increase from common to abundant in the case 

 of S. lyra. Among the pteropods most of the 

 species found in the Subtropic Zone were also 

 present in the Transition Zone, with a noticeable 

 increase in L. inflata frona comnnon to abundant 

 and a noticeable decrease in C. acicula fronn 

 abundant to present. Near its northern bounda- 

 ry the fauna resembled that of the Subarctic Zone 

 to some extent, with the appearance of the two 

 chaetognaths S. elegans and E. hamata and the 

 pteropod L. helicina. Only a few specimens of 

 the pteropods E. balatium and P. apicifulva , 

 which were not found in the Subtropic Zone, were 

 found in this zone, and the pteropod Cavolinia 

 longirostris, which was present in the Subtropic 

 Zone, was not found here. 



The Transition Zone was usually charac- 

 terized by the abundance of the chaetognath S^. 

 lyra and the common occurrence of S. minima . 

 Concerning S. minima , Pierce (1953), in his 

 study of the chaetognaths over the continental 

 shelf of North Carolina, stated that this species 

 occurred in greater abundance in a region of 

 mixing of different water masses. S. minima 

 was classed as "common" in the Transition Zone 

 (table 1), but it occurred in abundance in several 

 localities; whereas it was scarce or lacking in 

 the other two zones. Limacina inflata , which 

 was listed as common in nearly all of the sam- 

 ples from the Subtropic Zone, occurred inabun- 

 dance in several samples from the Transition 

 Zone, particularly near its northern boundary. 



Some of the samples collected in this zone 

 were very poor both in species and in volume, 

 containing very fewcopepods, chaetognaths, and 

 pteropods, but an unusually large percentage of 

 a heteropod, Atlanta sp. 



The northern boundary of the Transition 

 Zone, that is, the southern boundary of the Sub- 

 arctic Zone, was defined when the chaetognaths 

 S. elegans or E. hamata were found to be com- 

 mon in a sample. Figure 3 and table 5 show 

 that the boundary fluctuated slightly with change 

 in longitude in the summer and fall, occurring 

 between 43°N. and 45"'N. latitude on most lon- 

 gitudes, with the boundary slightly farther north 

 in the fall thain in the sunnmer. Sampling was 

 not continued far enough north to define the 

 northern boundary of the Transition Zone in 

 winter, but indications are that the boundary 

 moved slightly to the south. The southern 

 boundary of this zone coincides with the north- 

 ern boundary of the Subtropic Zone so it will not 

 be redefined. The Transition Zone lay roughly 

 between 33°N. and 45°N. latitudes during fall 

 and sumnaer, with surface temperatures 

 ranging from 78. 8°F, to 47. 3°F. from south to 



north, and surface salinities from 35.00%(j to 

 33.60%o. 



Subarctic Zone 



The northernmost subdivision of the area, 

 the Subarctic Zone, was characterized by a 

 marked decrease in species diversity for both 

 chaetognaths and pteropods (tables 1 and 2) and 

 great increase in biomass (fig. 1). The high 

 biomass (fig. 1) nnay be attributed to "blooms" 

 of copepods, euphausiids, and at times radio- 

 larians and diatoms (McGary et al. 1956, table 

 7). The southern limit of this zone was defined 

 by the occurrence in abundance of the two cold 

 water chaetognaths S. elegans or E. hamata, as 

 explained in the previous section. L. helicina 

 was the most abundant pteropod found in this 

 zone; L, inflata , E. pyramidata , E. balatium, 

 C. inflexa, and C. tridentata were also present. 

 Salinities in the Subarctic Zone were usually 

 less than 33. 60%o and temperatures lower than 

 68.4°F. in summer, whereas in the fall salini- 

 ties of less than 33.22%o and temperatures 

 lower than 47.3°F. were encountered. 



Fauna and Temperature-Salinity Relations 



The temperature-salinity (T-S) relation for 

 each station along the eastern and western me- 

 ridional sections of Hugh M. Smith cruise 30 is 

 shown in figure 4z.' , together with the relation 

 of the faunal zones to the North Pacific water 

 types (the western North Pacific Central Water 

 mass and the Subarctic Water mass) as redrawn 

 from Sverdrup et al. (1942: 741). The shape of 

 the temperature -salinity curve describes the 

 character or identity of the water mass at each 

 station (Sverdrup et al. 1942: 142). The spread 

 in the curves in the upper 200 meters of depth 

 (towards the higher temperatures in fig. 4) is of 

 particular importance to this study. There is a 

 natural segregation of the curves into three 

 groups: one with lower salinities and lower 

 temperatures characteristic of the Subarctic 

 Water mass; a second with higher salinities and 

 higher temperatures, features identifying the 

 Pacific Central Water nnass; and an intermedi- 

 ate group characteristic of the North Pacific 

 Current. 



— To construct this figure the observed 

 data were plotted according to a new method 

 described by Hans T. Klein in an unpublished 

 manuscript "A new technique for processing 

 physical o c e a n o g r a p h i c data, " Scripps 

 Institution of Oceanography. 



