lo82 FRESH-WATER BIOLOGY 



from forming by using filtered water instead of water laden with 

 plankton. 



Plankton and Fish Life. The occurrence of plankton in natural 

 waters has a definite and direct bearing upon the occurrence of 

 fish Hfe. Algae and protozoa and such organisms play an important 

 part in the cycle of changes which extend from the decomposition 

 of organic matter by bacteria to the food supply of man. This 

 cycle may be followed through the several elements of organic 

 matter, namely, nitrogen, carbon, sulphur, and phosphorus. The 

 proteid products of metabolism are consumed by bacteria; bacteria 

 are eaten by protozoa and the nitrate formed by bacterial action in 

 the presence of oxygen is utilized as food by algae; algae and proto- 

 zoa are consumed by rotifers and Crustacea and these latter form 

 the basis of the food of many fish. Some fish are provided with 

 special mechanisms for straining the plankton from the water, a 

 notable instance of this being the menhaden, a salt-water fish 

 which swims with its mouth open. The water enters through the 

 mouth and passes out through the gills, while the organisms that 

 are thus removed are carried to the stomach. The late Professor 

 Peck showed by experiments at Woods Hole that the abundance 

 and size of the menhaden are closely related to the abundance of 

 plankton. Similarly oysters have been shown to be dependent 

 upon the occurrence of diatoms in the waters which flow over the 

 oyster beds. Experiments made by the writer in the Great South 

 Bay, Long Island, showed that the best oyster beds were located 

 near mud flats where diatom life abounded. 



Intimately connected with the occurrence of the plankton and 

 the bacteria associated with decomposition of organic matter is 

 the presence or absence of dissolved oxygen and carbonic acid in 

 the water of lakes at different depths, and fluctuations in the occur- 

 rence of these gases profoundly affect fish life. If the amount of 

 organic matter at the bottom of a lake or pond is large the water 

 below the thermocline may lose most of its oxygen during periods 

 of stagnation. It is impossible for fish to live under such conditions, 

 so that lakes and ponds which undergo stagnation are not likely to 

 contain such fish as naturally seek the colder water found only at 

 great depths during the summer. Thus it is seen that plaiikton 



