46 



PLANKTON OF WISCONSIN LAKES 



sented; the latter was found in 1912. The highest maximum per- 

 centage, nearly half of the dry weight, was noted in 1915 and the lowest 

 in 1914. The maximum percentage of ash each year was correlated 

 with the appearance of large numbers of diatoms ; in four of the years 

 in which the observations continued from spring until late autumn or 

 early winter, the highest percentage of ash was found in September and 

 October, but in 1911 it came in August, being correlated with a crop 

 of diatoms which flourished during this month. The very low maxi- 

 mum in 1914 was due to the scarcity of diatoms during the vernal 

 period of that year and the observations did not cover the other diatom 

 season, namely, the autumn. 



Thus the mean percentage for 1914 is by far the lowest of all, but 

 that for 1917, in which year the observations also covered only the 



^mo 



^oo 



200 



too 



Fig. 15. — The quantity of dry organic matter and of ash in the net plankton 

 of Lake Mendota in 1911. Curve A represents the organic matter and curve 

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

 water. 



vernal period, is exceeded by only two other years. Excluding the two 

 parts of years, the mean percentage of ash shows a gradual increase 

 from 1911, the lowest, to 1915 and 1916, the highest, the last two years 

 being substantially the same. This is in accordance with the fact al- 

 ready pointed out that most of the samples having less than 20.0 per 

 cent of ash were collected during the period 1911 to 1914. The mean 

 percentage of ash for the entire series of 184 net samples is 23.5 per cent 

 of the dry material. 



The relative amounts of organic matter and of ash or inorganic 

 matter in the net plankton of the various years are shown graphically 

 in figures 15 to 20, inclusive. Both are indicated in milligrams per 



