PLANKTON OF THE GULF OF MAINE 387 



Bay and off Cape Cod during the last half of the month in 1920 (p. 458). Although 

 this is the only occasion on which we have actually observed this event, it is to be 

 expected equally in other parts of the gulf, where the peak of abundance for Phseo- 

 cystis may have chanced to fall between the dates of our successive cruises. 



These diatom flowerings of the Massachusetts Bay region are so short-lived 

 and dwindle so suddenly after they have attained their plurimum that we found 

 them reduced to an occasional Coscinodiscus only among a scanty community of 

 Ceratium, Peridinium, and Halosphaera on May 4 to 16 in 1915 25 and again in 1920, 

 although diatoms continue swarming in the central parts of the gulf and along 

 its northern shore line generally until considerably later. Diatoms vanish equally 

 from the waters along Cape Cod by the middle of May, where only an occasional 

 diatom was to be found among the small catches of Ceratium and Peridinium at 

 three stations on a line run by the Albatross from Cape Cod out to the north slope 

 of Georges Bank in 1920 (stations 10225 to 10227, May 16), though the water over 

 the southwestern part of the bank still supported much the same diatom com- 

 munity as the last week of February (p. 3S3). This late flowering was strictly 

 limited, however, to the shallows of the bank because our tow nettings over the 

 continental slope a few miles to the south yielded little except a sparse gathering of 

 peridinians (station 2012S). 



In the western side of the gulf the shrinkage of the diatom communities, fol- 

 lowing their season of abundance, which, as we shall see, foreshadows their eventual 

 disappearance from the plankton, proceeds progressively from south to north during 

 May. Thus tow-net catches made about the Isles of Shoals, where we were able 

 to follow the rise, culmination, and eclipse of the diatom flowerings at close intervals 

 during the spring of 1913, were still exceedingly abundant (almost purely diatom) 

 and very clean up until the first week in May in 1913, whereas there were very few 

 diatoms on the other side of Cape Ann as late as this. From that time forward, 

 however, the plankton of the Isles of Shoals area began to contain noticeable 

 amounts of diatom debris, and as the season advanced the relative amount of dead 

 specimens and variously fragmented remnants grew progressively greater until 

 the 25th of the month, when there were very few living diatoms (Bigelow, 1914a, 

 p. 406), though the nets still yielded large amounts of their debris. 



Peridinians, on the other hand, and especially the genus Ceratium, multiplied 

 as the diatoms dwindled (perhaps more relatively than absolutely), changing the 

 general composition of the phytoplanktonic community so rapidly, from rich diatom 

 at the beginning of May to peridinian with but few diatoms at the end of the month 

 in the area bounded on the south by Cape Ann, on the north by Cape Porpoise, 

 and offshore by Jeffrey's Ledge, that it is represented as "mixed diatom and 

 peridinian" on the accompanying chart for May (fig. 106). 



The duration of the spring flowerings of diatoms in the shoal waters off south- 

 western Nova Scotia is likewise brief, for though they filled our tow nets there on 

 April 15, 1920 (stations 20103 and 20105), we found a sparse Ceratium plankton 

 in that general region from May 7 to 10, 1915 (stations 10271 and 10272), with but 

 few diatoms. 



"Station 10266, May 4, 1915; station 10220, May 1 and 16, 1920. 



