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What Are Plankton Animals?

Plankton are an aggregate of small plant and animal organisms that float or drift in great numbers in the Earth's waters. These small organisms make up the most abundant food source in the aquatic animal kingdom. Animal plankton are called zooplankton and they range in size from protozoans, like microscopic Paramecium, to the large Portuguese Man-of-War seen floating on the ocean surface.

Taxonomy
- Zooplankton are classified by size or stage of development. Categories for size include: picoplankton, animal organisms measuring less than 2 micrometers (microscopic measurement), nanoplankton measure between 2-20 micrometers, mesoplankton are between 0.2-20 millimeters, macroplankton measure between 20-200 millimeters and megaplankton, any zooplankton over 200 millimeters (or 8 inches).
  
  Developmental stage classifications are indicated as meroplankton and holoplankton. Meroplankton are larvae that grow into worms, mollusks, crustaceans, coral, echinoderms, fishes or insects. Holoplankton remain plankton their entire life and include pteropods, chaetognaths, larvaceans, siphonophores and copepods.
Zooplankton Communities
- Zooplankton exist in both salt water and fresh water and, next to bacteria, are the largest animal organisms that exist on the planet. Zooplatonic communities consist of numerous phylum (species) which are uniquely adapted to environmental factors like temperature, salinity, light and turbulence of the water. As an example, the zooplankton on one coastal bank will differ from the community a mile down shore. For this reason, scientists use zooplankton communities to distinguish the particulars and statistics of one water source from another. For example, a change in zooplankton concentration can indicate a change in water quality.
Influential Factors
- Distribution of zooplankton is influenced by the amount of resources available, reproduction and communal interactions. Since zooplankton feed on phytoplankton (plant plankton), their numbers increase during the spring algae bloom.
  
  Zooplankton are also affected by levels of aluminum, pH, calcium and heavy metals in the water. Nutrients like phosphorus and nitrogen will affect the prey of zooplankton, indirectly affecting zooplankton survival.
Most Common
- The sub-class copepoda is the most common of the saltwater zooplankton. Copepods are a numerous collection of crustaceans including over nine thousand species. The average size of copepods is 0.5-15 millimeters, but some grow to an extraordinary 8 inches.
  
  Freshwater copepods are a zooplankton animal that is physiologically comparative to their saltwater cousins. These micro-organisms are most numerous in the summer, because of the vast amounts of phytoplankton available.

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