46 ilamre of Aquatic Environment 



plants and animals properly balanced there occur 

 regular diurnal fluctuations corresponding to their 

 respective periods of activity in these upper strata. 



Photosynthesis is, however, restricted to the better 

 lighted upper strata of the water. The region of 

 greatest carbon consumption is from one to three meters 

 in depth in turbid waters, and of ten meters or more in 

 depth in clear lakes. Consumption of oxygen, however, 

 goes on at all depths, wherever animal respiration or 

 organic decomposition occurs. And decomposition 

 occurs most extensively at the bottom where the organic 

 remains tend to be accumulated by gravity. With a 

 complete circulation of the water these two gases may 

 continue to be used over and over again, as in the exam- 

 ple just cited. But, as we have seen, there is no circula- 

 tion of the deeper water during two considerable periods 

 of the year; and during these stagnation periods the 

 distribution of these gases in depth becomes correlated 

 in a wonderful way with the thermal stratification of 

 the water. This has been best illustrated by the work 

 of Birge and Juday in Wisconsin. Figure 8 is their 

 diagram illustrating the distribution of free oxygen in 

 Mendota Lake during the summer of 1906. It should 

 be studied in connection with figure 7, which illustrates 

 conditions of temperature. Then it will be seen that 

 the two periods of equal supply at all levels correspond 

 to vernal and autumnal circulation periods. The 

 season opens with the water nearly saturated (8 cc. of 

 oxygen per liter of water) throughout. With the warm- 

 ing of the waters the supply begins to decline, being 

 consumed in respiration and in decomposition. In the 

 upper six or seven meters the decline is not very exten- 

 sive, for at these depths the algae continually renew the 

 supply. But as the lower strata settle into their sum- 

 mer rest their oxygen content steadily disappears, and 

 is not renewed until the autumnal overturn. For three 



