664 BULLETIN OF THE BUREAU OF FISHERIES 



reached later and later in the season, at greater and greater depths, with the water 

 continuing to warm at any given level until the autumnal cooling of the surface 

 brings the temperature of the overlying mass down nearly as low. Thus, the sur- 

 face is warmest in August, the 20-meter level about the first week of September, the 

 40-meter level not until October, and the 70-meter level in November, while the 100- 

 meter temperature probably does not reach the maximum for the year until the first 

 part of December. This has the interesting biologic complement that while any 

 animal living in the littoral zone, or pelagic close to the surface, encounters the 

 highest temperature while the solar illumination has fallen but little from its maxi- 

 mum intensity, for inhabitants of the deep water in 70 to 100 meters the summer, 

 as measured by temperature, falls when the illumination by the sun is nearing its mini- 

 mum for the year. 



Sometime in July the warming of the surface suddenly slows down as the sun's 

 declination falls lower and lower; but the cooling that takes place during September 

 no doubt is due more to vertical mixing than to the loss of heat by radiation from 

 the water, because the mean temperature of the air does not fall below that of the 

 surface until about the middle or end of October (p. 671) . The two dulling agencies 

 that aflFect the surface of the Massachusetts Bay region — i. e., the constantly lower- 

 ing temperature of the air and the incessant tidal stirring that becomes more and 

 more active as the stability of the water decreases — make the whole column vir- 

 tually homogeneous in temperature (about 9°) down to 100 meters depth by the 

 beginning of winter. From that date on we have never found the surface differing 

 by more than 2.5° in temperature from the bottom in any part of Massachusetts 

 Bay until March; and in depths of 70 meters, or deeper, the bottom water is usually 

 slightly warmer than the superficial stratum from the last half of December until 

 the middle of February, with the winter minimum for the whole column usually fall- 

 ing between 2° and 3°. At the mouth of the bay, 7 to 12 mUes off Gloucester, the 

 temperature is at its minimum about the middle of February in most years. 



2. BAY OF FUNDY 



The graph for Massachusetts Bay illustrates the thermal cycle for the coastal 

 zone of the gulf where least stirred, vertically, by the tides; that for the Bay of 

 Fundy shows the opposite extreme. Corresponding to this difference in circulation 

 under the influence of a much more severe winter climate and a somewhat cooler sum- 

 mer in the atmosphere, the graph of annual temperature in the Bay of Fundy (fig. 87) 

 shows a vertical range of only about 5° in the upper 100 meters in summer, contrast- 

 ing with 14° in Massachusetts Bay. Similarly, the annual range of surface temper- 

 ture is only about 10°; 17° or 18° at the mouth of Massachusetts Bay. At 100 

 meters,, however, the annual range (approximately 5°) is about the same for the two 

 localities. Although the Bay of Fundy is much less stratified, with regard to tem- 

 perature, than is Massachusetts Bay during the warm months, it is more so during 

 the winter, with the surface 1° to 1.5° colder than the 100-meter level between the 

 dates when the whole column becomes homogeneous in temperature in autumn and 

 again in early spring. 



In normal years the surface of the Bay of Fundy reaches its highest tempera- 

 ture in August or early September (slightly later than the date when the surface of 



