I 



The Plankton Algae of the Palisades Interstate Park 109 



may be made, therefore, by a determination of the fixed carbon 

 dioxid. When such determinations are made all gradations are 

 found from very soft waters with less than 5 c.c. of fixed carbon 

 dioxid per liter, through medium hard lakes with 6-22 c.c. per liter, 

 to hard-water lakes with more than 23 c.c. per liter. Since medium- 

 and hard-water lakes have this greater potential supply of carbon 

 dioxid it is not at all surprising to find in computing the plant popu- 

 lation in different types of lakes that those with soft waters have a 

 much smaller total bulk of plankton than do those with medium and 

 hard waters. The mass of material available for photosynthesis is 

 not, however, the sole regulating factor, for other chemical relations 

 have an influence upon the growth of algae, but these are much 

 more uncertain. 



Another method of approaching the conditions under which algae 

 grow is that of studying carefully controlled laboratory cultures, 

 where the different chemical factors can be modified to suit the wish 

 of the investigator. The literature on the subject of pure cultures 

 will be found in the papers of Rayss ('15) and Vischer ('19). 

 This promising field, particularly with reference to the blue-green 

 algae, has not been developed to the fullest, primarily because of 

 technical difficulties in the way of cultivating blue-green algae. 

 Pringsheim ('13) has carried on experiments with blue-green algae, 

 which unfortunately were not plankton species, under conditions of 

 pure culture, and finds that as far as the nitrogenous nutrition is con- 

 cerned the blue-green algae may use either ammonia, nitrites 

 or nitrates as the source of nitrogen. He also investigated a large 

 number of organic nitrogenous compounds as sources of nitrogen 

 but found that they were of little use to these algae. This latter 

 is of great importance because it is a well-known fact that blue- 

 green algae grov/ best in waters that are slightly contaminated, and 

 the more frequent occurrence of blooms as a region becomes more 

 settled and as swamp or town drainage enters adjacent lakes, has 

 been pointed out quite frequently. Up to a certain point it is un- 

 doubtedly true that a greater supply of nitrogen favors the growth 

 of algae, particularly the blue-green algae. 



The general opinion until recently has been that blue-green algae 

 can make use of disintegration products in water and that their 

 luxuriant growth in stagnant waters is due to the considerable 

 amount of nitrogenous organic matter present. Pringsheim's in- 

 vestigations show that these organic substances are not a source of 

 food, and that the blue-green algae grow in spite of them and not 

 because of them. It is because blue-green algae are able to grow 

 under conditions where other algae will not survive, especially in 

 waters that are poor in oxygen, that they are found in lakes rich in 

 organic matter. 



Temperature. Another factor influencing the growth of algae is 

 that of temperature. During the cold months of winter the tempera- 

 ture of the water is such that there is usually but little growth of 

 algae, but with the warming during the spring months there arrives 

 a condition favorable for their growth. With the cooling off of the 



