PHYSICAL OOEANOGKAPHY OP THE GULP OF MAINE 677 



and virtually all of this residue was absorbed shoaler than 50 meters. Grein's exact- 

 ing measurements, therefore, confirm Knott's (1904) conclusion that a. m. and p. m. 

 temperatures taken by the "Pola" at 16 pairs of stations, with thermometers grad- 

 uated to 0.1° C, showed no evidence of the penetration of direct solar radiation 

 deeper than about 20 meters. 



In more turbid northern seas we may expect the solar radiation to be absorbed 

 in a still shoaler surface stratum, depending largely on the character and abundance 

 of the plankton at the time. In Puget Sound, for example, Shelford and Gail (1922) 

 found the first meter of water absorbing about 20 per cent of the visible light that 

 actually penetrates below the surface, with only 8 to 1 per cent of even the shorter 

 wave lengths reaching a depth of 10 meters under average illumination. 



In the English Channel, Poole and Atkins (1926) found the illumination at 20 

 meters to be about 5.5 per cent as strong as just below the surface; while in the 

 Bay of Fundy, according to Klugh (1925), only about 1.5 per cent of the illumination 

 recorded just below the surface penetrates to 10 meters in August in bright sunlight. 



In Lake Seneca, New York (probably still more turbid), Birge and Juday (1921) 

 found that only 15 per cent of the solar energy that entered the water penetrated to 

 a depth of 2 meters, 5.4 per cent to 5 meters, and only 1 per cent to 10 meters. Per- 

 haps as striking an example as any in nature of the absorption of the sun's heat by 

 the uppermost stratum of water is afiForded by certain oft-quoted salt-water basins 

 along the west coast of Norway, in which the salinity is very low at the surface but 

 so high from the depth of 1 meter downward that the water is in extremely stable 

 equilibrium. Here solar radiation in summer induces temperatures as high as 20° 

 to 30° in the upper 2 meters of water but hardly affects the temperatiure deeper 

 than about 5 meters. (See Helland-Hansen, 1912a, p. 65, for a discussion of these 

 "Polls," as they are named locally.) 



Judging from the similarity in latitude and in general hydrographic conditions, 

 the penetration of solar radiation is probably of about the same order of magni- 

 tude in the ojien Gulf of Maine as in Puget Sound. If, then, the water of the gulf 

 were entirely without motion, and if heat were conveyed downward by no other means 

 than direct solar radiation, more than 90 per cent of such of the sun's radiant energy 

 as penetrated the water at all would be expended within 10 meters of the surface, 

 something like 98 per cent within 25 meters of the surface, and all but a fraction of 

 1 per cent at a depth of 100 meters. At times of year when the water was particu- 

 larly turbid — spring, for example, during the active flowerings of diatoms — the solar 

 radiation would be absorbed still more rapidly. 



We must also bear in mind that that part of the sun's insulation which is inter- 

 cepted by the superficial stratum of water does not act solely to warm the latter, but 

 that a part of its energy is expended directly in evaporating water vapor from the 

 surface (p. 680). 



Under the conditions existing in the gulf it seems that if direct solar radiation 

 warms the surface by 20° at any given locality in the gulf, the 10-meter level would 

 certainly warm by only about 2°, very probably the 50-meter level would warm 

 by no more than 0.2°, and the 100-meter level would not suffer change sufficient for 

 our most delicate deep-sea thermometers to record during the part of the year when 

 the water is gaining heat, unless this heat were carried downward into the deeps by 



37755—27 12 



