The number of plankton is uncountable, even if you were to tryto determine specie count rather than population.Plankton include many creatures - bacteria, algae, variousprotozoa, diatoms, etc - that drift in the ocean and are food formany marine animals. There are phytoplankton that make their ownfood via photosynthesis. Zooplankton eat phytoplankton or smallerzooplankton. Bigger animals eat the zooplankton, and so on.