398 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 39 



ton being greatest where run-off and renewal are least. In other 

 words, a swift stream allows little time for the production of plank- 

 ton, Avhereas a sluggish stream affords greater time. The plankton 

 productivity of a stream is proportional to the age of the water and 

 inversely proportional to its velocity, as Reinhard also found in his 

 study of the upper Mississippi River. 



Inorganic elements are required for the growth of the algae just 

 as they are essential for the growth of higher plants. In addition 

 to water, carbon dioxide, oxygen, nitrates, sulfates, and phosphates 

 are necessary parts of their food supply. The inorganic materials in 

 solution in the water vary in kind and concentration according to the 

 type of soil present in the drainage areas of the streams. Birge and 

 Juday, in their extensive studies of Wisconsin lakes, reported that 

 many soft-water lakes contain less plankton than the hard-water 

 lakes. The growth rate is increased in the hard-water lakes by the 

 presence of bicarbonates and greater amounts of carbon dioxide in 

 the water. On the other hand, soft water and bog drainage pro- 

 mote the growth of some green algae. An excess of chlorides is also 

 found where there is great production of algae. Although chlorides 

 are not utilized directly by the algae, an excess of chlorides in the 

 water usually indicates that the water contains a relatively large 

 mass of fertilizing substances coming from sewage contamination. 

 Small amounts of these fertilizing substances are beneficial for plank- 

 ton growth. Minder, in his biochemical stud}'^ of the Lake of Zurich, 

 concluded that nitrogen in the form of nitrates is the principal factor 

 that regulates the quantity of plankton. There is a direct relation- 

 ship between the diatom production and the silica content of the 

 water, since diatoms utilize silica to form their cell walls. Pearsall, 

 in his study of English lakes, reported that diatoms were more abun- 

 dant in the silted lakes than in the rocky lakes. 



Algae give off oxygen in the growth process. This sometimes 

 accumulates to the point of supersaturation in the water. The ani- 

 mal life, Protozoa as well as fish, use this oxygen and give off in 

 turn carbon dioxide, which the plants use in their manufacture of 

 food. When the balance of this mutually beneficial exchange of 

 gases is upset, either the plants or the animals suffer. If the algae 

 are destroyed in a pond supporting many fish, the subsequent lack of 

 oxygen affects the fish directly as well as by indirectly limiting their 

 food supply. 



Olson has made interesting observations on the interrelationships 

 of sunlight, algae, and fish. The amount of oxygen dissolved in 

 the water varies with the temperature from 7.63 parts per million at 

 86° F. to 14.62 parts per million at 32° F., showing that the solubility 

 of oxygen in water is greatest at low temperatures. In a lake or 



