QUALITY OF ALGAL FLORAS 



The physical-chemical factors of the environment act not only in 

 selecting the quality of a flora, but sometimes the very quaHty deter- 

 mines tlie quantity. A single example to illustrate this is the fre- 

 quently encountered case of two lakes which support quite different 

 quahties of phytoplankton, one predominantly chlorophycean, the 

 other conspicuously cyanophycean. The factors which have acted 

 selectively to determine the qualities of these floras are well known 

 and include those mentioned above. The cyanophycean lake is high 

 in nitrogen, there is a relatively large amount of phosphorus avail- 

 able, the water is alkahne, with a pH ranging from 7.2 to 9.5, and 

 there is an abundance of free carbon dioxide or half-bound carbon 

 dioxide. This is tiie eutrophic type of lake and because frequently it 

 is relatively shallow the summer temperatures are high (25-30 C.). 

 Thus, although the characteristics of the hard water lake are far 

 from being detrimental to a chlorophycean flora, the combination of 

 factors is such that an ultra-favorable habitat is provided for the 

 blue- green algae and the diatoms. It is well known that the phys- 

 iology of many species in these groups is such that they are able to 

 carry on cell division and vegetative reproduction at an astoundingly 

 high rate. In fact, certain species of blue-green algae are so much 

 more successful in this respect than others even in the same group, 

 that they completely take over at the expense of competing forms. 

 Some pelagic Anabaena spp., Aphanizomenon jios-aquae, Gloeo- 

 trichia echinulata, CoelospJmerium Naegelianum, or Microcystis 

 aeruginosa, for example, either singly or together, may constitute 

 nearly 100 per cent of both the quantity and the variety of the phyto- 

 plankton because of their abihty to reproduce rapidly. These organ- 

 isms are often responsible for water blooms, the excessive growth 

 being directly related to the quality of the flora. 



In the chlorophycean lake, which frequently is the oligotrophic 

 type, there is a minimum of nitrogen and phosphorus, the water is 

 deeper than in the cyanophycean lake, there is proportionately less 

 water in contact with the bottom, and the temperature is generally 

 lower. Although the surface temperature may be the same in the two 

 lakes at a given time in mid or late summer, the chlorophycean lake 

 reaches its maximum temperature at a later date than the eutrophic 

 or cyanophycean lake because of the greater amount of water in the 

 hypolimnion and because of the greater time consumed in the 

 spring-summer overturn. With little carbon dioxide available, there 

 is no opportunity for a luxuriant growth of phytoplankters. The flora 

 that does develop here may contain many blue-green and diatom 

 species, but the greatest number of forms are chlorophycean. Even 



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