RECENT FRESHWATER INVESTIGATIONS 329 



June; then a midsummer maximum in July, a late summer 

 minimum in August and an autumn maximum in September or 

 October, followed by a decline to the winter minimum of 

 December to April. 



The seasonal distribution of individual organisms has been 

 studied by a host of observers, prominent among whom is 

 Zacharias. In the first report of the Plon station (93) records 

 of certain species are given and others are added in each sub- 

 sequent volume. Calkins (93a) notes a definite culmination 

 for each organism, no two falling at the same time, though 

 most occur during the summer. The diatoms find a maximum 

 in the spring with low temperature of the water, the Cyanophy- 

 ceae at the end of the hot season with a high temperature of 

 the water and the algae in general at the time of the fall over- 

 turning. Zacharias (95d) and others find a considerable agree- 

 ment in the periodicity of organisms in successive years, while 

 Birge (97a) looks upon the periodicity as really biennial. 



At Plon Zacharias (96h) is able to distinguish a winter and a 

 summer plankton and also for a brief period a fall and spring 

 plankton. In October and November the Copepoda rule so 

 that there is nearly a pure copepod plankton; from March to 

 May the diatoms are almost alone and in enormous numbers. 

 This is related to temperature as Schroder (98a) shows that in 

 colder alpine lakes and in streams the diatoms rule while in 

 ponds and lakes of higher temperature their place is taken by 

 the Schizophyceae. Precise data on the seasonal distribution 

 of different Crustacea are contained in the work of Marsh (97) 

 and Birge (97a) who have traced individual species through 

 long periods. 



Lauterborn (98) has made observations of importance on the 

 limnetic Rotatoria — nearly half are eurythermic, or perennial; 

 about the same number are stenothermic of the summer variety 

 and only two stenothermic with preference for the winter tem- 

 perature. The summer and winter forms are all monocyclic, 

 while the perennial species are dicyclic or polycyclic, i. e. 

 producing males and "winter eggs" two or more times yearly. 



