toward phytoplankters and other chlorophyll-bearing organisms. As 

 is well known these, both directly and indirectly, contribute food to 

 the microfauna, which in turn provides nourishment for the benthic, 

 limnetic, and emergent faunas. ('Emergent' refers to the animal life 

 which passes part of its life cycle in aquatic habitats, later emerging 

 to join the land fauna.) In addition to their role in the food chain, 

 chlorophyll-bearing organisms supply shelter and breeding places 

 for many kinds of aquatic animals, and they also have important 

 hmnological bearings in that they alter the chemistry of the water, 

 interfere with illumination, affect color, etc. 



Some of the phytoplankton (members of the Cyanophyta) are 

 able to fix nitrogen and so affect the supply of nitrates and the nitro- 

 gen cycle. Infonnation on the nitrogen-fixing activities of the aquatic 

 algae is meager, but evidence indicates that some forms play a more 

 important role than is generally recognized in this connection. 



The quantity of phytoplankton is determined, in part at least, by 

 the abundance of available carbon dioxide contributed by the atmos- 

 phere, by half-bound carbon dioxide from bicarbonates, and from 

 springs, etc. Carbon dioxide is, of course, continually being supplied 

 also by respiration in all forms of life, including bacteria. At the 

 same time all life draws upon dissolved oxygen, which diffuses from 

 the atmosphere or is contributed as a result of photosynthesis. 



Other elements necessary for plants, such as phosphorus and nitro- 

 gen, are supplied through drainage and by the continual overturn of 

 organic matter by bacteria and other destructive organisms. Var- 

 ious edaphic, limnological, and geological factors which determine 

 water chemistry, contour of the bottom, and the presence or absence 

 of seasonal overturns of lake water. These, in turn, help to determine 

 the nature of the bacterial flora, and hence the kinds and amounts of 

 decomposition products and the completeness with which the or- 

 ganic matter is reconverted. 



Lakes and streams may receive run-off water from agricultural 

 lands and effluents of waste matter which are richly supplied with 

 critical nutritive elements and compounds. These become incorpo- 

 rated in the food cycle. 



To an unknown extent, substances produced by bacterial decom- 

 position also furnish nutrients to the microfauna. Also unknown is the 

 extent to which nitrates for the use of green plants are formed by 

 nitrifying bacteria. Information is not at hand which will permit a 

 definite statement concerning this portion of the nitrogen cycle, but 

 there is ample justification for assuming that nitrification by bacteria 

 occurs under favorable aquatic conditions as it does in terrestrial 

 soils. 



[49] 



