NET PLANKTON OF LAKE MENDOTA 



47 



cubic meter of water and the space between the two curves represents 

 the excess of organic matter over the inorganic matter. In these dia- 

 grams the curves for the former are marked A and those for the 

 latter B. 



The curves representing the ash possess, in a general way, the same 

 configuration as those of the organic matter ; that is, they show spring 

 and autumn periods of maxima and summer and winter periods of 



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Fig. 16. — The quantity of dry organic matter and of ash in the net plankton 

 of Lake Mendota in 1912. Curve A represents the organic matter and curve 

 B the ash. The amounts are indicated in milligrams per cubic meter of 

 water. 



minima, which have already been noted for the organic matter. The 

 ash is subject to certain irregularities in summer, however, just as 

 noted in the organic matter. A closer comparison of the curves for the 

 various years shows that more or less marked differences exist. In 

 1911, for example (fig. 15), the curve for ash does not reach its highest 

 point in August until nearly a week after the organic matter reaches 



