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Plankton Definitions

July 26, 2006      

Plankton (Greek: planktos= drifting) drift passively or swim so weakly that even modest currents push them around. They are often defined as small animals and plants less than 5 mm long (0.197 in) although a few ocean forms can be bigger than humans. An object 1 mm (1,000 µm) long is about as small as the unaided human eye can comfortably see. Plankton size is usually measured in microns. 1,000 microns (µm) equals 1 mm or 0.03937 inch.

It is estimated by some, that over 90% of the world's oxygen is produced by four groups of marine phytoplankton, which are the basic food supply for larger plankton and some larger animals. Diatoms and dinoflagellates are small, 10-243 µm, and picoplankton -- blue-green (cyanobacteria) and green (prochloriphytes) algae are even smaller, 0.2-2.0 µm long. The most commonly found zooplankton are arthropods (tiny crustaceans and insects) and the eggs and larval forms of aquatic animals such as mollusks, fish, and protozoans. Invertebrate larvae (63-560 µm) and copepoda (crustaceans) (153-2,000 µm) are common in fresh water.

Copepods - (Latin: cope= 1 shell) a subclass of crustaceans.

Crustacea - (Latin: Crusta= crust, shell) arthropods with a chitinous exoskeleton. These are the most abundant zooplankton which eat diatoms, copepods, fish eggs, larva, and other small organisms. Large freshwater daphnia (water fleas) may be from 153-5,000 µm. A 20 or 30 power magnifier will often allow easy viewing of their insides.

Cyanobacteria - The smallest plankton (< 0.2 µm) blue-green algae are abundant in the oceans and sometimes in freshwater. Their outer membrane is very hard to digest; not many larger plankton eat them until that membrane is destroyed by a few species of bacteria and virus. They also may form large algae cluster mats.

Diatoms - small mobile plants (algae) 0.5-200 µm long, with silicified (silica, sand, quartz) skeletons. They are the most abundant phytoplankton in the cold oceans. A 63 µm net will get a good sampling of available diatoms.

Ichthyoplankton - (Greek: ichthys= fish) includes fish eggs, newly hatched eggs (fry), young fish, and adults of small fish. Sizes are from 153 µm long to 5,000 µm or longer.

Macroplankton or Macroinvertebrates - (Greek: makro-= big) usually means zooplankton. Several lower size definitions exist; the Canadian E-MAN protocol indicates that these organisms are retained by mesh sizes of approx. 200-500 µm; the USEPA likes 500 µm, some US states prefer 425 µm, and much historical research used 363 µm.

Microcrustacea - Those crustaceans between 153 µm and 363 µm in length.

Microplankton - (Greek: mikro-= small) usually refers to phytoplankton.

Nannoplankton - (Greek: nanno-= dwarf or very small). Generally any plankton, usually plants, smaller than 80 µm; many important nannoplankton are only 0.2-2.0 µm long.

Net plankton - old term; plankton captured in a 80 µm net.

Phytoplankton - (Greek: phyton= plant). Generally, 63-153 µm long. Some algae form large clusters, clumps, and thread-like groups of considerable size, but individual cells are usually in the 63-153 µm range. Large algae clusters and clumps are not always defined as plankton.

Plankton - (Greek verb: planktos= to wander or drift). The passively floating or weakly swimming animal and plant life in either fresh or marine waters. Many plankton reproduce daily, some hourly, and some every 10 minutes when the temperature is optimal (often close to 0°C), and when essential nutrients and foods are present.

Rotifers - (Latin: rotatus= wheel). A multi-celled animal which has a food ingestion tube with an area of strong cilia whose motion gives the appearance of a rapidly revolving wheel. Fun to watch live under a microscope. Rotifer sizes range from 1-600 µm. Most are between 200-400 µm long, including spines.

Zooplankton - (Greek: zoi= animal life). Includes the eggs, young, and even small adult animals of all animal species. Zooplankton are generally longer than 153 µm, up to about 5,000 µm (5 mm) or about 0.2 inches.

References:

• Rosenberg, D.M., I.J. Davies, D.G. Cobb, and A.P. Wiens. 1997. Draft- Ecological Monitoring And Assessment Network (EMAN) Protocols For Measuring Biodiversity: Benthic Macroinvertebrates In Fresh Waters. Dept. Of fisheries & Oceans, Freshwater Institute, Winnipeg, Manitoba. 530, appendices.
• Wildlife Supply Company. 97-98. Aquatic Sampling Instruments & Equipment for Reliable Results. Saginaw, Michigan. 104pp.

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