928 BULLETIN OF THE BTTEEAU OF FISHERIES 



but little stability develops there, more or less overturniug of this sort probably 

 takes place along the coast of Nova Scotia even at the warmest season. The fre- 

 quency with which bottles have stranded there after drifting across the gulf may be 

 explained on this assumption. 



The upper 40 meters of the southwestern part of the gulf attains its highest 

 stability during the last half of July and first half of August, being then most stable 

 off Massachusetts Bay and out to Cashes Ledge (fig. 186); but the Bay of Fundy as 

 a whole is hardly more stable then than in winter, with a gradation from southwest 

 to northeast around the north shore of the gulf,*' paralleling the degree of stratification 

 of the water with respect to temperature (p. 596). 



These regional differences reflect corresponding differences in the vertical 

 circulation. In the one case this is active enough to prevent the development of 

 the stable state by keeping the water thoroughly stirred, but in the other mixing is 

 not rapid enough to prevent the formation of a warm, light, surface layer, which, as 

 it develops, retards vertical movements of any sort. The insulating effect that 

 results is responsible for the preservation of the low temperature of each preceding 

 winter well into the following summer, in the deep bowl off Gloucester and in the 

 trough between Jeffreys Ledge and the Isles of Shoals (fig. 70). 



The following rule, therefore, may be laid down for the summer season: Where- 

 ever in the gulf the surface temperature is lower than in the surrounding waters, 

 and the water is nearly homogeneous vertically (with little stability), vertical circu- 

 lation of some sort is active; but where the water is warmest at the surface and 

 most stratified as to temperature and density vertical circulation of any kind is 

 weakest. 



Nantucket Shoals, where tides run strong over and between the ridges and 

 channels, afford an interesting example of the thermal result of active mixing, the 

 surface temperatures there being lower but the bottom water warmer in summer 

 than in the neighborhood generally. These same criteria show active mixing on the 

 eastern side of Georges Bank; likewise, no doubt, about Georges and Cultivator 

 Shoals. This also applies to German Bank. Dawson (1905, p. 15) has pointed out 

 that pools or " wakes" of low surface temperature, extending north and south from 

 Lurcher Shoal off Yarmouth, result in the same way " from the stirring up of the 

 water." 



The Bay of Fundy is the classic example of violent tidal stirring for the Gulf 

 of Maine region, where the currents, running with great velocity over the shoals 

 at its entrance and among the islands off the New Brunswick shore, keep the water 

 mixed, top to bottom, throughout the summer, a fact referred to repeatedly in the 

 preceding pages. This peculiar circulatory state of the Bay of Fundy, made clear 

 by Huntsman, is of far-reaching biologic significance; for, as he points out, so low a 

 surface temperature is thereby maintained throughout the summer that "conditions 

 approximating those in the far North are produced in shallow water" (Huntsman, 

 1924, p. 281). 



The rush of the tides between the islands along the coast of Maine, east of Penob- 

 scot Bay, is similarly reflected in low stability and slight thermal stratification (p. 599) . 



" Only about one-third as stable near Mount Desert and one-tenth as stable near Grand Manan as at the mouth of 

 Massachusetts Bay. , 



