were made, the surface waters of Lake Mendota showed 28.5 ppm 

 of calcium and 5.0 ppm of carbonates. This lake lies in the limestone 

 region of the state and supports a eutrophic type of flora. 



Nitrogen and Phosphorus 



Special mention should be made of the significant roles of nitro- 

 gen in its various forms, and of phosphorus. The importance of these 

 nutrients has been measured, although somewhat incompletely. That 

 they are potential determiners of ultimate productivity is evidenced 

 clearly by the many hmnological studies which ha\'e related high 

 nitrogen and phosphorus readings to luxuriant phytoplankton floras 

 (Harvey, 1926; Wiebe, et al, 1929; Domogalla and Fred, 1926). 

 The negative correlation of relatively high concentrations of these 

 elements with periods of phytoplankton minima provides inferen- 

 tial evidence of their use by green plants. That is, when nitrates 

 and phosphorus are low the phytoplankton population is high, the 

 nutrients being consumed and stored in the organisms. When the 

 phytoplankton decreases through an accelerated death rate and 

 distintegration occurs, the elements are released and their per- 

 centages in the chemistry of the lake rises. Nitrogen enters di- 

 rectly into the phytoplankton cycle. Nitrogen content, in turn, is 

 dependent upon several physical processes in and around the body 

 of water (run-off from agricultural lands, for example). The 

 nature of the bacterial flora, therefore, the chemistry of the drain- 

 age water, and the presence or absence of nitrogen-fixing bacteria 

 and algae, are some of the more important factors determining 

 nitrogen content. High oxygen content permits a rich plankton 

 flora, but when nitrogen is low or absent, many kinds of algae are 

 excluded from the flora. Some species of the Cyanophyta are 

 especially rich in proteins and require, therefore, a highly nitro- 

 genous medium. Thus a dense bloom of Aphanizomenon flos- 

 aquae or Microcystis aeruginosa would account for a great fluctua- 

 tion in the free nitrogen and nitrate content of lake water at different 

 periods of the year. Nitrogen fixation by some species of blue-green 

 algae (see De, 1939; Fogg, 1942; Fritsch and De, 1938; Hutchinson, 

 1944) is also involved here. This specific relationship to nitrogen 

 fixation is a reminder of the many reasons that Hmnological studies 

 which involve analyses of the biota should include specific, not 

 merely generic, determinations of the organisms concerned. Lim- 

 nologists not infrequently list only algal genera in the published 

 results of their studies, and this is not in keeping with the best 

 scientific procedure. 



Here it might be well to recall that although nitrogen content, 

 carbon dioxide, phosphorus, and other elements are able to deter- 



[41] 



