A LIMNOIvOGICAIv STUDY OF THE FINGER LAKES. 
577 
but below that temperature the former is the only active agent. When the surface 
water cools below 4° it becomes lighter than the warmer water beneath it and tends to 
float on the latter; that is, there is a resistance to mixture owing to the difference in 
their temperatures. As a result, it requires a strong wind to disturb the water to any 
considerable depth. But in spite of this resistance to mixture the wind is able to disturb 
the water at all depths, even in the deepest lakes, and cause the late autumn and 
winter temperatures to fall well below 4°. (See table iv, p. 555.) The winter stratifi- 
cation is inverse ; that is, the coldest water is at the surface and the warmest at the bottom. 
In spring conditions again become favorable for an overturning and circulation of 
the water. In the lakes which freeze over in winter a preliminary step in this process 
is the removal of the covering of ice. Substantially all of the direct warming takes place 
close to the surface, and as the temperature rises this water becomes heavier than that 
below and tends to sink, thus producing convection currents; but this holds true only 
as long as the temperature remains below 4° or the point of maximum density. After 
the whole body of water reaches a temperature of 4° any warming of the surface layer 
makes it lighter than the cooler water below and it tends to float on the latter. This 
eliminates convection currents as a factor in producing a general circulation, but they 
still play a more or less important role in mixing the water of the upper stratum when 
cooling takes place at the surface at night or during cool periods. The wind is now 
the only agent involved in the production of a complete vertical circulation. 
As the season advances the temperature of the upper water rises so that it offers 
a greater and greater resistance to mixture with the cooler water below. As a conse- 
quence the tendency of the lower water to take part in the circulation grows correspond- 
ingly smaller and smaller. Finally the thermal resistance to mixture becomes so great 
that the wind is no longer able to mix the warm upper water with the cooler water below 
and the lake becomes separated into three distinct strata, viz, the epilimnion, the 
thermocline, and the hypolimnion. (See p. 547.) This is known as a direct thermal 
stratification, and it persists from early summer until the autumnal overturn takes place. 
The autumnal circulation is much more thorough than the vernal. This is due to 
the fact that the mere cooling of the water in the autumn, as long as its temperature 
is above 4°, produces convection currents which tend to mix the various strata. In 
the spring, however, general convection currents are formed only as long as the water 
remains below 4°, which is generally only a comparatively short period of time. Small, 
spring-fed lakes, in fact, in which the temperature of the bottom water rises to 4° before 
the ice disappears, and which are well sheltered from winds, may not experience a 
complete vernal overturning under favorable weather conditions. 
OXYGEN. 
Circulation periods . — During the autumnal and vernal periods of vertical circulation 
all of the dissolved gases, as well as other substances that may be held in solution, are 
uniformly distributed from surface to bottom; but during the succeeding stages there 
