50 



Nature of Aquatic Environment 



sumption of nitrates by plancton algae. Nitrates, be- 

 cause they furnish nitrogen supply in the form at once 

 available for plant growths, arc, in shallow waters at 

 . an index of the fertility of the water. As on 

 land, so in the water, the supply of these may be 

 inadr< [uate f< >r maximum productiveness, and they may 

 be added with profit as fertilizer. 



The carbonates — Lime and magnesia combine with 

 carbon dioxide, abstracting it from the water, forming 



PlG. 9. Environs of the Biological Field Station of the Illinois State Labora- 

 tory of Natural History, the scene of important work by Kofoid and others 

 on the life of a great river. 



solid carbonates (CaC0 3 and MgC0 3 ). These accumu- 

 late in quantities in the shells of molluscs, in the stems 

 of stoneworts, in the incrustations of certain pond 

 weeds, and of lime-secreting algae. The remains of 

 such organisms accumulate as marl upon the bottom. 

 The carbonates (and other insoluble minerals) remain; 

 the other body compounds decay and are removed. 

 By such means in past geologic ages the materials for 

 the earth's vast deposits of limestone were accumu- 



