Zooplankton belong to the Kingdom Protista. This kingdom contains many unicellular organisms which do not fit into any other kingdom. Protists are classified as any organism which is not an animal, plant, or fungus. Although this kingdom contains mostly unicellular organisms, it can also contain some multicellular organisms which include algae. Members of this kingdom can be either heterotrophic or autotrophic. The majority of these organisms live in water where a few may live in moist soil or even the human body. All organisms within this kingdom are eukaryotic.
Plankton can be split into two primary groups which include phytoplankton and zooplankton. Zooplankton are very tiny in size and typically arise within a variety of aquatic environments. They are classified according to size as well as developmental stage. For further details regarding how to taxonomically classify these organisms within the Kingdom Protista, please refer to provided resource.