The most important and always present representatives of phytoplankton (in ponds) are 

 the silicaceous algae Melosira and rragellaria . Among those appearing at times in enormous 

 masses are the Polycistis and the Volvox species, while blue-green algae — in the height of 

 summer — are the t:,'pical "water bloom" producers. The number of species present in negligible 

 amounts is very great. 



Bacteria 



In the previous pages we have dealt only with autotrophic plants, important from the 

 productive-biological viewpoint and producing autotrophic organic substances. But aside 

 from the flora of the pond, we have also to consider pond-bacteria and their destructive 

 and conversion functions. The knowledge of these functions is absolutely necessary since 

 the mode of treatment as well as of fertilizatiOTi of the respective ponds is determined 

 thereby. 



The functions of the bacteria within the metabolic cycle are chiefly heterotrophic, 

 caiising destruction of organic substances, hence may be considered as "reducing agents", 

 and for the sin^Dle reason that as consumers they also need organic substances for their 

 existence. 



The essential life conditions as well as the propagating possibilities for bacteria 

 are so favorable in a pond, that they are found practically everywhere and in enormous 

 quantities. The purest natxiral water can still contain 100 gems per 1 can, filthy water 

 as many as 10,000, and verj' filthy water even up to a million and more. In fact, v;e can 

 accept it as a rule that the multitude of bacteria is almost entirely dependent — aside 

 from warmth — upon the amount of assimilable organic substance. 



The p)ond-bacteria may verj'- well be grouped together with the above named flora, since 

 among then also we have free-floating species, sessile species — attached to objects or to 

 higher organisms — and species T^iich live upon the floor of the pond. Some feed upon 

 dissolved organic natter, others upon solid organic matter. 



The immensely important and useful functions of the bacteria consist in dissolving 

 (mineralizing) dead remnants of organic substances, upon which process all vegetation 

 depends. The lack of bacterial activity will lead to peat and morass conditions. 



In the process of deconposition of organic matter, the most important nutritional 

 item for plant life — caiton dioxide — is liberated. This in turn dissolves and binds 

 calcium (and other substances) so that carbon dioxide — in combined form — is preserved for 

 the pond and will thus aid in the assimilation of other iii^Jortant nutriments. 



The nitrogen, contained in waste protein is decomposed into urea and ammonia through 

 the activities of putrefactive bacteria and these substances again are converted into 

 nitrite and nitrate through the activities of nitrifying bacteria. (Plants consume all 

 nitrates with preference.) These biological processes are possible only in waters, rich 

 in oxygen and non-polluted by organic matter, therefore, such process will take place to 

 only a small extent in the uppermost mud layers of the pond floor and are of advantage by 

 stimulating productivity. 



The sulphur of proteins — ^when dissolved in the slime, is acted on by anaerobic 

 putrefactive bacteria, under exclusion of air — ^undergoes the transformation into hydrogen 

 sulphide (noticeable by its foul egg odor) and under the influence of air oxidizes into 

 sulphuric acid. 



All detritus and morass — especially mud, containing cellulose — is most quickly and 

 most conqjletely dissolved by bacteria under the free admission of oxygen, i.e. tiirough 

 aerobic putrefactive and fermentative bacteria. 



In thicker laj'ers of mud, where oxygen cannot penetrate deeply enough, this dissolving 

 process remains incomplete and leads to the development of marsh ^as and other gases, but 

 without reducing the masses of niud to any perceptible extent. 



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