386 



BULLETIN OF THE BUREAU OF FISHERIES 



diatoms swarming and peridinians practically nonexistent during the first half of 

 May in 1915 (Bigelow, 1917, p. 324), the result being that the vernal flowerings 

 of diatoms reach their widest expansion at this season. 



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Fia. 105.— Distribution of the more characteristic types of phytoplankton, April, 1920. 1, rich diatom; 2, sparse diatom; 

 3, sparse Coratium and diatom; 4, rich Phseoeystis and diatom 



The unicellular alga Phssocystis may also swarm, even to the extent of monopo- 

 lizing the surface waters locally, for a brief period during the month of April, but 

 shortly disappear once more, as occurred in the southern part of Massachusetts 



