Barney and Anson. — The Top-Minnoiv 273 



in ponds also has a tendency to modify plankton production, 

 as the plants become competitors with the plankton organisms 

 for the available nitrogen. In his conclusions, Kofoid says : 



Summer heat pulses often attend plankton increases. * * * Light 

 affects plankton production. The half year with more illumination and 

 fewer cloudy days produces from 1.6 to 7 times as much plankton as 

 that with less illumination and more cloudy days. Seasons of unusual 

 cloudiness are accompanied by depression in production. * * * Lakes 

 rich in submerged vegetation produce less plankton than those relatively 

 free from it.* 



In view of the fact that plankton production is modified by 

 temperature, light and the presence of vegetation in the water, 

 it is safe to say that our open-surfaced pond 2 produced more 

 plankton than pond 1 with submerged vegetation, and that 

 production in ponds 3 and 4 was smaller than in either 1 or 

 2, since the waters of the former ponds had lower temperature, 

 less light, and contained competitive plants. Inasmuch, how- 

 ever, as pond 1 produced more fish than pond 2, the conclu- 

 sion can be fairly reached that the quantity of food in each 

 was at least sufficient, and hence was not a determining factor 

 in Gambusia production. Very possibly the production of 

 Gambusia in the surface-covered ponds 3 and 4 was limited 

 because of scarcity of food. At least, this is probably one 

 cause for the small output of these ponds. 



Passing from the consideration of food supply we are 

 confronted with the question of the other important factor, 

 oxygen supply. Oxygen may become dissolved in water by 

 two methods, mechanical and natural. Oxygen is introduced 

 into the water through the surface by mechanical means, such 

 as the effect of the wind, waterfalls and current, by the addi- 

 tion of falling rain, by the movement of animals on or in the 

 water, or by means invented to churn the water and cause air 

 to bubble through it. The natural means are of equal, if not 

 of greater, importance in ponds and small lakes, oxygenation 



*Ibid., pp. 572, 573. 



