plankton

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun 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.

from The Century Dictionary.

• noun 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.
• noun The minor animals and plants that float passively in the water, considered collectively and in contrast with those that swim actively. See nekton.
• noun Specifically in phytogeography, an aquatic vegetation consisting of freely floating microscopic algæ.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Biol.) All the animals and plants, taken collectively, which live at or near the surface of salt or fresh waters.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun a generic term for all the organisms that float in the sea. A single organism is known as a plankter

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the aggregate of small plant and animal organisms that float or drift in great numbers in fresh or salt water