Plankton are small organisms that live in open water aquatic habitats, below the surface and above the bottom. They are the base of the aquatic food web, as plant-like members of the plankton (phytoplankton) that have chlorophyll-a and fix carbon through photosynthesis, and in turn are grazed upon by animal members of the plankton (zooplankton). These zooplankton are then eaten by larger animals such as fish (nekton).
The zooplankton community in estuaries, including the Delaware Bay, includes holoplankton which are animals that live their entire lives in the water column (such as copepods). There are other animals, called meroplankton, that are present in the water column as larva but then metamorphosis into juvenile and adult stages. Common meroplankton in Delaware Bay are blue crabs, which have a life cycle that starts as an egg and proceeds through several planktonic zoea larval stages before settling to the estuary bottom. Because of these different lifestyles, zooplankton are identified by their use of the water column as a habitat and a heterotrophic mode of obtaining energy, rather than by their being in any one taxonomic group.