The Collaborative International Dictionary of English v.0.48:

Plankton \Plank"ton\ (pl[a^][ng]k"t[o^]n), n. [NL., fr. Gr.
   plagto`n, neut. of plagto`s wandering, pla`zesqai to wander.]
   (Biol.)
   All the animals and plants, taken collectively, which live at
   or near the surface of salt or fresh waters. --
   Plank*ton"ic, a.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

plankton
    n 1: the aggregate of small plant and animal organisms that
         float or drift in great numbers in fresh or salt water

Moby Thesaurus II by Grady Ward, 1.0:

53 Moby Thesaurus words for "plankton":
   Iceland moss, Irish moss, Loch Ness monster, alevin, benthon,
   benthos, brown algae, cetacean, conferva, dolphin, dulse,
   fingerling, fish, fry, fucoid, fucus, game fish, green algae,
   grilse, gulfweed, kelp, kipper, lichen, man-eater,
   man-eating shark, marine animal, minnow, minny, nekton, panfish,
   phytoplankton, pond scum, porpoise, red algae, reindeer moss,
   rockweed, salmon, sargasso, sargassum, scum, sea monster, sea moss,
   sea pig, sea serpent, sea snake, sea wrack, seaweed, shark, smolt,
   sponge, tropical fish, whale, wrack