Centigrade for a period of 17 hours, cooled in a desiccator and 

 weighed. It was then ignited in an electric furnace for 30 minutes, 

 cooled, and weighed again. After correcting for the amount of 

 dissolved material in the water transferred from the centrifuge 

 bowl with the plankton, the loss of weight by ignition was designated 

 as the dry organic matter of the plankton. This method involves the 

 total plankton, that is, both animals and plants, but the plants 

 ordinarily contribute several times as much weight as the animals. 



The organic matter of the plankton was determined on eight 

 dates from September 20 to October 22. The average amount during this 

 period was 2,200 milligrams per cubic meter of waterj the smallest 

 was 1,800 and the largest 2,900. In Lake Mendota, for a similar 

 period of 1915 (Birge and Juday, 1922, Figure 3li), the average amount 

 was approximately 2,130 milligrams per cubic meter, with a range from 

 1,660 to 2,670, In the same period of I916 (Figure 35), the average 

 amount was approximately 2,310 milligrams per cubic meter, with a 

 range from 1,830 to 3,000. The two year average for the period con- 

 cerned was 2,220 milligrams per cubic meter, which was almost exactly 

 the same amount as in Western Lake Erie in the same period of 1929 

 (2,200 milligrams per cubic meter). 



There are reasons for believing that the plankton of 

 Western Lake Erie deri,ves more of its organic matter from the 

 so-called dust-fine detritus than does Lake Mendota. Western Lake Erie 

 is frequently disturbed by winds which bring into suspension the dead 

 and disintegrating plankters on the bottom, while in Lake Mendota many 

 of these forms sink to the bottom and remain undisturbed during a con- 

 siderable part of the period in question. That much detritus of 

 organic origin enters the plani:ton of Western Lake Erie is suggested 

 by the fact that there is much of inorganic origin. The presence of 

 a large amount of inorganic sediment is indicated by the brick-red 

 color of the ignited samples, and by the high percentage of ash. On 

 the average, the ash made up 72.1 per cent of the dry weight of the 

 plankton, and in one case it made up 82.3 per cent. In Lake Mendota, 

 the largest per cent of ash was about 75.0, and doubtless the average 

 was much less. Another point to be noted in comparing the two lakes 

 is that Lake Mendota shows high phytoplankton counts. Thus in 1925, 

 the total count in spring was about 9OO thousand units per liter and 

 in autumn was 1,100 thousand units (Domogalla, 1926), If the year 1925 

 was representative of normal conditions, it is obvious that Lake Mendota 

 has much more living material in its plankton than Western Lake Erie, 



177 



