GENERAL CONSIDERATIONS ON SEASONAL CHANGES.* 



It follows from the facts set forth in the preceding discussion that 

 in general each month of the year, characterized by a certain range 

 of hydrographic, thermal, and chemical conditions, and of illumi- 

 nation, has a plankton characterized as follows : 



1. There is a certain range of component species, some of 

 which are occasional stragglers and others more or less uniformly 

 present. 



2. There is a certain range of numbers of individuals, varying 

 with the species and profoundly affected by fluctuations in the 

 environmental factors, which change the proportions of the various 

 species from year to year. These proportions vary also from month 

 to month and constitute one of the main elements in the seasonal 

 changes of the plankton. 



3. Transitions from month to month are most profound at 

 seasons of greatest environmental change, as, for example, at the 

 times of vernal increase and autumnal decline in temperatures. 



4. Seasonal changes in the plankton follow the environmental 

 changes and not the calendar. Autumnal plankton is found when 

 autumnal temperatures arrive. 



5. In the main, but two types of plankton are found in the 

 Illinois River the summer, and the winter assemblage. The vernal 

 and autumnal types are only transitions between the two when 

 organisms from both are present. The winter plankton is charac- 

 terized by a small number of species peculiar to that season, and a 

 number of perennial forms; the summer, by a larger number of 

 summer organisms with the perennial types. 



LAKE VERSUS RIVER PLANKTON. 



Is the plankton of streams (potamoplankton) different from 

 that of lakes (limnoplankton) and ponds (heleoplankton) ? This 

 terminology, introduced by Zacharias ( '98 and '98a), seems to imply 

 a distinction which lies not only in the differences in the configura- 



* The detailed discussion of seasonal changes in the plankton is deferred to a 

 later paper. 



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