176 Wisconsin Academy of Sciences, Arts, and Letters. 



production, although in some of these cases the results are com- 

 plicated by the fact that the temperature of the surface water 

 has a direct effect on the reproduction. 



The matter of the annual distribution of the organisms of the 

 plankton is a very interesting question, but is also a very compli- 

 cated one. As has iust been stated, certain of the diatoms have 

 a distinct spring and fall maximum, and there are other organ- 

 isms which, because of their dependence upon the diatoms, have 

 similar periods of maximum gTowth. But there are other 

 causes at work which control the growth of individual organ- 

 isms, so that tlieir optimum periods may come at very different 

 times of the year. 



Generally speaking it is probably true that the largest amount 

 of plankton occurs in midsummer, although Yung says that the 

 maximum development of plankton in Lake Leman is in May 

 or June, and that the minimum is in March and September.. 

 Zacharias states that the maximum period for Lake Plon is 

 about August 1st. Measurements of a large number of collec- 

 tions from various lakes in Wisconsin indicate that the maxi- 

 mum of plankton occurs in these lakes in the latter part of July. 

 There may, however, be marked exceptions to this general state- 

 ment. For example, I have found in Green Lake a development 

 of diatoms in midwinter so enormous as to make the total plank- 

 ton even greater than in summer. The exact period of maxi- 

 mum development mav varv from vear to vear, and in different 

 lakes, because of varying local conditions. The reason for this 

 July maximum seems to be both because a considerable number 

 of organisms have their highest development, and because there 

 is a greater variety of forms at this time than at any other time 

 of the year. This is undoubtedly because of the peculiarly 

 favoring conditions of temperature. It is this time of the year 

 that seems to be especially favorable to the growth of the algai. 

 The "water-bloom'' may appear in June and remains sometimes 

 until into September, but it is in July and August that it is 

 present in the greatest quantity. This summer, on Lake Win- 

 nebago, it was most abundant during tlie latter half of August. 



The minimum of plankton development is in the winter 

 months, especially in January and February. At no time of 



