mine abundance of the algal flora, they also influence the variety, i.e., 

 the kinds, of species present. As mentioned before, it does not follow 

 that because water is a universal and standard medium selectivity is 

 not being exercised rigorously. So clear-cut are some qualitative 

 selections operating in algal ecology that the phycologist is able to 

 use the presence of certain species or groups of species as indi- 

 cators of physical-chemical conditions in a body of water. Without 

 hydrogen ion determination equipment but just by observing the 

 quantity and quality of the algal flora, especially the phytoplankters, 

 the experienced worker can estimate the acidity or alkalinity ( some- 

 what roughly to be sure) and the relative abundance of carbon 

 dioxide; can predict whether there is a rich or poor supply of nitrates; 

 and can tell something of other limnological features in a habitat. 



Bottom Deposits 



Another way in which modification of phytoplankton quantity 

 and quahty may come about is through the effects produced in 

 bottom sediments when organic matter decomposes. The composi- 

 tion of the silt, or 'Gyttja', determines the quality of the bacterial 

 flora, a group of organisms which is important in fixing the manner 

 and speed with which nutrient elements are returned to solution. 

 It is obvious, of course, that the kind and quantity of biota supported 

 in the water above, as well as on, the floor influence the physical and 

 chemical nature of the bottom sediments, which, in turn, acts in de- 

 termining the number and, to a certain extent the kinds of bacteria 

 in the sediments. Also, a lake bottom which supports dense popula- 

 tions of mollusks and midge larvae will be affected by the fauna. 



In a lake where a luxuriant phytoplankton has become established, 

 especially in a cyanophycean lake, a great accumulation of nitro- 

 genous matter may result. As mentioned above, some of the blue- 

 green algae are nitrogen fixers, and many of them are great accumu- 

 lators of this element. Thus their physiology may result in an 

 ever-increasing supply of organic nitrogen in a lake. With bacterial 

 turnover, nitrogen appears first in one form and then another, and 

 eventually alters the chemistry of the water above bottom sediments 

 and so influences the biota. 



Thus a closed cycle of interchanging and interacting phenomena is 

 seen to be operating within a lake. The processes which make up the 

 cycle are forever fluctuating. It is easy, therefore, to use the analogy 

 so often made between organic metabolism and the flux, reflux, and 

 pulsations within a body of water. The productivity of an aquatic 

 habitat is achieved by its metabolism.' 



[42] 



