564 BULLETIN OF THE BUREAU OF FISHEBIES 



BELOW THE SURFACE 



In the northern and western parts of the Gulf of Maine, to which the chilling 

 efifect of the cold Nova Scotian water does not reach and which are only indirectly 

 afiFected by the shoreward and seaward oscillations of the warm oceanic water out- 

 side the edge of the continent, the superficial stratum, down to say 20 meters, is 

 sensibly warmer by mid-May than in April. The surface, also, warms so much 

 faster than the water only a few meters down that a temperature gradient of several 

 degrees develops over all this part of the gulf by the end of May as the first step 

 in the transformation from the homogeneous state that characterizes the upper 100 

 meters at the end of the winter (p. 523) to the very steep gradient of summer (p. 596). 



Thus, the mean temperature of the 20-meter level of Massachusetts Bay was only 

 about 1° higher on May 20 to 22, 1925 (about 5.5°), than it had been on April 

 21 to 23, the difference between this depth and the surface having now increased to 

 about 3° to 5°, except around the shores of Cape Cod Bay, where tidal stirring was 

 active enough to maintain a more homogeneous state (FisJi Hawk cruise 13, stations 

 6 and 7) . Local differences of this sort, in the rate at which heat is transferred 

 ■ downward into the bay during the spring, were responsible for a regional variation 

 of about 6° (from 4° to 9.9°) in the temperature of its 20-meter level at this date, 

 and for a regional distribution (warmest in Cape Cod Bay) paralleling the sur- 

 face (fig. 28) ; but evidently they had not yet been effective much deeper than 20 

 meters, because the temperature of the bay still continued virtually uniform from 

 station to station at the 40-meter level and at nearly the same values (3.3° to 3.8°) 

 as it had a month earlier. 



While the deepest water of the bay (at 70 to 80 meters level) had warmed by 

 about 0.2° meantime, the source of heat in this case was probably the bottom water 

 offshore. Similarly, 'the 40 to 60 meter level of the bay warmed by only 0.6° in 

 1920 between April 9 (station 20090, 2.3°) and May 16 (station 20124, 2.9°); the 

 bottom water in 100 to 120 meters by only about 0.4° (from 2.3° to 2.7°), although 

 the surface temperature rose by about 6.4° meantime. In short, seasonal warming 

 is negligible at depths greater than 25 to 30 meters until after the third week of May 

 in the Massachusetts Bay region. 



This statement appUes equally to Ipswich Bay north of Cape Ann, where the 20- 

 meter level warmed from 1.94° to 4.18° between April 9 and May 7 to 8, 1920, and 

 the 40-meter level only from 2.45° to about 3.1° (stations 20092 and 20122), with no 

 appreciable change at depths greater than 60 meters, so that the vertical range of 

 temperature between the surface and 40 meters increased from only about 1° to 

 nearly 5° during the 4 weeks' interval (fig. 33) . 



In the basin off the northern part of Cape Cod, just outside the 100-meter con- 

 tour, the 40-meter temperature rose from 2.2° on March 24 (station 20088) to 3.78° on 

 May 16 (station 20125), while the temperature at 100 meters hardly changed appre- 

 ciably during this interval of nearly 8 weeks. Below that depth the water, which 

 had cooled slightly from March to April, then warmed fractionally, so that the 

 curves for March and May fall close together (fig. 3) at 140 meters (about 3°-4°). 

 In the southwestern part of the basin, where no observations were taken in April, 

 a similar difference obtains between records for May 17 and February 23, 1920, 



