PLANKTON OF THE GULF OF MAINE 55 



Rather scanty catches at the same relative position on the slope 100 miles far- 

 ther east on July 22, 1914 (station 10220), likewise included tropical animals (Rhin- 

 calanus, a phyllosome crustacean larva, Phronima, Doliolum, and four specimens of 

 the warm-water pteropod Limacina rangii) as well as boreal, while the tropical ele- 

 ment was similarly represented by Phronima and Sagitta enjlata in the plankton over 

 the slope off Marthas Vineyard a month later (August 26, stations 10260 and 10261), 

 although the catch was chiefly boreal (Bigelow, 1917, p. 245). In the cold summer 

 of 1916 the tropical water lay farther out from the edge of Georges Bank in July, 

 with the 50-meter temperature ranging from 4.85° to about 8° over the slope between 

 the 175 and 1,000-meter contours on the 23d (stations 10349-10351, and 10352). 

 Corresponding to this, the plankton along this zone was typically boreal (much the 

 same as in on the bank and in the gulf), Calanus finmarchicm dominating, with Pseu- 

 docalanus, Metridia lucens, Euchxta norvegica, large Euthemisto compressa and E. 

 bispinosa abundant (as is usually the case along the slope), Limacina retroversa, 

 Thysanoessa inermis, Th.raschii, and Sagitta elegans. Indicative of the zone of mix- 

 ture between coastal and ocean water was the fact that Sagitta serratodentata was about 

 as numerous as S. elegans over the 200-meter contour (station 10349) and Nematoscelis 

 megalops at the outer station; but the only planktonic animals or plants to which a 

 tropical origin could safely be credited were a few Salpa Jusiformis at station 10349, 

 many at station 10352, a single Physophora hydrostatica (station 10353), a large 

 Pyrosoma (station 10352), and a few fragments of gulfweed (Sargassum, station 

 10352). This poverty of warm-water forms contrasted strongly with what we had 

 found there in July, 1914, listed above (p. 54). 



None of our three lines off Cape Sable (where high temperatures are separated 

 from the slope by a still broader wedge of cold mixed water) has run out far enough 

 to reach Gulf Stream water. Nevertheless we have taken Rhincalanus and Sagitta 

 enjlata over the 500 to 1,000 fathom contours in summer even there (station 10233), 

 and have seen Physalia (June 24, 1915). No doubt the boreal forms would be left 

 behind altogether a few miles farther out to sea along this line in summer also, 

 to give place to tropical forms on the surface and to typically oceanic plankton in 

 the shadow zone of the mid-depths. 



In winter and early spring it is necessary to go considerably beyond the 1,000- 

 meter contour to find surface water as warm even as 10° or tropical pelagic animals 

 in any numbers abreast of the Gulf of Maine. For example, on February 22, 1920, 

 the only representatives of this community in hauls made off the western end of 

 Georges Bank (station 10244) were an occasional copepod (Rhincalanus) and amphi- 

 pod (Phronima), with Phronima and the medusan genus Rhopalonema at the 

 corresponding location off Cape Sable on March 19 (station 10277). The tow off 

 the southeast face of Georges Bank on March 12 (station 10269) produced no dis- 

 tinctively tropical forms, but by May 17 of that year the Gulf Stream community 

 had again approached so close to the western end of the bank that our nets yielded 

 several Salpse, subtropical copepods (Eucheirella), amphipods, and medusas 

 (Rhopalonema) among the boreal organisms of which the bulk of the plankton con- 

 sisted at the outermost station (20129). 



