578 
BULLETIN OF THE BUREAU OF FISHERIES. 
may be marked changes in the gaseous content of the different strata. As the water 
cools in the autumn its capacity for oxygen increases, and free expdsure to the air enables 
it to obtain additional amounts of this gas as circulation proceeds. As a result, the 
lakes enter the winter stage of their cycle substantially saturated with oxygen at all 
depths; that is, with about 9.0 cc. to lo.o cc. per liter of water. No winter observations 
were made on the Finger Lakes, but it is safe to assume from the results obtained on 
Wisconsin lakes that there is little change in the quantity of oxygen in the deeper lakes 
during the winter, more especially in those which show only a comparatively small 
decrease in the lower water in summer. In winter the life processes which furnish 
decomposable material are at a low ebb, and the temperature of the water is so low that 
decomposition goes on very slowly, even at the bottom, where organic material may be 
fairly abundant. In the shallower lakes, however, there may be a marked decrease of 
dissolved oxygen in the bottom stratum, and under favorable conditions it may even 
disappear entirely from some of the lower water. 
At the close of the vernal period of vertical circulation the oxygen has a fairly 
uniform distribution from surface to bottom, but more or less marked changes take place 
in the different strata during the direct stratification stage, so that the history of the 
dissolved gases is different for the different zones. The maximum difference is found 
during August, and for this reason a single set of observations during this month, such 
as made on the Finger Lakes, makes it possible to give the history of the dissolved 
gases with a very considerable degree of accuracy for the whole stratification period. 
In the epilimnion . — As already noted, the epilimnion is kept in circulation by the 
wind, which tends to keep the quantity of dissolved oxygen near the saturation point. 
But the amount is subject to variations in spite of this fact. As the temperature of 
the epilimnion rises in spring and early summer its capacity for holding oxygen in solution 
decreases so that the volume of this gas tends to decrease until the summer maximum 
of temperature is reached. But the loss of oxygen may not keep pace with the rise 
in temperature. That is, a certain quantity in excess of the amount required for satura- 
tion may remain for a while, since the water tends to retain the residual gas unless it is 
pretty thoroughly agitated by the wind. In such instances, however, the quantity 
of excess oxygen is never very great. This stratum is preeminently the zone of photo- 
synthesis and in this process some oxygen is liberated by chlorophyllous organisms. If 
the epilimnion is well populated with such organisms and the conditions are favorable 
for photosynthesis this stratum may become supersaturated with the oxygen that is 
liberated. Another small amount is obtained from the nitrites and nitrates which 
serve as a source of nitrogen for the chlorophyllous organisms. 
The appended table (table xviii, p. 602) shows that at the time of these observations 
the epilimnion of all of the Finger Lakes contained between 6 and 7.4 cc. of dissolved 
oxygen; with the exception of Conesus Lake it was a little larger than the amount 
required for saturation, the maximum excess being about 12 per cent. In most of the 
lakes a larger portion or perhaps all of this excess oxygen was most probably due to 
the photosynthetic activities of chlorophyl-bearing organisms since they were well 
