REACTION IN WATER 97 



understood to require comment. However, the ability of green plants 

 to utilize the half-bound carbon dioxide of the bicarbonates is not so 

 generally known, nor is the important consequence in modifying the 

 ion concentration of the water. The detailed reactions are closely 

 related to the times and layers in which the processes occur and espe- 

 cially to the annual cycle of the lake itself in terms of spring and 

 autumn overturn, the relations of epilimnion and hypolimnion, etc. 

 In general, the increase of oxygen and decrease of carbon dioxide 

 pertain to the upper layer in which the phytoplankton is concentrated ; 

 the zone of oxygen deficit is usually near the bottom where decom- 

 position is often limited by the access of this gas. When this occurs, 

 anaerobic processes result, in some measure at least, and a variety of 

 gases may be produced, such as methane and hydrogen sulphide (cf. 

 Birge, 1903; Birge and Juday, 1911). 



In general, reactions upon the mineral constituents, have to do 

 with their abstraction by plants and their return through decomposi- 

 tion. Related to this is the reciprocal conversion of carbonates and 

 bicarbonates, especially of calcium and magnesium, the first change 

 effected by the carbon dioxide released and the second by the de- 

 mands of the phytoplankton when the free gas is low or absent. 

 Plants alone react upon the medium by reducing the quantity of those 

 minerals that provide essential constituents of the protoplasm, bub 

 the demands of different families and genera are not the same. Thus, 

 among plants, only diatoms reduce the amount of silica materially; 

 they also make heavy demands upon nitrates, according to Pearsall 

 (1922:248), while the reaction of desmids upon the nitrates is much 

 less. Animal reactions by removal are due chiefly to shelled forms 

 which exert their major influence upon calcium carbonate. For the 

 most part, these minerals are carried to the bottom in the bodies of 

 certain dead organisms and are there again rendered available by the 

 decomposition reactions of bacteria in particular. In this process, 

 soluble organic compounds are also produced. 



Streams 



Reactions upon medium and bottom are much reduced in streams, 

 by comparison with lakes, owing to the fact that the current renders 

 accumulation difficult and at the same time brings in new materials. 

 At the same time the produccnt phytoplankton is less developed as a 

 rule, and the total quantity of reactors is correspondingly less. As a 

 consequence, the significance of reactions in streams depends largely 

 upon the current and its swiftness, sluggish and swift streams being 



