85 PLANKTON OF WISCONSIN LAKES 



the other months of the year also. In one set of observations in April, 

 1915, a maximum of somewhat more than 8 million colonies per liter 

 was noted. 



The other forms of blue-green and green algae were found chiefly 

 in the summer and autumn. Coelosphaerium and Oocystis were the 

 most regular in their appearance, but breaks in the curves representing 

 them show that they were not found at times. With respect to the 

 breaks in the various curves it may be said that a break does not mean, 

 necessarily, that this particular form had disappeared entirely for the 

 period covered by the break in the curve, but it does mean that the 

 form was so scarce that it was not noted in the samples for this period 

 of time ; by centrif uging a larger sample of water for the enumeration, 

 say 100 or perhaps 200 cubic centimeters, some of the breaks might 

 have been eliminated, but in all probability not many of them. Coelo- 

 sphaerium was more abundant as well as more regular in its appear- 

 ance than Oocystis ; the other forms of blue-green and green algae were 

 still more irregular in their appearance in samples as indicated by the 

 more numerous breaks in the curves representing them. 



Three genera of diatoms were found in the nannoplankton, namely, 

 Cocconeis, Cyclotella, and Stephanodiscus. Cocconeis appeared at ir- 

 regular intervals during the period from March to December, 1916, but 

 the number was relatively small. A small form of Cyclotella was 

 found in all of the 1915 samples and it was present in the 1916 catches 

 from February to October with the exception of one set of observations 

 during the latter part of August. In November and December it was 

 noted as irregular and it was not observed in any of the samples be- 

 tween the middle of January and the last week of April, 1917. 



Stephanodiscus astraea * showed a most interesting periodicity dur- 

 ing this series of observations as indicated in the diagrams. This disc- 

 shaped diatom is very small, the diameter ranging from 6.5/a to 9.0jtx 

 with an average of approximately 8.0/*. It appeared about the middle 

 of April in 1915, rose to a maximum during the second week in May, 

 and then declined in numbers, rather rapidly at first and then more 

 gradually until it finally disappeared about the first of July. In 1916 

 this diatom was found during the latter part of March, increased 

 rapidly in numbers in early April, and reached its maximum point in 

 the third week of this month. There was a rapid decrease in the last 

 week of April and a more gradual decline in May, the form disappear- 

 ing entirely by the first of June. In 1917 this diatom again made its 

 appearance in the latter part of March, but the number rose more 

 slowly so that the maximum was not reached until the end of the first 

 week in May. This was followed by a rapid decrease in numbers, but 



We are indebted to Dr. Albert Mann for the identification of this diatom. 



