PHYSICAL OCEANOGRAPHY OF THE GULF OF MAINE 543 



is no danger of temperatures lower than about 1.5° to 2°, however, either on the 

 slopes of the basin or in any one of the deep isolated bowls at depths of 100 meters 

 or more, nor of temperatures lower than 4° on the bottom of the basin. A corre- 

 sponding difference in the upper strata also may explain the disappearance of sundry 

 planktonic animals from the coastal zone in winter, though they occur the year 

 around in the gulf out at sea (Bigelow, 1926). 



The contour of this mass of comparatively warm bottom water in the deeps of 

 the gulf is graphically illustrated by a chart showing the isothermobath for 4° in 

 February and March (fig. 20), for wherever temperatures as high as this were 

 recorded within the gulf the underlying strata were still warmer. In 1920 (probably 

 this applies yearly) there was no water as warm as 4° at this season at any level in 

 the coastal zone, out to the 100-meter contour, on either side of the gulf. However 

 (without attempting to draw too close a parallel between the intricate contour of the 

 bottom and the temperature), the floor of the v/hole gulf at depths greater than 150 

 meters was bathed with water warmer than 4°, filling the whole basin below a 

 uniform level of 120 to 130 meters in the western side and rising to within 60 to 80 

 meters of the surface in the eastern, as a well-defined ridge extending northward 

 from the Eastern Channel, with a tendency to pool off the mouth of the Bay of Fundy. 



It is not likely that this warm water ever overflows Browns Bank or the eastern 

 half of Georges at that season, although not barred from them by the contour of the 

 bottom. Certainly it did not in March, 1920; but the whole column of water over 

 the western half of Georges Bank was then warmer than 4°, so that the chart (fig. 

 20) shows the isothermobath in question as rising to the surface there and dipping 

 steeply toward the basin to the northwest. A contrast of 5° to 6° in bottom tem- 

 peratures between the southwestern and southeastern parts of the bank (station 

 20046, 8°; station 20067, 2.8°) illustrates the wide differences in the physical condi- 

 tions to which animals living on bottom are subject in winter and early spring on 

 various parts of the bank. 



It seems that at this season the fauna of the so-called "warm zone," which 

 characterizes the upper part of the continental slope off southern New England and 

 farther west (p. 531), must meet its eastern boundary at about longitude 67°, because 

 the bottom temperature was only 4.9° at 190 meters oft the southeastern face of 

 Georges Bank on March 12 (station 20068), contrasting with 11.55° at a depth of 

 120 meters off its southwestern slope on February 22 (station 20045). 



ANNUAL VARIATIONS IN TEMPERATURE IN EARLY SPRING 



Slight variations are to be expected, of course, in the temperature of the gulf 

 from one winter and spring to the next, even in what we may roughly term "nor- 

 mal" years; still more so between the exceptionally cold and warm winters that no 

 doubt fall at intervals. The station data for 1920 and 1921 allow a thermal com- 

 parison for the northwestern parts of the gulf for early March of those years, ampli- 

 fied by the Fish Hawk survey of Massachusetts and Ipswich Bays in 1925 and by 

 readings taken at a few localities in 1913. 



At the head of Massachusetts Bay, off Boston Harbor, the readings for early 

 March, 1921, and for February 24, 1925, are from 1° to 2° higher at all levels 

 than those for 1920, although the dates were within a few days of one another. As 



