388 



BULLETIN OF THE BUREAU OF FISHERIES 



In the deep offshore waters of the gulf, diatoms do not attain their maximum 

 abundance for the year until some time during the last half of May or first week 

 in June, after which they diminish so rapidly in number that in 1915 (the only year 



Fig. 106— Distribution of the more characteristic types of phytoplankton, May, 1915 and 1920. 1, rich diatom; 3, Cer- 

 atium and diatom; 5, Ceratium. The heavy broken curve marks the offshore boundary to abundant Thalassiosira 



of record) we found that diatoms had practically vanished by mid-June from the 

 tow nettings made in the basin of the gulf south of the line Cape Ann-Cape Sable, 

 having been replaced there by a scanty peridinian plankton. Diatoms had also 



