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 grow1:hs, are, in shallow waters at 

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

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

 inadequate for 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 



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

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



on the life of a great river. 



solid carbonates (CaCOs and MgCOs). 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- 



