Plankton is a collection of pelagic aquatic organisms at the mercy of water currents. They are organisms unable to swim or, although they can swim, they have no power to resist these currents.

Microplankton - planktonic organisms that range in size from 50 µ to 1 mm (rotifers, many protozoa, small crustaceans, most algae).

From micro... and plankton.