50 



PLANKTON OF WISCONSIN LAKES 



in ash. This increase in the organic matter was due to a rise in the 

 number of Cyclops and Microcystis, both of which possess a relatively 

 low percentage of ash. The ash and organic matter rose simultaneously 

 about the middle of May and the former declined rapidly during the 

 latter part of the month, thus making a sharp peak in this curve ; but 

 the organic matter remained high during this time and rose to a maxi- 

 mum at the end of the first week in June, so that this curve is character- 

 ized by a broad apex covering this period. One other difference may 

 be noted here ; there was a marked decline in the ash during the second 

 week of October to which no similar decrease in organic matter cor- 

 responded. 



SOO 



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Fig. 19. — The quantity of dry organic matter and of ash in the net plankton 

 of Lake Mehdota in 1915. Curve A represents the organic matter and curve 

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

 water. 



In 1917, the curve for ash shows a prominent peak about the first of 

 May, but the organic matter shows a steady increase following this date 

 instead of a decline similar to the ash. The increase in the organic 

 matter during May was due chiefly to an increase in the Crustacea and 

 Aphanizomenon ; these forms yield a much smaller percentage of ash 

 than the diatoms which produced the rise in late April and early May. 



In general, then, whenever the increase in organic matter is due to 

 the Crustacea, the rotifers, and the algae, exclusive of the diatoms, the 

 accompanying rise in the ash is relatively small ; but when the diatoms 



