NANNOPLANKION OF LAKE MENDOTA 69 



after the quantity of organic matter showed considerable variation, 

 but remained relatively high until the last run was made on December 

 6, 1915. 



Between December 6, 1915, and February 11, 1916, the organic mat- 

 ter decreased from 1,719.0 milligrams to approximately 995.0 milli- 

 grams per cubic meter of water, a decline of 43.0 per cent. By March 

 8, 1916, the amount was almost double what it was on February 11, and 

 it rose to more than treble the amount on the latter date by April 15. 

 During the succeeding three weeks there was a marked decline so that 

 the vernal peak in the curve for this year is a very prominent one and 

 reaches the highest point of the entire series. 



A secondary peak covers the period from the second week in May to 

 the middle of June, 1916, and another reaches its maximum height 

 about the end of the first week in July. Following this a gradual de- 

 cline continued until the end of the first week in September; during 

 the remainder of this month there was a rapid rise which was followed 

 by an equally rapid decline in October. Thus, the curve for this period 

 possesses a sharp and prominent peak which gives it a very different ap- 

 pearance from the curve of 1915. This decline continued in 1916 until 

 the minimum amount of the year was reached on November 1. The fol- 

 lowing six weeks were characterized by two minor increases in the 

 organic matter which were succeeded by relatively small decreases, so 

 that the curve presents two small peaks covering this period of time. 



A decrease of about 8.0 per cent in the organic matter of the nanno- 

 plankton was noted between December 12, 1916, and January 18, 1917 ; 

 this decline continued until March 9, 1917, when the minimum of the 

 entire series was found. By April 18 the organic matter had nearly 

 doubled in amount and the increase continued until the vernal maxi- 

 mum was reached on May 7 ; this maximum was a little more than three 

 times as much as the minimum of March 9. There was a marked decline 

 between May 7 and May 16 so that the curve presents a well defined 

 vernal peak ; this decline was followed by a prominent increase on May 

 23 and this was followed by a decrease on June 1, 1917, when the ob- 

 servations were discontinued. 



In a general way the curves in figure 28 show that the annual cycle 

 of the nannoplankton is separated more or less definitely into four 

 phases corresponding to the four seasons of the year ; the same fact has 

 been noted for the net plankton. It will be observed, however, that the 

 autumnal phase may not be as prominent in the nannoplankton in some 

 years, such as 1915 for example, as it was each year in the net plankton. 

 A distinct vernal maximum was noted for each of the three years cov- 

 ered by these observations and it proved to be the maximum of the year 

 in both of the complete years, namely, 1915 and 1916. 



