602 BULLETIN OF THE BUREAU OF FISHERIES 



TEMPERATURE GRADIENT IN DEPTHS GREATER THAN 100 METERS 



The deeps of the gulf at depth's greater than 100 meters have shown interesting 

 variations, regional and annual, in the vertical distribution of temperature in sum- 

 mer. - In the bowl off Gloucester, isolated from the bottom water of the open gulf 

 by its barrier rim (p. 520), the temperature has either proved virtually homogeneous 

 vertically, from the 100-meter level downward, or has been fractionally coldest on 

 the bottom at that season. The water has also been slightly colder at the bottom 

 than at 100 meters at all our summer stations in the deep trough north of Cape Ann, 

 which is inclosed by the shoal ridge known as Jeffreys Ledge (fig. 6).^' 



In the open basin of the gulf, however, the bottom water may either be about the 

 same temperature as the mid-stratum or may be decidedly warmer and much salter, 

 depending, probably, on the amount of slope water flowing into the gulf at the time 

 (Bigelow, 1922, p. 165), and the records suggest a tendency for the one or the other 

 of these alternate states to persist over a period of years. 



In July and August, 1912, the western, northwestern, and northeastern parts of 

 the basin were virtually homogeneous in temperature (4.6° to 5.2°) from the 100 to 

 150 meter level down to the bottom in depths of 190 to 230 meters (stations 10007, 

 10023, 10024, 10036, and 10043); equally uniform vertically at depths greater than 

 75 to 100 meters in the eastern side (station 10028, 7.4°), or slightly colder on bottom 

 there (station 10027, 6°). 



During the summer of 1913, however, we found this type of vertical distribution 

 replaced by the alternate state just described, with the water of the basin coldest 

 at about 100 to 110 meters, warmer at greater depths, both in July and in August, 

 as foUows: 



Only at the head of the eastern trough (stations 10096 and 10097) and on the 

 northern slope of the basin off Monhegan Island (station 10102) was the bottom 

 slightly colder than the 100-meter level in that summer (fig. 8). 



The water was again coldest at about the 100-meter level at every deep station 

 in the inner parts of the gulf in July and August of 1914, and with the vertical 

 warming of the deep water not only much more pronounced than in 1913 but ex- 

 tending right down to the bottom in most cases. Only at one station (10249) for 

 that summer was the temperature slightly lower on bottom than at 150 meters, as 

 follows: 



" The 100-metcr temperature at this locality has ranged from 4.4° to 6.4° in August of 1913 and 1914 (stations 10104, 10105, and 

 10252), with 3.0° to 4.7° at 130 meters, 4.3° at 155 meters. On Aug. 7, 1923, the 30 to 80 meter stratum (about 4°) was 2° to 3° 

 colder. 



