3S 



BULLETIN OF THE BUREAU OF FISHERIES 



place to a rich Peridinian flora in summer, which is succeeded in turn by the limited 

 flowering of diatoms in autumn, as described in the chapter devoted to the phyto- 

 plankton (p. 383). 



No such seasonal alternation of dominance by one or other group takes place 

 among the planktonic animals of the gulf, however, though there is a very pro- 

 nounced oscillation in the total amount of zooplankton present there at different 

 times of year and in the abundance of its several members relative to one another. 

 Thus, we have never failed to find the Calanus community dominating the pelagic 

 fauna generally in the southwest part of the gulf, whether our trips thither were 

 made in the heat of summer, the cold of winter, in autumn, or in spring. Neverthe- 

 less, even in this region the varying seasons of reproduction of different animals, 

 which determine the presence or absence of their larva? and the abundance or scarcity 

 of the adults, with the local irregularities of distribution that always obtain for the 

 larger pelagic forms, added to the general ebb and flow in the abundance of the 

 zooplanktonic community as a whole, cause such variations from month to month as 

 appear in the following lists of the more abundant species in tow-net catches made 

 at the mouth of Massachusetts Bay in spring, summer, autumn, and winter. The 

 case is made still more complex by sporadic fluctuations in the abundance of one 

 species or another, for which we are not yet able to account. 



Tow-net catches at the mouth of Massachusetts Bay 

 [T>. dominating the plankton; X, occurred] 



