Plankton are broadly divided into 2 groups: zooplankton and phytoplankton. Both groups of plankton are defined by their ecological niche at the bottom of the food chain rather than by size or taxonomy. Where plankton are divided into trophic troups, a third group of plankton, the bacterioplankton, is added.
Animal or plant?
Because the main difference between zooplankton and phytoplankton is the abilty of phytoplankton to generate energy from photosynthesis or chemosynthesis, phytoplankton were originally named because they were considered to be plants. Zooplankton, which are incapable of photosynthesis or chemosynthesis, were considered to be animals. However, this division is not as clear as it seems.
It is better to think of phytoplankton as autotrophic and zooplankton as heterotrophic. An autotrophic organism is capable of generating complex organic compounds by using energy from light (photosynthesis) or other chemical reactions (chemosynthesis). A heterotrophic organism cannot do this, so it must obtain organic carbon from other sources.
One form of plankton, blue-green algae, was once considered a plant. They have been reclassified as cyanobacteria, but they are still clearly autotrophic. The cyanobacterium Prochlorococcus is responsible for half the photosynthesis in the entire ocean.
In trophic divisions, the bacterioplankton are divided from the other 2 groups of plankton on the basis of their food source. Here, the bacterioplankton are detrivores, which feed on non-living matter. According to this division, phytoplankton are the base-level producers of the ocean, zooplankton are the base-level consumers of the ocean, and bacterioplankton are the base-level recyclers of the ocean.
However, some species in all 3 groups overlap with each other. Prokaryotic phytoplankton are also bacterioplankton. Dinoflagellates can produce organic compounds through photosynthesis, but also consume organic compounds directly.
Phytoplankton are found in regions which are close to the kind of energy they need for photosynthesis or chemosynthesis. This limits them to the photic zone, which is the part of the ocean where light can penetrate, and to undersea vents. The depth of the photic zone varies, but is a maximum of around 800 feet.
Zooplankton are not tied to an external non-organic energy source. They have no depth limits. However, they are tied to their food sources, which means that 90% of all marine life lives in the photic zone.
The vast majority of both phytoplankton and zooplankton are single-celled organisms which can easily be seen under low magnification. Diatoms, which can be unicellular or colonies, are among the largest phytoplankton, and can grow large enough to be barely visible with the naked eye.
The size range for zooplankton is much greater. Zooplankton includes organisms ranging in size from microscopic to larger than a human being. The marine viruses which fall under femtoplankton are less than 0.2 micrometers across. At the other end of the scale, the medusa form of the giant Nomura's jellyfish (Nemopilema nomurai) can have a bell of more than 6 feet across, with a weight of up to 440 pounds.