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Mostly autotrophic microscopic algae which inhabit the illuminated surface waters of the sea, estuaries, lakes, and ponds. Many are motile. Some perform diel (diurnal) vertical migrations, others do not. Some nonmotile forms regulate their buoyancy. However, their locomotor abilities are limited, and they are largely transported by horizontal and vertical water motions.

A great variety of algae make up the phytoplankton. Diatoms (class Bacillariophyceae) are often conspicuous members of marine, estuarine, and fresh-water plankton. Dinoflagellates (class Dinophyceae) occur in both marine and fresh-water environments and are important primary producers in marine and estuarine environments. Coccolithophorids (class Haptophyceae) are also marine primary producers of some importance. They do not occur in fresh water.

Even though marine and fresh-water phytoplankton communities contain a number of algal classes in common, phytoplankton samples from these two environments will appear quite different. These habitats support different genera and species and groups of higher rank in these classes. Furthermore, fresh-water plankton contains algae belonging to additional algal classes either absent or rarely common in open ocean environments. These include the green algae (class Chlorophyceae), the euglenoid flagellates (class Euglenophyceae), and members of the Prasinophyceae.

The phytoplankton in aquatic environments which have not been too drastically affected by human activity exhibit rather regular and predictable seasonal cycles. Coastal upwelling and divergences, zones where deeper water rises to the surface, are examples of naturally occurring phenomena which enrich the mixed layer with needed nutrients and greatly increase phytoplankton production. In the ocean these are the sites of the world's most productive fisheries.

McGraw-Hill Concise Encyclopedia of Bioscience. © 2002 by The McGraw-Hill Companies, Inc.


Planktonic plant life.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
References in periodicals archive ?
Table II.- Phytoplankton cell abundance (Cells/L) and their percent (%) contribution during fish killing incident of Al-Arbain and Al-Shams lagoons at Jeddah coast, Saudi Arabia, the Red Sea.
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Water transparency (Secchi depth, SD) was measured and hydrochemical as well as phytoplankton samples were collected monthly during the growing season in 1997-2015.
A team of specialists from MOCCAE has been collecting and analyzing water samples from various coastal areas of the Arabian Sea and the Gulf of Oman to identify the types of phytoplankton present.
Since plants and phytoplankton both reduce the net carbon in the atmosphere, studying them and predicting their future has direct implications for where the Earth is headed in a time when human activity is adding carbon to the atmosphere at an unprecedented rate.
Different levels of disturbance affect community organization in various ways (Lopes, Ferragut, & Bicudo, 2009) and may interrupt, postpone, or redirect seasonal phytoplankton successions (Znachor, Zapomelova, Rehakova, Nedoma, & Simek, 2008).
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