244 U. S. BUREAU OF FISHERIES 



small bog lakes are omitted from consideration. For the area studied there 

 can be prepared maps showing the distribution of the items obseiTed along the 

 several lakes, and thus can be shown the regional limnology of the area in a way 

 that has not been possible before. 



There is no reason to believe that the lakes of the rest of the district differ 

 widely from those of its' central area, which has been covered by our study. 

 It was found that the lakes in the more distant parts of the area were like 

 those nearer to Trout Lake; the lakes still more distant are like the others in 

 area and depth, and their contents are probably also similar. 



In 1926 the Bureau of Fisheries, published a report on the organic content 

 of lake water (Bulletin of the Bureau of Fisheries, Vol. XLII ; Doc. No. 1012), 

 giving the results reached by us in the examination of the organic content of 

 dry residues obtained from large samples of lake water — 50 to 525 liters each. 

 These quantities of water from lakes of southeasteni Wisconsin yielded dry 

 residue in amounts varying from 1,000 to 10,000 milligrams. It was mani- 

 festly impossible to evaporate such large quantities of water from large num- 

 bers of lakes, and it was also impossible to handle such quantities in temporary 

 field laboratories. The survey therefore has applied to the study of lake 

 residues the methods of microanalysis whose principles were worked out by 

 Professor Pregi, of Graz, Austria. These methods are employed extensively in 

 Europe, but have received little or no attention in America, either in the teach- 

 ing of chemistry or in practice. Their essence lies in the use of refined methods 

 and of a vei-y sensitive and accurate balance, so that only 5 to 10 milligrams 

 of the substance need be used for an analysis. This survey imported such a 

 balance in 1925 and also Pregl's apparatus for carbon and nitrogen determina- 

 tions. It appeared that the apparatus had to be modified considerably before it 

 was suited to the special problems offered by lake residues, and it was not until 

 1926 that it could be put to active use. Up to October 1, 1927, about 325 de- 

 terminations of the organic nitrogen in residues were made by microanalytical 

 methods, and about 525 determinations of organic carbon. This work on the 

 residues from the summers of 1925 and 1926 is now nearly complete ; 225 deter- 

 minations of carbon have been made on residues of 1927, and 183 remain to be 

 done. Thei-e are fewer than 100 samples whose nitrogen must be determined. 

 In most cases during the past summer the organic nitrogen was determined 

 directly from the water. 



The employment of these refined methods of analysis have made it possible 

 to secure sufiicient material by evaporating relatively small quantities of water. 

 During the past summer from 2 to 4 liters of water constituted a sample. 

 The amount of residue ranged from a minimum of 9.2 milligrams per liter 

 to a maximum of 95.4 milligrams. The average for 408 samples was almost 

 exactly 40 milligrams per liter. It may be of interest to note that during the 

 field season of 1927 more than ly^ tons of water from the northern lakes was 

 evaporated, leaving a total dry residue of about 2.1 ounces. The amount of 

 kerosene oil required as fuel for the evaporation was nearly as great as the 

 amount of water. 



The study of these residues will be continued actively during the current 

 academic year. The University of Wisconsin has provided funds for four 

 research assistants, who will give half time to chemical work on the lakes,, 

 chiefly on these residues. The main work of one man is on the organic car- 

 bons. He will complete the work on the samples from the northern lakes and 

 also will analyze samples from southern lakes for comparison. He will deter- 

 mine carbon in plankton samples, also. We estimate that about 600 such de- 

 terminations will be needed. A second assistant studies the nitrogen and 

 phosphorus of the residues. He will complete the examination of the samples 

 and will check the colorimetric methods used in the field. If time permits, he 

 will check, in a similar way, the methods of determining soluble silica. A 

 third assistant is making mineral analyses of residues. He begins with the 

 larger samples, using macroanalytical methods. He will go on with micro 

 methods, especially for the determination of potassium and sulphates. While 

 micro methods for these last substances reduce the amount of material needed,^ 

 they do not cut down the time required for analysis. It will not be possible, 

 therefore, with the means at our command, to examine all or even the greater 

 part of our samples in this way. The fourth assistant is analyzing the bottom 

 deposits of the lakes, both as to organic and inorganic content. In addition^ 

 apparatus is being constructed to work out micro methods for the determina- 

 tion of ether extract (fat) from the residues. 



