596 BULLETIN OF THE BUREAU OP FISHERIES 



Passing eastward from the longitude of Nantucket, we find a more sudden tran- 

 sition from the comparatively cool water (18°) over the southwestern part of Georges 

 Bank to the high temperature of the oceanic water outside the 200 meter contour, 

 accompanied, however, by such irregularities as might be expected along the zone of 

 contact of waters differing in salinity as weU as in temperature. At times the 

 north-south gradation in sm-face temperature along this sector of the edge of the 

 continent is also interrupted by a cooler band. On July 20 to 21, 1914 (stations 

 10216 to 10218)," this was indicated by surface readings of 18.6°, 17.3°, and 20.48° 

 at three successive stations from north to south on a line crossing the southern slope 

 of the bank. 



Such data as are available point to an abrupt increase in the breadth of the 

 cool wedge eastward from Georges Bank between the edge of the continent and the 

 warm oceanic temperatures of >20°, to the seaward of the latter. Thus the surface 

 was only about 17° at our outermost station off Shelburne on July 28, 1914 (station 

 10233), while the Canadian Fisheries Expedition crossed a band of 17° to 19.7° 

 water some 70 miles wide outside the 200-meter contour in the offing of Cape Sable 

 on July 22, 1915 (Bjerkan, 1919; Acadia stations 41 to 44). Unfortunately no tem- 

 peratures have been taken off the slopes of Georges or Brow^ns Banks during the 

 last half of August of late years, but even if the isotherm for 18° should encroach a 

 few miles farther inward by the end of the month than is represented on the chart 

 (fig. 46) , there is no reason to suppose that the surface temperature rises higher than 

 20° inside the 100-meter contour on the banks anywhere east of Nantucket Shoals 

 at any season, except possibly for brief periods following persistent southerly winds. 



TEMPERATURE GRADIENT IN THE UPPER 100 METERS 



A differentiation in the vertical distribution of temperature between the western 

 and eastern sides of the gulf begins to develop in June, widening with the advance 

 of summer, until the extremes, as represented by the western basin on the one hand 

 and by the Bay of Fundy and coastal banks off western Nova Scotia on the other, 

 yield graphs differing widely in the upper 100 meters by August. 



The most striking feature of the western type, as exemplified by the basin off 

 Cape Ann (fig. 48) and by the bowl at the mouth of Massachusetts Bay off Gloucester 

 (fig. 4), is that the water cools very rapidly from the surface down to a depth of 

 40 to 50 meters, succeeded by only a slight fall in temperature down to the 100- 

 meter level. Whether increasing depth is accompanied by a further slight cooling 

 or by a slight warming depends on the locality, the topography of the bottom, and 

 to some extent on yearly fluctuations, as discussed later (p. 602). In August we have 

 found the 40-meter level averaging from 10° to 14.5° cooler than the surface in the 

 western side of the basin and 9° to 13° cooler at the mouth of Massachusetts Bay 

 (figs. 4 and 5), illustrating the remarkably sudden change that any animal would 

 experience there, from warm water to cold, by sinking down for a few meters only. 

 Observations taken farther up the bay on August 22 to 24, 1922 (stations 10630 to 

 10645), showed a similar vertical chilling down to 50 meters or so, except that the 



•• This cool band is more clearly marked, by temperature, at deeper levels, as described on page 608. 



