NET PLANKTON OF LAKE MENDOTA 27 



The results obtained for the organic matter in the net plankton of 

 Lake Mendota are summarized in table 6 (p. 187) in which the maxi- 

 mum, minimum, and mean quantities are indicated for the different 

 years. The data are given in detail in the general table. No. 43, p. 202. 

 During the period of these observations the amount of dry organic 

 matter was smallest in 1911 ; the mean for this year was only 175 milli- 

 grams per cubic meter of water, or about half as much as in the other 

 full years. Next in order come 1914 and 1917, but in these years the 

 observations were discontinued respectively on July 2 and June 1, so 

 that data are available for only the first half of these years. The 

 results for these two years, therefore, are not comparable with those 

 covering a full year. Adding the catches obtained between July and 

 December, 1913, to the 14 secured during the first half of 1914 gives a 

 mean of 363 milligrams of organic matter per cubic meter of water, 

 while the catches taken between June 1, 1916, and June 1, 1917, give an 

 average of 324 milligrams. Both of these averages are comparable in 

 amount with those obtained during the full-year periods of 1912, 1913, 

 1915, and 1916, in which the means range from 343 milligrams to 393 

 milligrams per cubic meter of water. Thus, for the four full-year pe- 

 riods the largest mean exceeded the smallest by less than 15.0 per cent. 



The seasonal changes in the amount of organic matter in the net 

 plankton are shown in two diagrams, figures 7 and 8. The curves 

 for 1913-14 (fig. 7) and for 1916-17 (fig. 8) cover all four seasons 

 of the year; since they cover the complete annual cycle, they will 

 be considered in some detail here. In these diagrams the vertical 

 spaces represent the number of milligrams of organic matter per cubic 

 meter of water, the scale being indicated in the left margin. The hori- 

 zontal spaces represent the months of the year, each month being di- 

 vided into four equal parts. In the curves the results obtained in the 

 various observations are shown by the circles; the curves themselves 

 have been constructed simply by connecting these points without at- 

 tempting to round them off into more symmetrical diagrams. These 

 diagrams are based on the data given in table 43. 



The first sample obtained in 1913, namely, on April 16-18, yielded 

 185.1 milligrams of organic matter per cubic meter of water. Follow- 

 ing this the amount rose rapidly to a maximum of 462.3 milligrams on 

 May 7-9 and it then remained high until June 2-6. A marked decrease 

 followed, a minimum of 126.9 milligrams being found on June 30-July 

 3. This was succeeded by a decided rise in July and early August, the 

 amount reaching a maximum of 534.0 milligrams on August 4-7. This 

 summer maximum was 71.7 milligrams larger than the vernal maxi- 

 mum. A very marked and rapid decrease to 87.2 milligrams on Au- 

 gust 19-22 came next; this amount proved to be the minimum of the 

 year. 



