72. What is plankton? 



The word "plankton" is derived from a Greek word meaning wander- 

 ing. Plankton includes all sea animals and plants too small or weak to do 

 anything but drift with the currents. The plants are known as phyto- 

 plankton and the animals as zooplankton. Both are important food 

 sources for fish and other animals. 



The single-celled plants known as diatoms make up more than half 

 the plankton in the ocean. A cubic foot of sea water may contain 

 20,000 plants and only 120 animals or eggs. Phytoplankton uses the 

 nutrient salts and minerals in sea water as food. It, in turn, is food for 

 many animals, which are themselves part of the "food chain." 



Plankton "blooms" in the spring when nutrient-rich bottom water is 

 brought to the surface by storms. Longer days provide more light to 



stimulate plant growth and increase numbers rapidly. Phytoplankton 

 may spread over miles of ocean, discoloring the water with shades of 

 yellow, brown, or green. 



Many planktonic organisms are sensitive to changes in temperature 

 and salinity. Sudden changes can cause mass mortality, not only to the 

 plankton, but also to the animals that feed on it. 



Beside planktonic plants and animals, there is another group with 

 some characteristics of each. This group includes the dinoflagellates 

 which manufacture their own food, but also eat other organisms and 

 have means of locomotion. 



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