312 HENRY B. WARD: 



the greatest importance in its bearing on conditions of existence 

 in a lake. Drown (93) found that in water basins in winter 

 under the ice there is a deficiency in the amount of oxygen 

 present, which increases from the surface downward. In some 

 reservoirs the bottom water becomes even malodorous and as 

 poor in quality as during the summer stagnation period. This 

 was true only of lakes rich in organic material. The careful 

 and extended investigations of Hoppe-Seyler (96) on Lake 

 Constance, Switzerland, show a deficit of oxygen in deeper 

 waters above the calculated amount. The amount present, 

 however, is still sufficient to satisfy the respiratory needs of 

 the abyssal animals, even the most sensitive fish, such as trout. 

 Knauthe (98) maintains that in somewhat turbid waters the 

 micro-organisms demand more oxygen than fish and larger 

 forms and in stagnant waters far more than is contributed by 

 the atmosphere. In daylight the microscopic green plants 

 give off oxygen to the water so abundantly that in strong sun- 

 light the maximum is reached in a few hours. Even moon- 

 light causes an appreciable increase in the quantity of oxoygen; 

 but in darkness the amount sinks in five or six hours of sum- 

 mer temperature to the minimum necessary for the Cyprinidae. 



Calkins (!»3) groups the odors of freshwater into three 

 classes (1) those of chemical or putrefactive decomposition, (2) 

 those of growth, i. e., excretory products, and (3) those of 

 physical disintegration. All evidence points to oil globules as 

 the specific cause of those odors grouped under the last two classes. 

 Certain odors are associated with definite organisms. Jackson 

 and Ellms (97) were able to add to the evidence concerning 

 natural odors and the organisms producing them and to dis- 

 tinguish sharply between the natural odor and that produced 

 by the decomposition of the same organism. Here also 

 Whipple (94a). 



The geological and physical features of individual lakes have 

 been studied by Ule (94a) at Plon, Pero (95) in Italy, Large 

 ('.'7i in Indiana, Wagner (97) in Bohemia, and Lorenz von 

 Liburnau i 98). 



Among the articles noted on the phytoplankton Schroter (97) 



