One of the disadvantages of the centrifuge method of concentrat- 

 ing the plankton is that certain organisms lose their normal appearance 

 and are difficult to identify under the low power of a microscope. With 

 most organisms this loss of normal appearance results merely in a slow- 

 ing of the counting process, but with Aphanizomenon the result may be 

 more objectionable, Aphanizomenon normally occurs in bundles of fila- 

 ments, but the bundles are commonly broken during the process of cen- 

 trifuging. The individual filaments have a close superficial resem- 

 blance to those of Oscillatoria, and may be counted as such. In all 

 orobability the counts of Oscillatoria in this report have been increased 

 to some extent by the accidental inclusion of filaments of Aphanizomenon. 



Acknowledgments 



Dr. Albert Mann, U. S. National Museum, identified certain of 

 the diatoms collected in 1929, and Dr. Helen Brown, The Ohio State 

 University, identified many of the algae taken in 1930. Statistical 

 analysis of the plankton curves was done by Dr. Ralph Hile, U. S. Bureau 

 of Fisheries. The writers are glad to a clmowledge the help of these 

 investigators. 



Data and discussion 



Qualitative data 



The plankton algae other than diatoms have been treated in a 

 recent paper (Tiffany, 1933), and a list of them is not necessary here. 

 In number of genera and species, the list is headed by the 

 Chlorophyceae, with 36 genera and 77 species. The Diatomeae are second 

 with 20 genera and 3? speciesj the Myxophyceae third with 12 genera and 

 2? species. The remaining classes have few representatives; Heterokonteae, 

 3 genera and 5 species. Chrysophyceae, 3 genera and 5 species; 

 Dinophyceae, 2 genera and 3 species; Euglenineae, h genera and 6 species. 

 The entire list comprises 80 genera and l50 species. It is hoped that the 

 diatoms will soon be studied in as great detail as the other groups. 



Quantitative data 



Island Section 



Horizontal listribution 



The question of the degree of uniformity in horizontal distribu- 

 tion of plankton in lakes has been a controversial one since the beginning 

 of plankton investigations, Without attempting to trace the history of 



1U7 



