748 FRESH-WATER BIOLOGY 



As already noted, certain species appear in temporary pools only 

 in the spring season. In those that occur in lakes, there is some- 

 times a pronounced seasonal distribution. For example, in Green 

 Lake, Wis., on which extended studies have been made, Diaptomus 

 sicilis is common in the winter, but rarely found in the summer, 

 while most forms, as would be expected from the favorable food 

 conditions, are more abundant in the summer months. 



Excepting the few winter forms, the maximum numbers of any 

 species occur in the months from May to September or early 

 November. Sometimes there are two maxima, one in the spring 

 and one in the fall. Generally speaking, the maximum develop- 

 ment occurs when the waters reach their highest temperature, but 

 other factors may modify the time. Generally speaking, also, the 

 maximum development in numbers is somewhat later in deep 

 lakes than that in shallow lakes, corresponding to the general law 

 for the development of the total plankton. 



The great controUing factor in the distribution of the Copepoda 

 is, without doubt, temperature. That Diaptomus sicilis should be 

 found in Green Lake only in the winter is a matter of temperature, 

 for it is found in the cold waters of the Great Lakes throughout the 

 summer. Limnocalanus macrurus, which, in small, deep lakes, is 

 found during the summer only below the thermocline, comes to 

 the surface in the winter months when the surface water is colder. 

 In Wisconsin, Cyclops bicuspidatus is a common limnetic species in 

 the deeper lakes, but is rarely found in the shallower lakes except 

 in the winter season. It is evident that it prefers the colder waters. 

 On the other hand, Epischura lacustris and Diaptomus oregonensis 

 are distinctly summer forms, disappearing, for the most part, in 

 the winter months. 



Partial studies have been made which have disclosed some very 

 interesting facts in regard to the vertical distribution of Copepoda 

 in our lakes. In general it may be said that most of these forms 

 are confined to the upper waters, above the thermocline, some 

 having very distinct vertical migrations, caused by changing con- 

 ditions of light and temperature. It has long been known that 

 not only are the open waters of our lakes peopled with myriads 

 of these minute creatures, which can readily be collected by a tow- 



