452 BULLETIN OF THE BUREAU OF FISHERIES 



from April 12 to 17 in 1920; nor did we find it at all in the eastern side of the gulf 

 in the first half of May in 1915. 



Considering the gulf as a whole, Thalassiosira attains its plurimum abundance 

 as well as its widest range during the last half of April, but it remains so typically 

 neritic throughout its vernal flowering period that it is always most plentiful close 

 in to the land, where it may monopolize the surface waters locally. Such, for example, 

 was the case off Gloucester on April 3, 1913 (station 10055), when the mass of diatoms 

 taken in a short tow was almost exclusively composed of two species of Thalassiosira — 

 T. gravida and T. nordenskioldi — with only occasional examples of Chsetoceras densum, 

 Gh. atlanticum, Oh. contortion, Biddulphia aurita, Coscinosira polychorda, Tlialassio- 

 thrix nitschioides, and Rhizosolenia semispina. Even more monotonous and equally 

 abundant catches of Thalassiosira were made by Welsh between Cape Ann and 

 Cape Elizabeth early in May, 1913. On the 2d he wrote: 08 "The water yesterday 

 and to-day full of green slime," and on the 3d, "the water is full of greenish-brown 

 algae," which on examination proved to consist almost altogether of Thalassiosira 

 (in Bigelow 1914a, p. 406). This genus was equally predominant, and in great 

 abundance, off Penobscot Bay on May 12, 1915 (station 10276), and at St. Andrews 

 Fritz found it far outnumbering all other diatoms combined on April 20 and May 1, 

 1917, the dates of its maximum abundance. 



Even in the centers of greatest abundance for 'Thalassiosira along the western 

 and northern shores of the gulf we have usually found a considerable mixture of the 

 several species of Chsetoceras in the catches of the tow nets, especially of Gh. debile, 

 Gh. decipiens, Gh. diadema, and of various other diatoms as well. Farther out at 

 sea, in the basin of the gulf, Thalassiosira has never been notably abundant and has 

 been both outnumbered and outbulked by Chastoceras at most of the stations. 

 This was the case on Platts Bank (station 20094) on April 10 and in the western side 

 of the basin (station 20115) on the 18th in 1920. Near Cashes Bank, however, 

 Thalassiosira was a large element in the plankton — though hardly to be described 

 as dominant — the day previous (station 20114). Possibly this shoal ground is a 

 local flowering center. 



These observations suggest that Thalassiosira first spreads to the basin of the 

 gulf as flotsam from the coastal zone and to some extent from Georges Bank, but 

 that it continues to multiply as long as the physical state of the water with which it 

 drifts continues favorable for its existence and reproduction. 



Thalassiosira did not dominate the diatom community at any of our stations off 

 western and southern Nova Scotia during the spring of 1920, though it was both 

 plentiful and widespread there in April, as I have just remarked. 



It is probable that the geographic range of Thalassiosira in the Gulf of Maine 

 begins to contract, from the sea shoreward and from south to north along the 

 western shore, about the 20th to the 25th of April in most years. Our stations for 

 1915 and 1920 combined show that it entirely vanished from the Cape Cod- 

 Massachusetts Bay region by the first week of May. It was confined to the northern 

 coastal zone, from Cape Ann to the Bay of Fundy, by the second week of the month 

 in 1915. In the zone between Cape Ann and Cape Elizabeth, where it was so 



" In his field notes. 



