of as much as 2.4. This is, of course, a logarithmic expression of in- 

 crease and represents an enormous change in water chemistry with 

 far-reaching biological eflFects. 



Oxygen 



It is obvious that oxygen is one of the primary limiting and deter- 

 mining factors in phytoplankton ecology, as for all other forms of 

 life. Because of their photosynthetic activities, plants in daylight are 

 practically independent of free oxygen in solution. When carbon di- 

 oxide is present in sufficient quantities and other factors are favor- 

 able, chlorophyll-bearing organisms can automatically maintain the 

 required amount of oxygen needed for their own respiration. But at 

 night plants are required to draw upon free oxygen in the surround- 

 ing medium for this process. When there are excessive growths of 

 algae, particularly in warm shallow water when the oxygen content 

 is low, the available supply of oxygen may be reduced to a point be- 

 low the amount normally required by the fauna. Thus, by increasing 

 or decreasing the oxygen content, algae act as agents in determining 

 the quantity and kinds of animal life which a body of water may 

 support at different levels. Photosynthesis, however, is regulated by 

 such factors as carbon dioxide, discussed above, and light. 



Light 



Illumination as an ecological factor determines that most algae, 

 particularly plankters, occupy what is termed the photosynthetic 

 zone, the upper 2-5 meters of water. Turbidity, color, and amount of 

 disturbance at the surface all help to determine the depth to which 

 light favorable for photosynthesis will penetrate. Because of the 

 great amount of light lost at the surface through reflection and be- 

 cause of further reductions by absorption and diffusion, photosyn- 

 thetic plants are required to carry on their activities in the upper 

 levels. This explains the (usually) greater quantity of dissolved oxy- 

 gen in this stratum. The exhaustive studies of Birge and Juday 

 (1911) describe the gas content and its fluctuations in 156 lakes. 

 Their graphs clearly illustrate this relationship between oxygen and 

 the photosynthetic zone. In their Figure 135 (p. 243), curves are 

 shown for the oxygen content of Lake Mendota, Wisconsin, in July. 

 At this time there was a heavy growth of phytoplankters (Coelos- 

 phaerium sp., ApJianizomenon fios-aquae) of more than 5000 organ- 

 isms per liter. At the surface, the oxygen content was 6.6 ppm. From 

 the surface flora the number of algae was irregularly reduced to 

 about 1000 per liter at 20 meters, where the oxygen content was only 

 0.1 ppm. In May of the same year in which the above readings 



[40] 



