The word “plankton” is an umbrella term for organisms that live their lives adrift in the water and are unable to move independently. The term comes from an Ancient Greek word which means “floating,” and these organisms do indeed float through bodies of water both fresh and salty around the world. Primarily, plankton live in the sunlit zone of the ocean, although some species are found in much deeper water. They are very important to life on Earth, as they serve as the bottom of the food chain. They nourish larger animals, which are in turn eaten by even bigger animals, and so on up to organisms like humans at the top of the food chain. Plankton are also responsible for the Earth's atmosphere, thanks to the efforts of billions of photosynthesizing phytoplankton.
There are three loose categories of plankton, although the nebulous identities of some species make them difficult to categorize. Phytoplankton are those with plant-like characteristics, such as diatoms. They photosynthesize for energy, and are sometimes found in large groups termed blooms. Many are unicellular, and are very simple animals.
Zooplankton possess animal-like characteristics, and can sometimes get very large. Jellyfish, for example, are zooplankton because, while they are larger than many other plankton, they too are at the mercy of the ocean's currents. Most eat phytoplankton for energy, and in turn serve as an energy source for larger animals in the ocean, such as whales. As organisms like the dinoflagellates demonstrate, some zooplankton are capable of limited motion, but they are still unable to resist currents of water.
Bacterioplankton are the third group. Like their counterparts on land, they consume waste products from other organisms. They can also photosynthesize for energy, and some species, such as those found by hydrothermal vents, are capable of chemosynthesis. They are also eaten by zooplankton.
Once categorized, there are two more divisions of these organisms. The first is the group of holoplankton, organisms that remain plankton for their entire lives. The second is meroplankton, larval forms of sea creatures like shellfish, fish, crustaceans, and many others. In their early stages, these larvae drift through the water, usually powerless to move, until they develop into older animals.
Although small, plankton play an important role in the health of the planet. Just like other organisms, they require nutrients and a balanced environment to live in. Their absence in a body of water indicates an environmental imbalance, as does a disproportionate number of unusual plankton. For this reason, some scientists spend a lifetime researching them, and new species are constantly being discovered and studied.