NANNOPLANKTON OF LAKE MENDOTA 85 



Minute flagellates were found in the nannoplankton of Lake Mendota 

 at all seasons of the year. Most of them, perhaps all of them, belonged 

 to the form described by Lewis 3 in 1913 under the name of Chloro- 

 chromonas minuta. Lewis' description is based on material obtained 

 from Lake Mendota, but the form has been noted in lakes in Iowa and 

 New York as well, which indicates that this flagellate is rather widely 

 distributed. In Lake Mendota it was found in very considerable num- 

 bers at times, especially during the month of May. During the third 

 week in May, 1916, the number reached two and a quarter million per 

 liter of water in two sets of observations. The average number for 137 

 samples in which this form was found, was 237,000 per liter. 



Cryptomonas was found pretty regularly in the various counts, but 

 the breaks in the curves show that it was not found at certain periods 

 both in winter and in summer. It was most abundant in November, 

 1915, when more than 300,000 individuals per liter of water were noted 

 in two samples. The average for 112 samples in which this form was 

 noted, was 45,000 individuals per liter of water. Another flagellate, 

 Euglena, was found in 1915 ; it appeared chiefly during the months of 

 September and October, but the numbers were relatively small, ranging 

 from about 1,000 to 13,000 individuals per liter of water. 



Ciliated protozoa of various sizes were found in the majority of the 

 catches, but they were rather irregular in their distribution as indicated 

 by the breaks in the curve representing them. A fairly large anaerobic 

 ciliate appeared in the lower water each year during the latter part 

 of the summer. 4 



Several genera of the blue-green algae (Cyanophyceae or Myxophy- 

 ceae) and of green algae (Chlorophyceae) were represented in the ma- 

 terial as indicated in the diagrams. A few other forms appeared from 

 time to time, but they were found in such small numbers and so irre- 

 gularly that they have not been shown in the figures. 



The most important alga belonging to the blue-green or green group 

 of forms is an Aphanocapsa with very minute cells, probably Aphano- 

 capsa delicatissima West. It was found at all seasons of the year and 

 at all depths of the lake. This alga was obtained at a depth of 170 

 meters (558 feet) in Seneca Lake, New York, and it seems to have a 

 wide range geographically, since it has been noted in the nannoplank- 

 ton of various Wisconsin lakes, in Seneca and Cayuga Lakes, New York, 

 and in West Okoboji Lake, Iowa. Usually the colonies are rather small, 

 having only 15 to 50 cells, but sometimes a colony is found which has 

 100 cells or more. This Aphanocapsa was most abundant in Lake 

 Mendota in April and May, but the numbers were fairly large during 



"Archiv fur Protistenkunde, Bd. 32, 1913, pp. 249-256, 1 pi. 

 4 See Juday, Biological Bulletin, Vol. 36, 1919, pp. 92-95. 



