standing crop (the number of individuals living 

 at one instant) in any area is limited by the 

 amount and combination of nutrients in the 

 water; by temperature; by the quality, dura- 

 tion, and penetration of light; and by the total 

 mortality of the organisms. The standing crop, 

 therefore, varies sharply from one place to 

 another at a given time and from time to 

 time in the same place. 



Among the smaller and more abundant 

 forms of algae in the phytoplankton are the 

 diatoms. Areas where diatoms occur in heavy 

 concentrations often are referred to as "pas- 

 tures of the sea"; here diatoms may contrib- 

 ute as much as 98 percent of the standing crop 

 of organic matter. The fish and even the great 

 whales depend ultinnately on them for sus- 

 tenance. Fossil remains of diatoms indicate 

 that in eons past, as at present, billions, 

 possibly trillions, of tons of them must have 

 been produced annually in excess of the enor- 

 mous amounts required to feed the animals of 

 the sea. 



Diatoms surround themselves with walls of 

 silicon dioxide in the form of two valves, one 

 of which overlaps the other. These'walls are 

 ornamented by characteristic patterns-- some 

 of exceptional beauty-- which taxonomists use 

 to identify species. After death of the diatom, 

 bacteria dispose of the soft body, but the walls 

 rennain intact and sink to the bottom. The 

 accumulation of shells may reach a consider- 

 able thickness over a long period of time. 

 One deposit at Lompoc, Calif., formed on the 

 bottom of a prehistoric sea but now a part of 

 the land area as a result of geological changes 

 in the earth's crust, extends over many square 

 miles and is more than 700 feet thick in places. 

 One scarcely can conceive the myriads of in- 

 dividuals required to produce such immense 

 bulk. 



Bacteria, too, are very small andare pres- 

 ent almost everywhere in the aquatic and 

 terrestrial worlds. Bacteria are much like 

 algae but lack chlorophyll and must secure 

 their food ready made from the bodies of 

 other organisms or their products. Vegetative 

 cells of bacteria grow to a maximum size and 

 then divide by fission into two bacteria of 

 equal size. Some surround themselves with 

 a thick wall and thus become spores or rest- 

 ing cells that are resistant to far more 

 unfavorable conditions than are the vegetative 

 cells. No estimate of the abundance of bacteria 

 in the plankton has been made, but many in- 

 vestigators believe that the total bulk of 

 bacteria would be considerably more than that 

 of the diatoms. 



Together with yeasts and molds, bacteria 

 cause the decay of plant and animal tissues. 

 Through this process, bacteria release the 

 carbon, oxygen, and phosphorus that are locked 



^1/50,000 to 1/12,500 Inch long and 1/125,000 to 

 1/50,000 Inch wide. 



into the tissues and reduce them to simple 

 compounds such as carbon dioxide, water, ni- 

 trates, sulphates, and phosphates that can be 

 used again as food by living phytoplankters. 



USES OF PLANKTON 



Manufacture of Basic Nutriment 



Extensive investigations have shown that a 

 great preponderance of the basic or primary 

 nutriment in the aquatic environment is man- 

 ufactured by the unicellular microscopic algae 

 in the phytoplankton. These tiny plants contain 

 chloroplasts that can produce carbohydrates 

 from carbon dioxide and water in the presence 

 of light (a process called photosynthesis). The 

 plant cells then synthesize complex organic 

 compounds from suitable salts of essential 

 inorganic substances--chiefly carbon, phos- 

 phorus, and nitrogen. The plants that perform 

 this miracle of chemical conversion then be- 

 come the food of the zooplankton, which, by 

 themselves, are incapable of gaining nourish- 

 ment from inorganic substances in the water. 

 Whereas most primary producers are minute 

 single-celled plants from the microplankton, 

 the consumers are comparatively large single- 

 or many-celled animals from the macroplank- 

 ton. 



Food for Aquatic Animals 



The zooplankton that browse on diatoms 

 become the food of the larval young of nearly 

 all species of fish, and of adult fish that 

 remain plankton feeders throughout life (e.g., 

 minnows, anchovies, and menhaden). These in 

 turn, are food for large, carnivorous (flesh- 

 eating) fish and the whalebone (baleen) whales. 



Fish that have floating eggs must spawn 

 either on the feeding grounds for the young 

 or in places where the winds and water cur- 

 rents will take the eggs to the feeding grounds 

 by the end of the incubation period. The tiny, 

 newly hatched fish (less than 1/3 inch long) 

 feed on small plankters and will perish unless 

 food of appropriate size and variety is avail- 

 able to them a few hours to a few days after 

 they hatch. As the young fish grow, they eat 

 proportionately larger forms of plankton. 



Mackerel are voracious feeders. They fre- 

 quently cram their stomachs until they become 

 distended when certain types of plankton are 

 abundant. The young eat chiefly immature 

 copepods and ostracods, larvae of shrimp, 

 crabs, and some small fish. At times during 

 summer and fall, adult mackerel fill their 

 stomachs with Sagitta , an arrow- shaped worm, 

 and mollusks known as pteropods, in localities 

 where these animals are plentiful. During 

 spring in the North Atlantic, mackerel conn- 

 monly eat large quantities of copepods of the 

 genus Calanus , which are known as "red feed" 



