48 



PLANKTON OF WISCONSIN LAKES 



its maximum. Likewise in early October of that year the rise in the 

 ash was not proportionately as rapid as in the organic matter. 



The marked vernal rise in the organic matter in 1912 (fig. 16) was 

 not accompanied by an equally marked increase in the ash. Between 

 the first of May and the end of the first week in June, the former rose 

 from 83.6 milligrams per cubic meter of water to 574.4 milligrams, 

 almost a sevenfold increase, while the ash, during the same interval of 

 time, increased from 18.1 milligrams to 63.7 milligrams, the latter being 

 only three and a half times the former. The increase in organic matter 

 at this time was due largely to an increase in the number of copepods 

 and usually these organisms possess a low percentage of ash. During 

 the last week of July in this year there was also a distinct rise in the 



Fig. 17. — The quantity of dry organic matter and of ash in the net plankton 

 of Lake Mendota in 1913. Curve A represents the organic matter and curve 

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

 water. 



organic matter which was not accompanied by a corresponding increase 

 in the ash ; in this case, the rise of the former was due chiefly to a crop 

 of Ceratium and the percentage of ash in this organism is low. The rise 

 in organic matter in late August and early September was due to dia- 

 toms and it was accompanied by a similar rise in the ash. During the 

 remainder of this year the two curves are similar, but the ash showed 

 a proportionately greater decrease in November than the organic 

 matter. 



The vernal increase of ash in 1913 (fig. 17) did not begin as early 

 as that of the organic matter and the curve for the former does not 



