678 BULLETIN OF THE BUREAU OF FISHERIES 



some other process. The warming by dh-ect solar radiation would therefore be vir- 

 tually negligible during a single summer at depths greater than about 50 meters if 

 there were no vertical circulation, this limit varying with varying states of turbidity 

 and with the roughness or smoothness of the surface of the water as well as with the 

 cloudiness of the sky, the haziness of the atmosphere, the percentage of foggy days, etc. 



DISPERSAL OF HEAT DOWNWARD INTO THE WATER 



With at least nine-tenths of the solar energy that enters the water of the gulf at 

 all absorbed within 10 meters of the surface, and virtually all of it shoaler than 30 

 to 50 meters, the importance of vertical circulation in carrying down into the deeps 

 water that has been warmed at the surface, and by bringing cold water up within the 

 influence of the sun from below, becomes at once apparent.** 



The vertical circulation of the gulf is discussed in another chapter (p. 924). It 

 concerns us here, however, as the factor that chiefly governs the temperature of the 

 mid-stratum between the depths of, say, 25 and 100 meters. In different parts of the 

 gulf and at different seasons we find all gradations from water so stable, vertically, 

 and with currents so weak that virtually no interchange takes place between the 

 different strata, to the opposite extreme where the whole column is kept so thoroughly 

 churned by tidal currents that the heat absorbed by the surface is uniformly dis- 

 persed downward. This last state characterizes nearly the entire area of the gulf 

 during the first days of spring and is responsible for the fact that the whole 

 upper stratum, down to 100 meters, at first warms at so nearly uniform a rate. 



The vertical uniformity of temperature that characterizes Nantucket Shoals, 

 locally, too, Georges Bank, parts of the Bay of Fundy, and the coastal belt along 

 the west coast of Nova Scotia, results similarly from tidal stirring so active that it 

 overcomes the tendency of the water to become stable as the spring progresses. Off 

 the western shores of the gulf, however, where tidal stirring is not active enough to 

 counteract the increasing stabiHty of the column induced by the warming of the 

 surface, the development of a light stratum at the surface tends more and more to 

 insulate the deeper strata of water from the effects of solar warming as the season 

 advances. The more stable the water becomes, the more effectively are the deeper 

 strata protected in this way from thermal influences from above. 



It is this obstacle, which the stable state of the water opposes to vertical circu- 

 lation during the warm half of the year, which is responsible for the fact that the 

 temperature rises so much more rapidly and to so much higher a value at the sur- 

 face than only a few meters down, and which allows the persistence of much lower tem- 

 peratures at depths of only 50 to 100 meters all summer. However, there is always 

 enough vertical movement of the water everywhere in the Gulf of Maine to prevent 

 this insulation of the deeper strata from becoming as effective as it is along the coast 

 from New York, southward, during some springs (Bigelow, 1922). 



Observations taken during our first cruises in 1912 (Bigelow, 1914) pointed to 

 local differences in the strength of the tidal currents as chiefly responsible for the 

 fact that the surface is so much colder, but the bottom, depth for depth, so much 

 warmer along the coast of Maine east of Penobscot Bay and in the Bay of Fundy 



" Conduction and the radiation of heat from one particle of heat to the next are negligible in this respect. (Wegemann, 1905; 

 Eriimmel, 1907.) 



