The term plankton is derived from the Greek word planktos, meaning wandering, and is used to describe any of a diverse array of prokary-otic (Bacteria and Archaea) and eukaryotic (Eukaryota) aquatic organisms that possess limited mobility and thus float, or passively drift, in the water column. Many taxa are holoplanktonic and spend their entire life cycle in the plankton, while others are mero-

planktonic, spending only a portion of their life cycle in the plankton. Planktonic organisms are found in both marine and freshwater ecosystems. Marine ecosystems physically dominate the planet, inasmuch as oceans cover approximately 71 percent of the earth's surface. In comparison, freshwater ecosystems cover less than 1 percent of the surface of the earth but contain a larger proportion of life's diversity.

Plankton have traditionally been subdivided into two main groups—phytoplankton and zooplankton—based on their mode of nutrition. Phytoplankton manufacture their own food through photosynthesis (photo-autotrophs) and are represented by a number of taxonomically diverse groups, such as cyanobacteria and other photosynthetic bacteria, and single-celled and multicellular photo-synthetic eukaryotes. Phytoplankton are, therefore, limited to dwelling in the upper, sunlit waters of a body of water. These surface waters, known as the photic zone, constitute a layer of water that extends from the air-water interface to the depth at which 99 percent of all sunlight is absorbed. In the clear tropical waters of the world's oceans, the photic zone may extend as deep as 200 m, whereas in inland freshwater lakes, the maximum depth to which light can penetrate is around 100 m.

Zooplankton are heterotrophic organisms and thus must obtain their nutrition by feeding on other organisms or by maintaining intracellular symbiotic associations with unicellular photosynthetic organisms. A diversity of organisms are classified as zooplankton, including numerous single-celled eukaryotes (flagellated and nonflagellated), and meta-zoans. As collecting techniques have become more sophisticated throughout the years, it has been recognized that smaller-size organisms dominate plankton communities. Bacterio-plankton include both heterotrophic and autotrophic bacteria; viroplankton include viruses that infect both prokaryotic organisms (bacteriophages) and eukaryotic organisms (viruses).

Although most plankton are less than 1 cm in size, plankton span a broad size range. The size classes used to describe plankton include: megaplankton (20-200 cm), meso-plankton (0.2 to 20 mm), microplankton (20-200 pm), nanoplankton (2-20 pm), picoplankton (0.2-2 pm), and femtoplank-ton (0.02-0.2 pm).

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