Plankton are a diverse collection of organisms found in water, including marine and freshwater ecosystems, that are unable to propel themselves against a current or wind. They are carried by tides, currents, and other forces, and cannot swim well enough to move against these forces. Plankton range in size from tiny microbes, which are invisible to the naked eye, to jellyfish meters long. The name plankton comes from the Greek word planktos, which means "wandering" or "drifting".

Plankton are classified into two groups: phytoplankton (plants) and zooplankton (animals) . Phytoplankton are microscopic plants that perform photosynthesis to convert the suns rays into energy to support them, and they take in carbon dioxide and produce oxygen. Zooplankton include microscopic animals, the young of larger invertebrates and fish, and weak swimmers like jellyfish. Most zooplankton eat phytoplankton, and most are, in turn, eaten by larger animals.

Plankton are an important food source for many large and small ocean creatures, and they play a vital role in absorbing carbon dioxide and producing oxygen in the ocean. They are also threatened by microplastics, which can block phytoplankton from receiving enough sunlight to survive. Without plankton, there would be few living organisms on earth, and certainly no Great Barrier Reef.