PLANKTON OF LAKE MONONA 113 



cubic meter, was found about the middle of June. Thus, the summer 

 minima were much higher in Lake Mendota in both years than in Lake 

 Monona; the minimum in the former lake which came nearest to the 

 summer minima of the latter was the one obtained on March 9, 1917, 

 when the nannoplankton of Lake Mendota yielded only 795.2 milli- 

 grams of dry organic matter per cubic meter of water. The range of 

 variation in the amount of organic matter in the nannoplankton of Lake 

 Monona is much greater, therefore, than in that of Lake Mendota. 



The results for organic matter in the nannoplankton of Lake Monona 

 can not be indicated by a diagram for the year 1915 because the ob- 

 servations were not made frequently enough in that year for this pur- 

 pose, but the collections were made with sufficient regularity in 1916 

 for the construction of a curve showing the results. The curve marked 

 C in figure 37 indicates the number of milligrams of dry organic matter 

 per cubic meter of water in the nannoplankton samples obtained in 

 1916. This diagram shows that there was a decline in the organic 

 matter between May 12 and June 14, with only a slight rise between the 

 latter date and July 6. By July 18, however, there was an appreciable 

 increase in the quantity and this rise continued thereafter until it 

 reached a maximum point on October 12. A very marked decrease 

 followed on October 29 and on November 9, so that the curve shows 

 a sharp peak covering the autumnal period. Attention should be called 

 to the fact here that, in late June and early July, curve C falls below 

 curve B in this diagram ; the former curve ( C ) represents the organic 

 matter in the nannoplankton and the latter (B) that in the net plank- 

 ton. That is, during this period of time the nannoplankton yielded a 

 smaller amount of organic matter per cubic meter than the net plank- 

 ton. This phenomenon was noted in two samples obtained in 1915 and 

 in two collected in 1916. 



According to curve C the story of the organic matter in the nanno- 

 plankton of Lake Monona was comparatively simple in 1916. The first 

 observation of that year served to show that there was a more or less 

 marked vernal maximum ; this was followed by a decline to a summer 

 minimum in late June and early July. Thereafter the organic matter 

 steadily increased in amount until an autumnal maximum was reached 

 on October 12 ; a marked decrease was noted during the following month 

 and this decline continued, doubtless, until a winter minimum was at- 

 tained. The annual cycle, therefore, consisted of four phases, namely, 

 summer and winter minima separated by spring and autumn maxima, 

 just as has been noted for Lake Mendota. A comparison of curve C 

 of this diagram with curve B in figure 35 shows, though, that the vari- 

 ous changes were much less complex in Lake Monona than in Lake Men- 

 dota during. the same period of time in 1916, so that the curve is much 

 simpler for the former lake than for the latter. 



