228 !Wisconsin Academy of Sciences, Arts, and Letters. 
time where it may obtain more oxygen in ease its supply of this 
gas is deficient. Through the influence of the wind this aerated 
water is forced down into the lower strata and mixed with the 
deeper water, thus carrying down a supply of dissolved oxygen. 
This process continues during the period of complete circula¬ 
tion and results in a pretty thorough aeration of the water at 
all depths. The upper water also receives some oxygen through 
the process of photosynthesis, since in this process the chloro¬ 
phyll-bearing organisms, under the influence of light, take up 
carbon dioxide and liberate oxygen. The decrease in the tem¬ 
perature of the water in autumn and winter increases its 
capacity for oxygen, since this gas is more soluble in cold than in 
warm water. As a result of the circulation and the increase in 
the capacity of the water for this gas, the largest quantity of 
oxygen, held in solution by the waters of these lakes is found 
just at the close of the circulation period. 
As soon as the lower water ceases to take part in the circula¬ 
tion it is cut off from further addition to its supply of dissolved 
oxygen because this gas diffuses so slowly through water that 
the amount transferred in this way is negligible. Hence the 
supply is limited to the amount which this water contains at the 
time that it ceases to take part in the circulation. Soon an 
appreciable decrease in the dissolved oxygen of the lower water 
is noted. This decrease is the resultant of two processes, namely, 
respiration and decomposition. The lower strata are inhabited 
by various organisms which consume dissolved oxygen in their 
process of respiration and liberate carbon dioxide. But prob¬ 
ably a much larger portion is consumed in the decomposition of 
organic material. Especially is this true when the upper water 
supports a large population of plankton organisms, more particu¬ 
larly phytoplankton. The decrease is most rapid at the bottom 
because decomposable material is most abundant there. 
But decomposition takes place at all depths. The floating 
devices possessed by the various phytoplankton forms reduce 
their specific gravity so that it is but little greater than that 
of the water. Thus such organisms sink very slowly when they 
are dead and they may pass through the early stages of decay, 
at least, at almost any depth. When this decomposition takes 
place in the upper water the dissolved oxygen which is con¬ 
sumed in this process may be replenished because this stratum 
is kept in circulation by the wind. Also, the photosynthesis 
