquatic life.
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Noun
1. the aggregate of small plant and animal organisms that float or drift in great numbers in fresh or salt water
(hypernym) organism, being
(member-holonym) aggregate, congeries, conglomeration
(member-meronym) phytoplankton
(part-meronym) copepod, copepod crustacean

n. plankton, microscopic organisms which live in water and serve as a basic food source for aquatic animals

n. plankton