BIOLOGICAL STUDY OF CHESAPEAKE BAY WATERS 
309 
The cooler weather of fall brings with it a lowering of the temperature of the 
upper layers of Chesapeake waters, while those below, as pointed out by Hjort (1896) 
for Norwegian fjords, often retain their summer condition, thus resulting in a warmer 
layer being found between upper and lower cooler ones (mesothermous). Such a 
vertical range of temperatures was widespread over the bay during the cruise of 
September, 1916. This condition is well illustrated at area A: Surface 23.4, 7 meters 
24.4, 14 meters 23.9, 22 meters 24.1, 32 meters 23.6, 42 meters 23.4; at area J, surface 
24.1, 5 meters 25.0, 10 meters 24.7, 17 meters 24.6, 27 meters 24.6, 37 meters 24.6; 
and at area X, surface 23.9, 7 meters 24.9, 14 meters 25.0, 24 meters 24.8, 34 meters 
24.8. According to the United States Weather Bureau, this was a decidedly cool 
September except during the first week. In fact, on the day the cruise was begun 
(September 8) the air temperatures at Baltimore, Washington, and Norfolk reached 
94° F., 93° F., and 90° F. (34.4° C., 33.9° C., 32.2° C.), respectively, but a drop of 
12, 5, and 4 degrees, respectively, occurred the next day and decreases in temperature 
continued for several days after that. On the other hand, October, 1920, was a warm 
month, and at the same areas the following readings were obtained: At area A, 
surface 20.4, 10 meters 20.1, 20 meters 20.0, 30 meters 19.4, 40 meters 19.3, 44.8 
meters 19.3; at area J, surface 20.0, 10 meters 19.8, 20 meters 19. 3, 30 meters 19.5, 
40 meters 19.1, 43.0 meters 19.2; and at area X, surface 19.4, 10 meters 19.9, 27.5 
meters 20.2. The lower bay, in general, showed a decreasing range of temperatures 
from the surface downward which would be in keeping with the high air temperatures. 
The surface temperatures, however, at X and at a few other areas, especially in the 
northern part of the bay, were found to be a little lower than those of the layers 
immediately below. 
During the December cruise of this same year (1920) the autumnal condition had 
taken on the winter condition (katothermous), with a few exceptions. 
SEASONAL DISCONTINUITY OF VERTICAL DISTRIBUTION OF TEMPERATURE 
Under the section in which the vertical distribution of temperature from the 
surface to the bottom is discussed, it has been pointed out that the thermocline (dis- 
continuity layer, Sprungschicht) is frequently seen in Chesapeake Bay and that at 
times its position coincides with a similar discontinuity in salinity. Such a condition 
has been noted in the Norwegian fjords, the Baltic Sea, and other localities. (See 
Krummel, 1907, p. 395.) The thermocline is most marked during the warmer months, 
since it is the heat of the summer sun’s rays that brings about the increase in tempera- 
ture of the upper layer, thus separating it from the cooler mass of water below. 
During the cruises of April and June, 1916, the thermocline was evident at many areas 
on the bay. But such a condition is not necessarily limited to the warmest months, 
for during the cruise of March, 1922, when the waters were still cold, a distinct thermo- 
cline was noted. The data for areas U, L, and G are given in Table 7. At area U 
observations were made during the night as well as during the day and the tempera- 
tures were almost invariably higher at the surface than at 5 meters. 
Table 7. — Temperatures and salinities at various depths on March, 1922, cruise 
Date 
Area 
Tempera- 
ture, ° C. 
Salinity 
Depth, 
meters 
Date 
Area 
Tempera- 
ture, ° C. 
Salinity 
Depth, 
meters 
u 
9.0 
5.22 
0 
Mar. 27, 1922 
L 
5.6 
17. 12 
30 
6.4 
8.22 
5 
5.4 
17.22 
40 
5.0 
10. 33 
11 
Mar. 25, 1922 
G 
8.9 
18. 36 
0 
Mar. 27, 1922 
L 
7.2 
12. 21 
0 
6.6 
30.65 
10 
5.8 
14. 54 
10 
6.5 
31. 10 
24 
5.7 
16. 65 
20 
