(m& 



14 PLANKTON OF WINNEBAGO AND GREEN LAKES. 



lar fall growth, and that other growths may occur at irregular 

 intervals as the wind happens to stir up the water ; that the two 

 most important conditions for the growth of the diatoms are 

 a sufficient supply of nitrates and a free circulation of air, and 

 that both these conditions are found at those periods of the year 

 when the water is in circulation. Whipple gives facts in re- 

 gard to the annual distribution of diatoms in Massachusetts 

 ponds which seem thoroughly to substantiate ■ his contention. 

 I have already referred to this work in a former paper (Marsh 

 '99), accepting these conclusions. In looking over the occur- 

 rence of the diatoms in Green lake and Lake Winnebago I 

 find nothing to contradict this theory in the occurrence of Cyclo- 

 tella, Melosira, Synedra, Fragilaria, or Stephana discus. The 

 occurrence of Cyclotella in Lake Winnebago especially seems 

 to confirm his statements. 



The occurrence of Asterionella in Green lake, however, dif- 

 fers distinctly from his results in Massachusetts waters. His 



e^z? general conclusion in regard to Aslevionella is that its two max- 

 imum periods come after the spring and fall overturning when 

 the spores, — if diatoms have spores, — are brought to the sur- 

 face accompanied by the food materials that have been forming 

 in the abyssal waters during the stagnation period. ISTow in 

 Green lake the maxima, as found in three winters, came in 

 the depth of winter when the lake was covered with eighteen 

 inches or more of ice, and at a period midway between the two 

 overturnings of the water. It is evident that Whipple's ex- 

 planation does not apply in this case. Why there should be 



> <eg»- this enormous production of Asterionella in mid-winter I do not 

 at all understand. Voigt (Voigt '02) reports Asterionella as 

 having a similar winter maximum in Trammer-See and Ede- 

 berg-See. 



xH 



,<l 4 Clostermm. 



5 Two or three species of Closterium- occur in both lakes as 



occasional members of the plankton. They cannot be considered 

 as true limnetic species, but rather as migrants from the littoral 

 flora. 



