Plankton are a type of drifting water dwelling lifeform. Plankton refers to a life-style of moving as water currents dictate and can be animals, plants or bacteria. (DS9 novel: The Tempest)
Cilas XII is home to a species of flowering plankton. (DS9 novel: Devil in the Sky)
Plankton loaf is traditional Japanese food made from Plankton which Keiko O'Brien regularly ate in her youth. (TNG episode: "The Wounded")