plankton

The collection of mostly small or microscopic organisms that drift or swim weakly in a body of water, including bacteria, diatoms, jellyfish, and various larvae. Plankton is an important food source for fish and other larger organisms.

In biology: The minor animals and plants, including especially the lower organisms, that float or swim together in the water, considered collectively and in contrast, with those that live upon the bottom under the water, or on land.

The minor animals and plants that float passively in the water, considered collectively and in contrast with those that swim actively. See nekton.

Specifically in phytogeography, an aquatic vegetation consisting of freely floating microscopic algæ.

All the animals and plants, taken collectively, which live at or near the surface of salt or fresh waters.